1

Ammonium0. 1 – 1.5 g

Buffering the urine

2

AMINO ACID METABOLISM• Amino acids are required for the synthesis of proteins,

peptides, nucleotides, neurotransmitters, other amino acids

• Free amino acids can be provided to cells either from the digestion of dietary proteins or the degradation of defective or aged cellular proteins

• Amino acids are catabolized into components that can directly join energy production pathways or be changed to glucose, fatty acids or ketone bodies; this happens:

when the amount of amino acids obtained from digestion and degradation is more than what is needed for biosynthesis

during starvation or uncontrolled diabetes mellitus• The catabolism of amino acids produces:

amino group – removed as urea carbon skeleton – seven types of intermediates

3

The digestion of proteins • The digestion of proteins begins in the stomach and

continues in the small intestine o Please refer to the notes on enzymes for the types,

activation and specificity of the proteases involved in the digestion

• Rennin (chymosin) is important in infants because it breaks a specific peptide bond in casein (a milk protein) curdling the milk and increasing transit time in the stomach

• Proteases can be endopeptidases or exopeptidases • The endopeptidases are specific for different types of

peptide bonds and produce fragments of varying sizes• Exopeptidases take over the job:

carboxypeptidase A and B – produced by the pancreas and cleave at the C-terminal

brush border aminopeptidases – act on the N-terminal of oligopeptides yielding free amino acids and di- and tripeptides

4

5

• Digestive enzymes themselves are digested and contribute to the amino acid pool

• Free amino acids are transported into intestinal epithelial cells by Na+ - dependent secondary transport

• Di- and tripeptides enter the epithelial cells through symport with H+ the H+ gradient is maintained by the Na+ - H+

exchanger peptidases in the epithelial cells change the di- and

tri- peptides into free amino acids • Amino acids enter the portal vein by facilitated transport• Many cells use Na+ - dependent secondary transport and

to some extent facilitated transport in order to absorb amino acids

o premature activation of zymogens inside the pancreas results in acute pancreatitis

6

7

Protein turnover• The degradation and resynthesis of proteins• The half-lives of eukaroytic proteins may vary from 30

seconds to many days ornithine decarboxylase – approx. 11 minutes;

hemoglobin – lifespan of red blood cells; crystallin (lens protein) – life span of the organism

• Rapidly degraded proteins include those proteins that are defective due to wrong insertion of amino acids or damage accumulated during normal functioning, regulatory enzymes,

• A protein's half-life correlates with its N-terminal residue

• Proteins with N-terminal Met, Ser, Ala, Thr, Val or Gly have half lives greater than 20 hours

• Proteins with N-terminal Phe, Leu, Asp, Lys or Arg have half lives of 3 min or less

8

autophagosome autophagic vacuole

(lysosome)

• PEST proteins having domains rich in Pro (P), Glu (E), Ser (S) and Thr (T) are more rapidly degraded than other proteins

• There are two ways of intracellular degradation of proteins1. lysosomal degradation - of endocytosed proteins or proteins undergoing autophagy

• In autophagy, part of the cytoplasm may become surrounded by two concentric membranes •Fusion of the outer membrane of this

autophagosome with a lysosomal vesicle results in the degradation of enclosed cytoplasmic structures and macromolecules

• the enzymes responsible for the

degradation are cathepsins

9

2.ATP- dependent cytosolic degradation• The ubiquitin – proteasome pathway• Ubiquitin is a small protein having 76 amino

acids only• It is present in all eukaryotes (hence the name)

and its amino acid sequence is highly-conserved

• Ubiquitin marks proteins for death the carboxyl terminal of a ubiquitin forms

an isopeptide bond with the ε-amino group of a lysine residue of a protein to be destroyed

• Three enzymes, are involved in the attachment of ubiquitin:

Initially the terminal carboxyl group of ubiquitin is joined in a thioester bond to a cysteine residue on Ubiquitin-Activating Enzyme (E1) ; this is an ATP-dependent process

10

The ubiquitin is then transferred to a sulfhydryl group on a Ubiquitin-Conjugating Enzyme (E2)

Ubiquitin-Protein Ligase (E3) then promotes the transfer of ubiquitin from E2 to the ε-amino group of a Lys residue of a protein recognized by that E3

• The substrate-specificity of this system comes from the various combinations of the types of E2 and E3

• More ubiquitins are added to form a polyubiquitin chainThe terminal carboxyl of each ubiquitin is

linked to the ε-amino group of a lysine residue (Lys 29 or Lys 48) of the adjacent ubiquitin

A chain of 4 or more ubiquitins targets proteins for degradation in proteasomes

11

12

• The proteasome is a complex of multiple proteases

• Constitutes nearly 1 % of cellular protein• It contains two main types of subcomplexes: a

barrel-like core particle (20S) and regulatory 19S particles on both ends of the barrel

the catalytic core particle and the regulatory particles make up the functional 26S proteasome

• The 19 S particles may unfold proteins and translocate the unfolded proteins into the 20 S particle; energy from ATP is consumed in the process

• The 19 S particles also cleave isopeptide bonds and free ubiquitin; ubiquitin is recycled

• The protein is degraded by the 20 S particle and free amino acids are released into the cellular space

13

14

• Proteasomal degradation of particular proteins is an essential mechanism by which cellular processes are regulated, such as cell division, apoptosis, differentiation and development progression through the cell cycle is

controlled in part through regulated degradation of proteins called cyclins that activate cyclin-dependent kinases

• Inability to degrade proteins that activate cell division (or rapid degradation of those proteins that suppress tumor formation) can lead to cancer

• Diseases like Alzheimer's, Parkinson’s , type II diabetes,… are associated with the deposition on tissues of non-degradable protein aggregates known as amyloid

15

The catabolism of amino acidsThe fate of amino acid nitrogen • α-amino groups are removed from amino acids

mostly through transamination reactions the amino acid becomes a keto acid when it

donates the amino group to α-ketoglutarate (changing it to glutamate)

• All amino acids except lysine and threonine undergo transamination reactions

• The enzymes involved are known as transaminases or aminotransferases

• Pyridoxal phosphate (PLP) is the cofactor • The glutamate thus derived collects the amino

groups and gives them off for biosynthesis or excretion

16

17

NH

CO

P

OO

O

O

CH3

HC

H2

N

(CH2)4

Enz

H+

RHC COO

NH2

Enzyme (Lys)-PLP Schiff base

Amino acid

NH

CO

P

OO

O

O

CH3

HC

H2

N

HC

H+

R COOEnz LysNH2

Amino acid-PLP Shiff base (aldimine)

NH

CO

P

OO

O

OH

CH3

CH2

NH2

H2

R C COO

O

Enz Lys NH2

Pyridoxamine phosphate (PMP)

α-keto acid

NH

CO

P

OO

O

O

CH3

HC

H2

N

HC

H+

R COOEnz LysNH2

Amino acid-PLP Shiff base (aldimine)

18

• Free ammonium can be released in different ways:

A.The oxidative deamination of glutamate by glutamate dehydrogenase - the only enzyme that can use NAD+ or NADP+ as electron acceptor

The reaction is reversible and takes place in the mitochondria

B. Serine and threonine dehydratases release NH4

+; histidine can be directly deaminated to give NH4

+

C.Intestinal bacteria produce NH4+ from amino

acids or urea; the ammonia enters the portal vein

D.Glutamine and asparagine lose their side chain amino groups through deamidation

E. The purine nucleotide cycle in the brain – aspartate is used as a substrate and fumarate and ammonium are released

19

20

The urea cycle• Nitrogen balance is the difference between the

amount of nitrogen consumed and excreted per day

• Positive nitrogen balance in growing individuals• Negative nitrogen balance during protein

deficiency or starvation• In healthy adults, the amount of nitrogen

consumed and excreted is approximately equal• Nitrogen is excreted in the form of urea, uric acid,

ammonium, creatinine, hippurate, creatine • Humans are ureotelic (excrete excess nitrogen

mainly in the form of urea) ammonotelic (ammonia); uricotelic (uric

acid)• Ammonium made available in the liver from

different sources enters the urea cycle

21

• Since the enzymes of the urea cycle are present in the liver only, amino groups from other tissues should be transported to the liver

• Two mechanisms of transport:1.the skeletal muscles export alanine synthesized

from the transamination of pyruvate (glucose catabolism) by glutamate the amino group donated by glutamate was

obtained from the breakdown of amino acids in the muscle

22

2.Glutamate can accept another amino group through an ATP-dependent reaction catalyzed by glutamine synthetase Glutamine is used by most tissues to transport ammonium

Glutamine travels to the liver, kidneys and the intestine and is deamidated the ammonium released by the deamidation is

used as a buffer (in the kidneys) or enters the urea cycle in the liver; glutamine can be used as an energy source by the intestine

23

o The two nitrogen atoms of urea enter the urea cycle as NH4

+ and as the amino N of aspartate1. The synthesis of carbamoyl phosphate • The NH4

+ and HCO3- (carbonyl C) that will be part

of urea are incorporated first into carbamoyl phosphate

The cleavage of 2 ATP molecules is needed to form the high energy carbamoyl phosphate

Carbamoyl phosphate synthetase (CPS I) is a mitochondrial enzyme; the cytosolic isozyme is involved in pyrimidine synthesis

• CPS I has an absolute requirement for the allosteric activator N-acetylglutamate

• This derivative of glutamate is synthesized from acetyl-CoA and glutamate when cellular glutamate is high, signaling an excess of free amino acids due to protein breakdown or dietary intake

24

2.The formation of citrulline• Carbamoyl phosphate reacts with ornitihine to give

citrulline; catalyzed by ornithine transcarbamoylase

3. The entry of the second N• Citrulline leaves the mitochondria in exchange for the

entry of ornithine from the cytosol• Citrulline reacts with aspartate producing

argininosuccinate • Argininosuccinate synthetase requires the splitting of

ATP to AMP and PPi

25

ATPAMP+ PPi

argininosuccinatesynthetase

+Pi

ornithine trans-

carbamoylase

+

26

4. The formation of arginine• Argininosuccinate lyase produces arginine and

fumarate• The arginine produced by the urea cycle is enough

for adults• The carbons of fumarate are those that were

obtained from aspartate Fumarate be changed to oxaloacetate by

enzymes of the citric acid cycle The oxaloacetate will receive an amino group

from glutamate and be changed to aspartate; aspartate reenters the urea cycle

The TCA and urea cycles constitute a bicycle: The Krebs bicycle

5. The production of urea and the regeneration of ornithine

• The action of arginase produces urea and ornithine

• Urea travels to the kidneys and excreted through the urine

27

argininosuccinase

+

28The urea cycle

29

The Krebs bicycle

The stocihiometry of the urea cycleNH4

++ CO2 + 3 ATP+ 2H2O→urea + fumarate + 2 ADP + AMP+ 4Pi

30

In addition to the allosteric effects of N-acetylglutamate, the synthesis of the enzymes of the urea cycle can be induced during periods of increased metabolism – protein rich diet or prolonged fasting

Urea cycle abnormalities• Hereditary deficiency in any one of the urea cycle

enzymes or liver cirrhosis lead to the increase in the blood of ammonia (hyperammonemia) or urea cycle intermediates

• The total lack of any urea cycle enzyme is lethal • Elevated ammonia is toxic, especially to the brain• Why is ammonia toxic to the brain? Hypotheses:

1. High ammonia levels would drive glutamine synthetase; this would deplete glutamate – a neurotransmitter and precursor for the synthesis of GABA• Glutamine may exert osmotic effects leading

to the swelling of the brain

31

2. Depletion of glutamate and high ammonia levels would drive the glutamate dehydrogenase reaction in the reverse direction; the resulting depletion of α-ketoglutarate, inhibits the production of energy

The treatment of urea cycle defects• limiting protein intake to the amount barely

adequate to supply amino acids for growth, while adding to the diet the α-keto acids of essential amino acids• liver transplantation; gene therapy has also been

tried • If the defect occurs after the synthesis of

argininosuccinate, argininosuccinate can be used as a carrier for the removal of nitrogen (because it has incorporated both amino groups)

the problem in this situation would be one of regenerating ornithine

If arginase is not deficient, the intake of high amounts of arginine would provide ornithine

32

• If the defect occurs at a point before the synthesis of argininosuccinate, substances are used that form conjugates with amino acids and are excreted in the urine

The body has to use ammonia to replace the excreted amino acids; ammonia levels decrease

Drugs: Benzoic acid - reacts with glycine to give hippurate

Phenylbutyrate - first changed to phenylacetate and then reacts with glutamine to produce phenylacetylglutamine

• The most common defect is in ornithine transcarbamoylase

• In the rare cases of arginase deficiency, arginine should be excluded from the diet

• Deficiency of N-acetyl glutamate can be corrected by administering an analog, carbamoyl glutamate

33

34

The fate of the carbon skeleton of amino acids• The deamination of most amino acids yields α-

keto acids that, directly or via additional reactions, feed into major metabolic pathways

• Amino acids are grouped into two classes based on whether or not their carbon skeletons can be converted to glucose: glucogenic or ketogenic

• Carbon skeletons of glucogenic amino acids are degraded to pyruvate or a 4-C or 5-C intermediates of the Krebs cycle

• Glucogenic amino acids are a major carbon source for gluconeogenesis when glucose levels are low

• They can also be catabolized for energy production or converted to glycogen or fatty acids for energy storage

• Carbon skeletons of ketogenic amino acids are degraded to acetyl-CoA or acetoacetate

35

• Carbon skeletons of ketogenic amino acids can be catabolized for energy or converted to ketone bodies or fatty acids

• leucine and lysine are strictly ketogenic• isoleucine, threonine, phenylalanine, tyrosine and

tryptophan and both glucogenic and ketogenic• The remaining thirteen amino acids are glucogenic

amino acids producing pyruvate can be considered ketogenic because pyruvate can be changed to acetyl-CoA

• One carbon transfer is a common theme in amino acid metabolism

• Three cofactors are used to transfer different one carbon groups between intermediates

Tetrahydrofolate (THF); S-adenosylmethionine/SAM/ado-Met; Vitamin B 12 (5’-deoxyadensoyl/methyl cobalamin)

o Biotin is involved in the transfer of the most oxidized form of carbon – CO2

36

37

Tetrahydrofolate • Folic acid/folate/folacin is composed of a pteridine

nucleus, para amino benzoic acid and one or more glutamic acid residues

• Once folic acid is absorbed by the intestine, it is converted to the biologically active form, tetrahydrofolate , by dihydrofolate reductase

only one glutamic acid remains; 4 hydrogens added

• THF travels to the liver and glutamic acid residues are added

• Most of the THF is released into the bile and recirculates just like the bile acids

• The carbon units carried by THF are attached to N5

and/or N10 of the pteridine ring • One carbon units carried by THF are:

Most reduced: - CH3 (methyl) Intermediate: - CH2 - (methylene)

Most oxidized: - CHO (formyl) - CHNH (formimino) - CH = (methenyl)

38

39

• The collection of one carbon units attached to THF is known as the one – carbon pool

• While they are still attached to THF, the one carbon units can be oxidized or reduced

• The main source of carbon units for THF is the carbon removed during the conversion of serine to glycine producing N5, N10-methylene THF

• Although THF can carry a methyl group at N5, the transfer potential of the methyl group is insufficient for most biosynthetic reactions; another cofactor is used as a carrier of methyl groups

S-adenosyl methionine • It contains an activated methyl thioether group;

donates methyl groups to oxygen or nitrogen• Its synthesis to be discussed under the synthesis

of cysteine

40

Vitamin B 12• The unmodified form of vitamin B 12 is known as

cobalamin • It has a corrin ring similar to porphyrin but unlike porphyrin two of its pyrrole rings are

joined directly (no bridges); cobalt takes the place of iron

• Cobalamin in the diet can be found in a free form or bound with proteins

• Free cobalamin is then bound by salivary or gastric secretions known as haptocorrins; protein-bound cobalamin is first freed of the proteins and then haptocorins bind it • Haptocorin bound cobalamin is changed to free

cobalamin in the intestine • Cobalamin is then bound by intrinsic factor which

assists in the absorption into the intestine; travels to the liver bound with transcobalamin II

41

• Vitamin B 12 is involved in two reactions in the body:1.The intramolecular rearrangement of a proton

during the formation of succinyl-CoA from propionyl-CoA

the coenzyme form of vitamin B 12 used in this case is 5’- deoxy -adenosyl cobalamin – 5’- deoxyadenosine attached to cobalt

2.The regeneration of methionine (to be discussed) methyl is attached to cobalt to give methyl

cobalamin

The

tran

spor

t of v

itam

in B

12

42

X = 5’- deoxyadenosine or methyl

Vitamin B 12

43

Intr

amol

ecul

ar r

earr

ange

men

t

44

Essential and non-essential amino acids• Eleven amino acids are considered to be non-

essential because they can be synthesized in the body• arginine, cysteine, tyrosine and histidine are

conditionally essentialArginine and histidine needed in the diet of

children and pregnant women; in adults arginine from the urea cycle is enough and histidine is effectively recycled

tyrosine and cysteine are synthesized from the essential amino acids phenylalanine and methionine, respectively; if these precursors are absent in the diet, then the products become essential

• The remaining essential amino acids are lysine, leucine, isoleucine, valine, tryptophan, threonine and histidine

• Except tyrosine and cysteine, essential amino acids can be synthesized from glucose and ammonia (or another amino acid)

45

Amino acids related with intermediates of glycolysis

• In the synthesis of serine, 3-phosphoglycerate is sequentially oxidized, transaminated and dephosphorylated

• In addition to the serine dehydratase reaction, serine can be changed to pyruvate through transamination followed by reduction and phosphorylation to give PEP (PEP then changed to pyruvate)

• The main pathway of glycine synthesis is from serine – serine hydroxymethyl transferase catalyzes the reaction which involves PLP and N5, N10 methylene THF

Glycine can be degraded by changing it to serine and then to pyruvate

A second way for the degradation of glycine is the production of glyoxylate by D-amino acid oxidase

D-amino oxidase is thought to act in the detoxification of D-amino acids from bacteria or cooked foodstuffs

46

47

48

• Glyoxylate can react with α- ketglutarate and be directed to energy production

• It can also be oxidized to oxalate by hepatic lactate dehydrogenase

• This oxalate, along with the oxalate obtained from the diet, contribute to the formation of kidney stones

3/4th of kidney stones is composed of calcium oxalate

• The third and major approach to degrading glycine is by glycine cleavage enzymeGlycine degraded to NH4+, CO2 and –CH2–

(carried by THF)If glycine cleavage enzyme is deficient, non-

ketotic hyperglycinemia results; mental retardation and early death probably due to the increased inhibitory effects of glycine on the nervous system

49

• The carbon skeleton and the amino group of cysteine are derived from serine; the sulfur is transferred from methionine •The sulfur of methionine attacks the 5’ carbon of the ribose of ATP releasing all the three phosphates in the process; SAM (Ado met) is the product •SAM donates CH3 and becomes S-adenosylhomocysteine •The hydrolytic removal of the adenosine gives homocysteine•Serine reacts with homocysteine to give cystathionine – catalyzed by cystathionine-β –synthase• Cystathionine is cleaved by cystathionase to give cysteine and α- ketobutyrate (which will be changed to propionyl-CoA and then succinyl-CoA)•Homocysteine can be changed to methionine: N5-methyl THF donates CH3 to cobalamin to give methyl cobalamin; methyl cobalamin donates the CH3 to homocysteine and methionine will be formed

50

51

• Cysteine allostericaly inhibits cystathionase and represses the expression of the cystathionine -β-synthase gene

• The deficiency of cystathionine -β-synthase or poor binding to its cofactor PLP would lead to increased amounts of methionine and homocysteine in the blood Homocysteine would then dimerize to

homocystine that is excreted in the urine leading to homcystinuria

excess homocysteine levels have been asssociated with mental retardation and atherosclerosis (homocysteine may damage the blood vessels and stimulate the proliferation of smooth muscle cells)

• During the degradation of cysteine, the sulfur can be disposed of in two ways

the production of sulfuric acid the formation of PAPS (activated sulfur)

o The transamination of pyruvate yields alanine

52

Amino acids related to Krebs Cycle intermediates

1. α- ketoglutarate • Glutamate by transamination or glutamate

dehydrogenase reaction • From glutamate glutamine is synthesized by

amidation; glutamine synthetase is one of the only three enzymes of humans that can fix free ammonia to organic molecules – the other two enzymes are glutamate dehydrogenase and CPS I

• An intermediate known as glutamate γ semialdehyde is synthesized from glutamate

• Proline and ornithine (which is precursor to arginine) can be synthesized from or changed to glutamate and degraded through glutamate γ semialdehyde

if arginine is consumed in protein synthesis, more ornithine would be synthesized from glutamate

53

54

55

• Although histidine is effectively recycled in humans, when degraded, five of its carbons give rise to glutamate

Formiminoglutamate obtained from histidine is changed to glutamate by transferring the formimino group to THF

56

2. Oxaloacetate • Oxaloacetate is transaminated to aspartate and

aspartate receives an amino group from glutamine and is changed to asparagine• Asparagine is broken down to aspartate and NH4

+ by asparaginase and aspartate is transaminated back to oxaloacetate

3. Fumarate• the urea cycle and the purine nucleotide cycle

change the carbons of aspartate to fumarate

57

• Fumarate can be obtained from the breakdown of tyrosine (meaning phenylalanine also can give fumarate)

4. Succinyl-CoA • Methionine, threonine, valine and

isoleucine are degraded to propionyl-CoA that will be changed to succinyl-CoA

methionine produces propionyl-CoA from α-ketobutyrate during the synthesis of cysteine

Threonine dehydratase also gives off α-ketobutyrate

• the main site of branched chain amino acid metabolism is the muscles

• After transamination, the keto acids of all three amino acids (valine, isoleucine and leucine undergo oxidative decarboxylation; branched-chain α-keto acid dehydrogenase complex – an analog of the PDC complex

58

• The subsequent reactions from all three amino acids give the reduced equivalents NADH and FADH2• Valine produces propionyl-CoA only• Isoleucine degradation gives acetyl-CoA in

addition to propionyl-CoA • leucine produces acetyl-CoA and acetoacetate

(purely ketogenic)• Defects in branched-chain α-keto acid

dehydrogenase complex will lead to maple syrup urine disease; it can progress to mental retardation• Methylmalonyl CoA-mutase , an enzyme involved

in the processing of propionyl-CoA to succinyl-CoA, can be deficient leading to methylmalonic acidemia (a rare but deadly disorder)

59

60

Amino acids that form acetyl-CoA and acetoacetate

• Phenylalanine is hydroxylated to tyrosine by phenylalanine hydroxylase, a mixed function oxidase

Tetrahydrobiopterin (BH4) is the cofactor used

• Tyrosine is transaminated to p hydroxyphenylpyruvate• p hydroxyphenylpyruvate is decarboxylated to

homogentisate by a dioxygenase • Homogentisate 1,2- dioxygenase converts

homogentisate to maleylacetoacetate ; after two more steps, fumarate and acetoacetate are obtained

• Phenylketonuria (PKU) results from the deficiency of phenylalanine hydroxylase in this case, a minor pathway of phenylalanine metabolism, transamination to give phenylpyruvate, becomes dominant phenylalanine and phenylpyuvate in the blood and urineMental retardation ensues; exclude phenylanine from diet

61

62

Tyrosinemia III

63

• homgentisate accumulates when homogentisate 1,2- dioxygenase is deficient: alcaptonuria

Relatively benign; the urine turns black on standingLater in life, the accumulation of homogentisate on

the joints may lead to arthritis Sir Archibald Garrod pioneered the study of inborn

errors of metabolism based on his observations on alcaptonuria

• Tyrosinemias I-III may result from the deficiencies of the other enzymes in tyrosine metabolism

• Tryptophan produces alanine from the non-ring carbons and acetyl-CoA and formate from the ring structure

• The ring could also be used in the synthesis of NAD+ and NADP+ - decreases the need for niacin in the diet

• A minor pathway of threonine degradation in the liver can produce glycine and acetyl-CoA

• Lysine degradation produces acetyl-CoA and acetoacetate

64

65

66

The relation between THF, Vit B12 and SAM• N5-methyl THF is the most stable form of THF• The only reaction in which methyl is removed from

THF is through transfer to vit B 12 during the synthesis of methionine• If vit B 12 is deficient, N5-methyl THF would

accumulate and eventually most of the THF in the body would be found in the form of N5-methyl THF - “Methyl Trap” hypothesis reactions that utilize folate would be compromised

• pernicious anemia has hematopoeitic and neurologic components• The hematopoeitic problems are thought to arise from a secondary deficiency of folate resulting from the primary deficiency in vit B 12 (absence of intrinsic factor)• The neurologic disorders are caused by the absence of the regenerating effect of vit B 12 on SAM; SAM is needed for methylation reactions in the nervous tissue and also, methylmalonyl-CoA competes with malonyl-CoA

67

68

Specialized products synthesized from amino acids

oMost neurostransmitters are either amino acids or derivatives of amino acids

Amino acids: glycine, glutamate, aspartate and γ - aminobutyric acid (GABA)

• GABA is the most important inhibitory neurotransmitter in the central nervous system

• It is synthesized through the decarboxylation of glutamate

• a characteristic feature of the production of biological amines is decarboxylation which requires PLP

69

• Histidine is decarboxylated to histamineHistamine is a mediator of allergic and other

inflammatory reactions, stimulator of gastric acid production and an excitatory neurotransmitter in the brain

• Dopamine (D), epinephrine (E) and norepinephrine (NE) are collectively known as catecholamines

D and NE are excitatory neurotransmitters in the brain; E and NE are also secreted by the adrenal medulla and the peripheral nervous system

the first step in the synthesis of catecholamines is the BH4 dependent hydroxylation of tyrosine to form 3,4-dihydroxyphehnylalanine (DOPA)

DOPA is then decarboxylated to dopamine Dopamine undergoes vitamin C dependent

hydroxylation to yield norepinephrine SAM methylates norepinephrine to

epinephrine

70

71

• Parkinson’s disease is associated with low levels of dopamine in the brain

L- Dopa is used for treatment in the late stages of the disease. Dopamine cannot cross the blood- brain barrier; once L-Dopa gets into the brain, it will be changed to dopamine

• The decarboxylation of tyrosine may produce tyramine which binds to NE receptors and causes headaches and hypertension if present in high quantities

cheese, beer, red wine,… contain high amounts of tyramine

72

• Tryptophan is converted to the neurotransmitters serotonin and melatonin in the pineal gland

• Tetrahydrobiopterin-dependent hydroxylation and PLP-dependent decarboxylation are involved

• The conversion reactions are sensitive to light• Serotonin accumulates in the brain during the

daytime and it is converted to N-acetylserotonin and then melatonin in the dark

• Serotonin inhibits feeding and elevates the mood Prozac, an anti-depressant acts by inhibiting

serotonin reuptake into the presynaptic neuron • Melatonin may be involved in male sexual

maturation; it has roles in controlling the biological clock (cricadian rhythm) and serves as an anti-0xidant

73

74

• The catecholamines, tyramine and serotonin are inactivated by oxidative deamination catalyzed by monoamine oxidase (MAO) by methylation through the action of catechol – O – methyl transferase (COMT)

• Histamine is first methylated by SAM and then acted upon by MAO followed by another oxidation step

• GABA is inactivated by changing it back to glutamate and then α-ketoglutatrate

• MAO produces H2O2 while degrading neurotransmitters

• MAO inhibitors are used in the early stages of Parkinson’s disease and as antidepressants if people taking MAO inhibitors consume

tryramine-rich foods, the tyramine will not be degraded and this will lead to serious hypertension – the “cheese effect”

75

76

• Nitric oxide (NO) is produced from arginine• Nitric oxide synthase (NOS) catalyzes a five

electron oxdiation of the guanidino nitrogen of arginine

• Two successive monoxygenation reactions occur to generate the intermediate Nω–hydroxy-L-arginine

• NOS has got five prosthetic groups: FMN, FAD, heme, BH4 , and Ca2+ - calmodulin

• There are three tissue specific isozymes of NOS: neuronal (nNOS), endothelial (eNOS) and inducible (iNOS)

• nNOS and eNOS activities are tightly regulated by Ca2+

• Ca2+ has no effect on iNOS, rather by cytokines; this is the isoform that is involved in the production of the NO that is produced by macrophages in order to kill microorganisms

• The NO produced by nNOS and eNOS acts in low concentrations in the control of blood pressure, neurotransmission, learning and memory

77

• NO brings about vasodilation by activating guanylate cyclase which increases the cellular level of cGMP

• Normally a phosphodiesterase terminates the action of NO by changing cGMP to GMP

• Sildenafil (viagra) blocks a specific isozyme of phospho-diesterase and perpetuates the vasodilation

78

o Amino acids are also used in the synthesis of substances other than neurotransmitters

• Arginine and glycine are involved in the synthesis of creatine

• in the kidney, the guanidino group of arginine is transferred to glycine to give ornithine and guanidoacetate in the liver, SAM methylates guanidoacetate to

creatine ATP then donates a phosphate group to creatine

to give phosphocreatine Creatine phosphate is used as a donor of

phosphate in the regeneration of ATP from ADP Phoshpocreatine or creatine can be non-

enzymatically converted into the cyclic compound creatinine

• the urinary creatinine excretion of a person is extremely constant from day to day and is proportional to the muscle mass. 95 % of the creatine in the body is found in the skeletal muscles and the remaining part in the heart, brain and testes

79

80

• Creatinine Clearance Test: compares the level of creatinine in urine (24 hrs.) with the creatinine level in the blood

• it is used for assessing kidney function• Polyamines are positively charged molecules with

multiple amino groups that are found in high concentrations in cells

• The decarboxylation of ornithine produces putrescine

• Putrescine then reacts with decarboxylated SAM (from methionine) and produces spermidine; spermidine is changed by the same process into spermine

Increase in the number of amino groups• The decarboxylation of lysine and arginine would

lead to cadaverine and agmatine , respectively• Polyamines may stabilize DNA by interacting with

the negatively charged phosphate groups of nucleotides

• They may also, to some extent, replace for cellular K+ and Mg2+ and control nucleic acid and protein synthesis

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• Melanin is a family of polymeric pigments of different colors synthesized from tyrosine

• tyrosinase catalyzes the conversion of tyrosine to DOPA and DOPA to dopaquinone

• A number of intermediates follow from DOPA to finally produce polymerized melanin

• Melanocytes produce melanin and carotene which blend and give rise to the color of the skin, hair and eyes

• Melanin granules are uniformly distributed in melanocytes and offer protection by absorbing ultra violet rays

• The deficiency of tyrosinase or other enzymes in the synthesis pathway of melanin leads to albinism lack of pigment in the skin, eyes and hair;

sensitivity to sunlight

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