Synthesis and degradation of fatty acids

Zdeňka Klusáčková

Fatty acids (FA)

mostly an even number of carbon atoms and linear chain

in esterified form as component of lipids

in unesterified form in plasma binding to albumin

Groups of FA:

according to the number of double bonds

no double bond

one double bond

more double bonds

saturated FA (SAFA)

monounsaturated FA (MUFA)

polyunsaturated FA (PUFA)

according to the chain length

<C6

C6 – C12

C12 – C20

>C20

short-chain FA (SCFA)

medium-chain FA (MCFA)

long-chain FA (LCFA)

very-long-chain FA (VLCFA)

Overview of FA

Triacylglycerols

main storage form of FA

acylglycerols with three acyl groups

stored mainly in adipose tissue

function: energy storage in the form of TAG

acyl-CoA and glycerol-3-phosphate

TAG incorporation into very low density lipoproteins (VLDL)

entry of VLDL into the blood circulation

TAG transport from the liver to other tissues via VLDL

synthesis of TAG in liver

(especially skeletal muscle, adipose tissue)

FA biosynthesis in the excess of energy (increased caloric intake)

FA biosynthesis

mainly in the liver, adipose tissue, mammary gland during lactation

localization: cell cytoplasm (up to C16)

endoplasmic reticulum, mitochondrion

enzymes: acetyl-CoA-carboxylase (HCO3- - source of CO2, biotin, ATP)

fatty acid synthase (NADPH + H+, pantothenic acid)

primary substrate: acetyl-CoA

final product: palmitate

(elongation = chain extension)

(always in excess calories)

FA biosynthesis

repeated extension of FA by two carbons in each cycle

on the multienzyme complex – FA synthase

to the chain length C16 (palmitate)

palmitate, a precursor of saturated and unsaturated FA:

saturated FA (> C16) elongation systems

unsaturated FA desaturation systems

FA biosynthesis

Acetyl-CoA1.

source: oxidative decarboxylation of pyruvate (the main source of glucose)

NADPH 2.

source: pentose phosphate pathway (the main source)

the conversion of malate to pyruvate (NADP+-dependent malate dehydrogenase - „malic enzyme”)

transport across the inner mitochondrial membrane as citrate

the conversion of isocitrate to α-ketoglutarate (isocitrate dehydrogenase)

degradation of FA, ketones, ketogenic amino acids

Precursors for FA biosynthesis

Formation of malonyl-CoA

HCO3- + ATP ADP + Pi

enzyme-biotin enzyme-biotin-COO-

enzyme-biotin

acetyl-CoA

malonyl-CoA

biotinyl-enzyme carboxybiotinyl-enzyme

+

1 carboxylation of biotin 2 transfer of carboxyl group to acetyl-CoA

formation of malonyl-CoA

enzyme – acetyl-CoA-carboxylase

FA biosynthesis

Regulation at the level of ACC

acetyl-CoA malonyl-CoA palmitateglucose citrate palmitoyl-CoA

acetyl-CoA carboxylase

protein kinase A AMP-dependent

insulin AMPcAMP

glucagon adrenaline

FA biosynthesis

protein kinase A

FA synthase

FA biosynthesis

FA biosynthesis

The course of FA biosynthesis

acetyl-CoA malonyl-CoA

acetyltransacylase

acyl(acetyl)-malonyl-

CoASH CoASH

transacylation

-enzyme complex

malonyltransacylase

acyl(acetyl)-malonyl-enzyme complex 3-ketoacyl-enzyme complex

3-ketoacyl-synthase

CO2

condensation

(acetacetyl-enzyme complex)

FA biosynthesis

The course of FA biosynthesis

3-ketoacyl-enzyme complex(acetoacetyl-enzyme complex)

3-hydroxyacyl-enzyme complex

NADPH + H+ NADP+

3-ketoacyl-reductase

H2O

3-hydroxyacyl- dehydrase

2,3-unsaturated acyl-enzyme complex acyl-enzyme complex

NADPH + H+ NADP+

enoylreductase

first reduction dehydration second reduction

FA biosynthesis

The course of FA biosynthesis

Repetition of the cycle

acyl-enzyme complex(palmitoyl-enzyme complex)

CoASH

malonyl-CoA

FA biosynthesis

The release of palmitate

palmitoyl-enzyme complex

H2O

+

palmitate

thioesterase

FA biosynthesis

The fate of palmitate after FA biosynthesis

palmitate palmitoyl-CoA

acylglycerols

cholesterol esters

acyl-CoA

esterification

elongation

desaturation

acyl-CoA-synthetase

ATP + CoA AMP + PPi

FA biosynthesis

FA elongation

microsomal elongation system1.

in the endoplasmic reticulum

malonyl-CoA – the donor of the C2 units

extension of saturated and unsaturated FA

palmitic acid (C16) FA > C16 elongases (chain elongation)

mitochondrial elongation system2.

in mitochondria

acetyl-CoA – the donor of the C2 unit

NADPH + H+ – the donor of the reducing equivalents

not reverse β-oxidation

FA biosynthesis

fatty acid synthase

Microsomal extension of FA

acetyl-CoA malonyl-CoA 3-ketoacyl-CoA

3-hydroxyacyl-CoA 2,3-unsaturated acyl-CoA acyl-CoA

CoASH + CO2

NADPH + H+ NADP+ H2O NADPH + H+ NADP+

+

synthase

reductase hydratase reductase

CoASH + CO2

+

NADPH + H+ NADP+

H2ONADPH + H+ NADP+

palmitoyl-CoA malonyl-CoA

stearoyl-CoA

Example:

FA biosynthesis

FA desaturation

in the endoplasmic reticulum

process requiring O2, NADH, cytochrome b5

FA biosynthesis

FA degradation

function: major energy source

(especially between meals, at night, in increased demand for energy intake – exercise)

release of FA from triacylglycerols in adipose tissue into the bloodstream

binding of FA to albumin in the bloodstream

transport to tissues

entry of FA into target cells activation to acyl-CoA

transfer of acyl-CoA via carnitine system into mitochondria β-oxidation

Most important FA released from adipose tissue:

palmitic acid

oleic acid

stearic acid

long-chain FA (LCFA, C12 – C20)

unsaturated FA

odd-chain-length FA

very-long-chain FA (VLCFA, > C20)

FA with C10 or C12

long-chain branched-chain FA

mitochondrial β-oxidation

mitochondrial β-oxidation

modified

peroxisomal β-oxidation

peroxisomal α-oxidation

ω-oxidation

FA degradation

Mechanisms of FA degradation

α-oxidation ω-oxidation

β-oxidation

Mechanisms of FA degradation

FA degradation

mainly in muscles

localization: mitochondrial matrix

peroxisome

enzymes: acyl CoA synthetase

carnitine palmitoyl transferase I, II; carnitine acylcarnitine translocase

substrate: acyl-CoA

final products: acetyl-CoA

β-oxidation

dehydrogenase (FAD, NAD+), hydratase, thiolase

propionyl-CoA

FA degradation

repeated shortening of FA by two carbons in each cycle

oxidation of acetyl-CoA to CO2 and H2O in the citric acid cycle

generation of 8 molecules of acetyl-CoA from 1 molecule of palmitoyl-CoA

cleavage of two carbon atoms in the form of acetyl-CoA

complete oxidation of FA

PRODUCTION OF LARGE QUANTITY OF ATP

production of NADH, FADH2 reoxidation in the respiratory chain to form ATP

FA degradation

β-oxidation

Activation of FA

fatty acid ATP

pyrophosphate (PPi)

acyl-CoA AMP

acyl-CoA-synthetase

acyl-CoA-synthetase pyrophosphatase

acyl adenylate

fatty acid+ ATP + CoASH acyl-CoA + AMP + PPi

PPi + H2O 2Pi

2Pi

FA degradation

The role of carnitine in the transport of FA into mitochondrion

FA transfer across the inner mitochondrial membraneby carnitine and three enzymes:

carnitine palmitoyl transferase I (CPT I)

acyl transfer to carnitine

carnitine acylcarnitine translocase

acylcarnitine transfer across the inner mitochondrial membrane

carnitine palmitoyl transferase II (CPT II)

acyl transfer from acylcarnitine back to CoA in the mitochondrial matrix

FA degradation

acyl-CoA

trans-Δ2-enoyl-CoA

L-β-hydroxyacyl-CoA

β-ketoacyl-CoA

acyl-CoA acetyl-CoA

acyl-CoA-dehydrogenase

enoyl-CoA-hydratase

L-β-hydroxyacyl-CoA-

β-ketoacyl-CoA-thiolase

Steps of cycle:

dehydrogenation

oxidation by FADcreation of unsaturated acid

hydration

addition of water on the β-carbon atomcreation of β-hydroxyacid

dehydrogenation

oxidation by NAD+

creation of β-oxoacid

cleavage at the presence of CoA

formation of acetyl-CoAformation of acyl-CoA (two carbons shorter)

β-oxidation

-dehydrogenase

FA degradation

Oxidation of unsaturated FA

3 acetyl-CoA3 rounds of β-oxidation

β-oxidation 1 acetyl-CoA

linoleoyl-CoA

NADPH + H+

NADP+

enoyl-CoA-isomerase

enoyl-CoA-isomerase

dienoyl-CoA-reductase

acyl-CoA-dehydrogenase

cis-Δ3, cis-Δ6

trans-Δ2, cis-Δ6

cis-Δ4

trans-Δ2, cis-Δ4

trans-Δ3

trans-Δ2

cis Δ9, cis-Δ12

4 rounds of β-oxidation

5 acetyl-CoA

the most common unsaturated FA in the diet:

degradation of unsaturated FA by β-oxidation to a double bond

conversion of cis-isomer of FA by specific isomerase to trans-isomer

continuation of β-oxidationto the next double bond

oleic acid, linoleic acid

elimination of double bond between C4 and C5 by reduction

formation of double bond between C2 and C3 by dehydrogenation

intramolecular transfer of double bond

continuation of β-oxidation

FA degradation

Oxidation of odd-chain FA

propionyl-CoA

D-methylmalonyl-CoA

L-methylmalonyl-CoA

succinyl-CoA

HCO3- + ATP

ADP + Pi

propionyl-CoA carboxylase (biotin)

methylmalonyl-CoA mutase (B12)

methylmalonyl-CoA racemase

shortening of FA to C5

formation of acetyl-CoA and propionyl-CoA

carboxylation of propionyl-CoA

epimerization of D-form into L-form

intramolecular rearrangement to form succinyl-CoA

entry of succinyl-CoA into the citric acid cycle

stopping of β-oxidation

FA degradation

Peroxisomal oxidation of FA

A) very-long-chain FA (VLCFA, > C20)

Differences between β-oxidation in the mitochondrion and peroxisome:

1. step – dehydrogenation by FAD

mitochondrion: electrons from FADH2 are delivered to the respiratory chain where they are transferred to O2 to form H2O and ATP

peroxisome: electrons from FADH2 are delivered to O2 to form H2O2,

which is degraded by catalase to H2O and O2

3. step – dehydrogenation by NAD+

mitochondrion: reoxidation of NADH in the respiratory chain

peroxisome: reoxidation of NADH is not possible, export to the cytosol or the mitochondrion

transport of acyl-CoA into the peroxisome without carnitine

FA degradation

Differences between β-oxidation in the mitochondrion and peroxisome:

4. step – cleavage at the presence of CoA

mitochondrion: metabolization in the citric acid cycle

peroxisome: export to the cytosol, to the mitochondrion (oxidation)

acetyl-CoA

a precursor for the synthesis of cholesterol and bile acids

a precursor for the synthesis of fatty acidsof phospholipids

Peroxisomal oxidation of FA

FA degradation

B) long-chain branched-chain FA

blocking of β-oxidation by the alcyl group at Cβ

α-oxidation

hydroxylation at Cα

cleavage of the original carboxyl group as CO2

methyl group is in the position α

transfer of FA in the form of acylcarnitine into the mitochondrion

shortening of FA to 8 carbons

complete of β-oxidation in the mitochondrion

Peroxisomal oxidation of FA

FA degradation

Refsum's disease

rare autosomal recessive hereditary disease

phytanic acid a product of metabolism of phytol (part of chlorophyll)

in milk and animal fats

decreased activity of peroxisomal α-hydroxylase accumulation of phytanic acid

(in tissues of nervous system and serum)

ataxia, night blindness, hearing loss, skin changes etc.

ω-oxidation of FA

minor pathway of FA oxidation

in the endoplasmatic reticulum

repeated oxidation of ω-carbon

-CH3 - CH2OH -COOH

formation of dicarboxylic acid

entry of dicarboxylic acid into β-oxidation

reduction of FA to adipic acid (C6) or suberic acid (C8)

excreted in the urine

FA degradation

Regulation of β-oxidation

acetyl-CoA malonyl-CoA CPT I β-oxidation

ACC

A) by energy demands of cell

by the level of ATP and NADH:

FA can not be oxidized faster than NADH and FADH2 are reoxidized in the respiratory chain

B) via carnitine palmitoyl transferase I (CPT I)

CPT I is inhibited by malonyl-CoA, which is generated in the synthesis of FA by acetyl-CoA carboxylase (ACC)

active FA synthesis inhibition of β-oxidation

FA degradation

Comparison of FA biosynthesis and FA degradation

in the liver

localization: mitochondrial matrix

substrate: acetyl-CoA

products: acetone

acetoacetate

D-β-hydroxybutyrate

conditions: in excess of acetyl-CoA

function: energy substrates for extrahepatic tissues

Ketone bodies

Ketogenesis

Ketone bodies

Ketogenesis

acetoacetate

spontaneous decarboxylation to acetone

conversion to D-β-hydroxybutyrate by D-β-hydroxybutyrate dehydrogenase

waste product (lung, urine)

energy substrates for extrahepatic tissues

Ketone bodies

Ketogenesis

Utilization of ketone bodies

citric acid cycle

energy source for extrahepatic tissues

(especially heart and skeletal muscle)

in starvation - the main source of energy

energy

production

water-soluble FA equivalents

Ketone bodies

for the brain

Production, utilization and excretion of ketone bodies

acetyl-CoA

oxidation in the citric acid cycle (liver)

conversion to ketone bodies

release of ketone bodies into blood

transport to tissues

Ketone bodies

(liver - mitochondrion)

lipolysis

FA in plasma

β-oxidation

excess of acetyl-CoA

ketogenesis

increased ketogenesis:

starvation

prolonged exercise

diabetes mellitus

high-fat diet

low-carbohydrate diet

utilization of ketone bodies as an energy source

to spare of glucose and muscle proteins

(skeletal muscle, intestinal mucose, adipocytes, brain, heart etc.)

Ketogenesis

Ketone bodies

http://www.hindawi.com/journals/jobes/2011/482021/fig2/

Marks, A.; Lieberman, M. Marks' basic medical biochemistry: a clinical approach. 3rd edition. Lippincott Williams & Wilkins, 2009.

Meisenberg, G.; Simmons, W. H. Principles of medical biochemistry. 2nd edition. Elsevier, 2006.

Matouš a kol. Základy lékařské chemie a biochemie. Galén, 2010.

Devlin, T. M. Textbook of biochemistry: with clinical correlations. 6th edition. Wiley-Liss, 2006.

Murray et al. Harper's Biochemistry. 25th edition. Appleton & Lange, 2000.

Bibliography and sources