Amino acid oxidation and the production of urea

Catabolism of proteins and aa nitrogen

• How the nitrogen of aa is converted to urea and the rare disorders that accompany defects in urea biosynthesis

• Normal condition- nitrogen intake match nitrogen excreted

• Positive nitrogen balance- an excess of ingested over excreted nitrogen- during growth and pregnancy

• Negative nitrogen balance – output exceeds intake- during surgery, advanced cancer or malnutrition

Oxidative degradation occur in 3 diff metabolic circumstances:

1)During normal synth and degradation of cellular protein- aa that are released from protein breakdown are not needed for new prot synthesis-degraded

2)Taking a protein rich diet- aa intake exceeds the body’s need for prot- degraded

3)During starvation or in uncontrolled DM – when carb cannot be utilized, proteins are used as fuel

• Under all this metabolic conditions- aa lose their amino groups to form α-ketoacids (the carbon skeleton of amino acids)

• The α-ketoacids undergo oxidation to CO2 and H20 and provide 3-4C for gluconeogenesis

1) Metabolic fates of amino groups2) Metabolic fates carbon skeletons

Metabolic fates of amino groups

• Degradation of ingested proteins to aa occurs in gastrointestinal tract

• Entry of protein dietary will stimulate the hormone gastrin – will stimulate the production of HCl- kill most bacteria, denaturing agent for protein, unfolding globular proteins, and make internal peptide bonds more accessible to enzymatic hydrolysis

• Pepsin is activated to cleave the polypeptide to smaller peptide

Dietary protein is enzymatically degraded to amino acids

• Once protein are degraded to amino acids, aa are transported to liver- to remove the amino groups – by aminotransferases or transaminases

• The amino groups are transferred α-ketoglutarate- forming glutamate

Glutamate releases its amino group as ammonia in the liver

• Amino groups must be removed from glutamate for excretion

• In hepatocytes, glutamate is transported from cytosol to mitoch – undergoes oxidative deamination- cat by glutamate dehydrogenase produced NH4+

• NH4+ is transported by glutamine to liver to be secreted thru urea cycle

Excretion of excess nitrogen• Excess nitrogen – excreted in one of three forms: ammonia

(as ammonium ion), urea and uric acid

• Fish – excrete ammonia – protected by the toxic activity through excretion and rapid dilution by environment

• Terrestrial animals- excrete urea- water soluble compounds

• Birds- excrete uric acid – insoluble in water- to avoid excess weight

• High blood urea level as a consequence (not a cause) of impaired renal fx

Urea cycle• Central pathway in nitrogen metabolism- urea cycle

• Start with the reaction of ammonium ion and C02 to produce carbamoyl phosphate

• Step 1- Carbamoyl phosphate + ornithine →citrulline (carbamoyl phosphatase 1

• Citrulline + Nitrogen (2nd)→arginino succinate

• Arginino succinate → fumarate and arginine

• Arginine → urea and regenerate ornithin

• Fumarate enter the TCA cycle

Ammonia intoxication • Ammonia produced by enteric bacteria and produced

by tissues are rapidly cleared from circulation by the liver and converted to urea

• Ammonia is toxic to central nerveous system

• Ammonia levels may rise to toxic levels in impaired hepatic function – cirrhosis

• Ammonia is toxic to brain – reacts with α-ketoglutarate to form glutamate – depleted levels of α-ketoglutarate – impair fx of TCA cycle in neurons

• Mammals with genetic defects in any enzyme involved in urea formation cannot tolerate protein rich diet- as free ammonia cant be converted to urea- lead to hyperammonemia

• Protein free diet is not an option. Mammals are incapable of synthesizing all 20 amino acids, thus must come from diet

Ammonia intoxication

Amino acid catabolism

• Involve transferring the amino nitrogen to α-ketoglutarate to produce glutamate- leaving behind the carbon skeleton

• After removal of their amino groups, the carbon skeleton of aa undergo oxidation to compounds that can enter the TCA cycle

• In liver

The fate of carbon skeleton

Amino acid catabolism

The fate of carbon skeleton

• Glucogenic aa yields pyruvate or OAA on degradation -----> gluconeogenesis

• Ketogenic aa breaks down to acetyl-CoA or acetoacetyl-CoA- leading to the formation of ketone bodies

Six aa are degraded to pyruvate

• Alanine, tryptophan, cystein, serine, glycine, and threonine

• All are converted to pyruvate

• Pyruvate can either be converted to acetyl-CoA (Ketone bodies precursor)

• Or converted to OAA (gluconeogenesis)

Glycine degradation

• Can be degraded in 3 ways – only one yield pyruvate

1)Involve conversion of glycine to serine via catalysis of serine hydroxymethyl transferase. Serine is then converted to pyruvate

2) Glycine undergoes oxidative cleavage to CO2, NH4+ and a methylene group – catalysed by glycine cleavage enzyme (or glycine synthase)

• This pathway is critical in mammals

• Defects in glycine cleavage enzyme- lead to elevated serum of glycine – inhibit neurotransmitter- mental retardation

Glycine degradation

3) Conversion of glycine to glyoxylate by D-amino acid oxidase. Glyoxylate is further oxidized to oxalate

• Crystals of calcium oxalate account for 75% of all kidney stones

Glycine degradation

Ketogenic amino acid

• Phenylalanine, tyrosine, isoleucine, leucine, tryptophan, threonine, and lysine

• Breakdown of trp – precursor to synthesis NAD and NADP in animalsSerotonin – a neurotransmitter in vertebrate

Phenylalanine catabolism

• Phenylalanine- precursor for dopamin (a neurotransmitter) and hormones (norepinephrine and epinephrine)

• Breakdown of phenylalanine- catalysed by phenylalanine carboxylase

• Deficiency of this enzyme lead to phenylketoneuria (PKU) disease – due to elevated levels of phenylalanine

PKU• Defiency of phenylalanine carboxylase cause phenylalanine

undergoes transamination with pyruvate – yield phenylpyruvate –

• Accumulate in blood and tissues- excreted thru urine

• Other than being excreted as urine directly, some of phenylpyruvate are reduced to phenylacetate – give odor to urine – nurses have traditionally used to detect PKU in infants

• Accumulation of phenylalanine in early life impairs normal development of the brain- mental retardation

• Due to excess of phenylalanine competing with other aa for transport across the blood brain-barrier- deficit required metabolites

• Mental retardation can be prevented by rigid diet- provide only enough phenylalanine

The fate of branched amino acid

• Isoleucine, leucine, and valine

• Degraded only in extrahepatic tissues – oxidized as fuels in muscles, adipose, kidney

• Due to the presence of branched-chain α-ketoacid dehydrogenase complex- absent in liver

Maple syrup urine disease• Due to the defective branched-chain α-ketoacid

dehydrogenase complex-

• Lead to accumulation of leucine in blood- and excreted to urine – smell like maple syrup

• Untreated lead to abnormal development of the brain, mental retardation, and death in early infacy

• Treatment include limiting the intake of valine, isoleucine, and leucine

Metabolic disorders related to urea cycle

• Defects in urea synthesis results in ammonia intoxication

• More severe if the metabolic blocks occur at reactions 1 or 2 – some intermediate compound have been synthesized

• Due to defects in any several enzyme involved in the cycle