Synthesis and degradation of fatty acids
Martina Srbová
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 chain length
<C6
C6 – C12
C14 – C20
>C20
short-chain FA (SCFA)
medium-chain FA (MCFA)
long-chain FA (LCFA)
very-long-chain FA (VLCFA)
 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)
Overview of FA
FA biosynthesis
 mainly in the liver, adipose tissue, mammary gland during lactation
(always in excess calories)
localization:  cell cytoplasm (up to C16)
 endoplasmic reticulum, mitochondrion
(elongation = chain extension)
enzymes:
 acetyl-CoA-carboxylase
(HCO3- - source of CO2, biotin, ATP)
 fatty acid synthase
(NADPH + H+, pantothenic acid)
primary substrate:
 acetyl-CoA
final product:
 palmitate
Precursors for FA biosynthesis
1.
Acetyl-CoA
source:
 oxidative decarboxylation of pyruvate (the main source of glucose)
 degradation of FA, ketones, ketogenic amino acids
 transport across the inner mitochondrial membrane as citrate
2.
NADPH
source:
 pentose phosphate pathway (the main source)
 the conversion of malate to pyruvate
(NADP+-dependent malate
dehydrogenase - „malic enzyme”)
 the conversion of isocitrate to α-ketoglutarate
(isocitrate dehydrogenase)
Precursors for FA biosynthesis
Acetyl-CoA
+
HSCoA
OAA - oxaloacetate
FA biosynthesis
Formation of malonyl-CoA catalysed by acetyl-CoA-carboxylase (ACC)
HCO3- + ATP
ADP + Pi
enzyme-biotin-COO-
enzyme-biotin
1
carboxylation of biotin
2
transfer of carboxyl
group to acetyl-CoA
acetyl-CoA
formation of malonyl-CoA
+
enzyme – acetyl-CoA-carboxylase
malonyl-CoA
enzyme-biotin
FA biosynthesis
 on the multienzyme complex – FA synthase
 repeated extension of FA by two carbons in each cycle
 to the chain length C16 (palmitate)
ACP – acyl carrier protein
FA biosynthesis
The course of FA biosynthesis
acetyl-CoA
malonyl-CoA
CoASH
CoASH
acetyltransacylase
malonyltransacylase
transacylation
acyl(acetyl)-malonyl-enzyme complex
FA biosynthesis
The course of FA biosynthesis
3-ketoacyl-synthase
CO2
condensation
acyl(acetyl)-malonyl-enzyme complex
3-ketoacyl-enzyme complex
(acetacetyl-enzyme complex)
FA biosynthesis
The course of FA biosynthesis
NADPH + H+
+
NADPH + H+ NADP
NADP+
H2O
3-ketoacyl-reductase
first reduction
3-ketoacyl-enzyme complex
(acetoacetyl-enzyme complex)
3-hydroxyacyldehydrase
dehydration
3-hydroxyacyl-enzyme complex
enoylreductase
second reduction
2,3-unsaturated acyl-enzyme complex
acyl-enzyme complex
FA biosynthesis
Repetition of the cycle
malonyl-CoA
CoASH
acyl-enzyme complex
(palmitoyl-enzyme complex)
FA biosynthesis
The release of palmitate
thioesterase
+
H2O
palmitate
palmitoyl-enzyme complex
FA biosynthesis
The fate of palmitate after FA biosynthesis
acylglycerols
cholesterol esters
ATP + CoA
AMP + PPi
palmitate
acyl-CoA-synthetase
esterification
palmitoyl-CoA
elongation
desaturation
acyl-CoA
FA biosynthesis
FA elongation
1.
microsomal elongation system
 in the endoplasmic reticulum
 malonyl-CoA – the donor of the C2 units
NADPH + H+ – the donor of the reducing equivalents
 extension of saturated and unsaturated FA
FA > C16
elongases
(chain elongation)
2.
mitochondrial elongation system
 in mitochondria
 acetyl-CoA – the donor of the C2 unit
 not reverse β-oxidation
palmitic acid (C16)
fatty acid synthase
FA biosynthesis
FA desaturation
 in the endoplasmic reticulum
 enzymes: desaturase, NADH-cyt b5-reductase
 process requiring O2, NADH, cytochrome b5
stearoyl-CoA + NADH + H+ + O2
4 desaturases:
double bonds at position  4,5,6,9
linoleic, linolenic – essential FA
oleoyl-CoA + NAD+ + 2H2O
FA biosynthesis - summary
• Formation of malonyl-CoA
• Acetyl-CoA-carboxylase
• FA synthesis
 Palmitic acid
• FA Synthase– cytosol
 Saturated fatty acids(>C16)
 Elongation systems- mitochondria, ER
 Unsaturated fatty acids
 Desaturation system - ER
-
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

1
entry of FA into target cells

3
transfer of acyl-CoA via carnitine system into mitochondria
4
5
2
activation to acyl-CoA
β-oxidation
In the liver , acetyl CoA is converted to ketone bodies
FA degradation
-carbon
-oxidation
C10 , C12
http://che1.lf1.cuni.cz/html/Odbouravani_MK_3sm.pdf
β-carbon
-carbon
β-oxidation
-oxidation
Branched FA
VLCFA
FA degradation
β-oxidation
 mainly in muscles
localization:  mitochondrial matrix
 peroxisome (VLCFA)
enzymes:
 acyl CoA synthetase
 carnitine palmitoyl transferase I, II; carnitine acylcarnitine translocase
 dehydrogenase (FAD, NAD+), hydratase, thiolase
substrate:
 acyl-CoA
final products:
 acetyl-CoA
 propionyl-CoA
FA degradation
β-oxidation
 repeated shortening of FA by two carbons in each cycle
 cleavage of two carbon atoms in the form of acetyl-CoA
 oxidation of acetyl-CoA to CO2 and H2O in the citric acid cycle
complete oxidation of FA
 generation of 8 molecules of acetyl-CoA from 1 molecule of palmitoyl-CoA
 production of NADH, FADH2
reoxidation in the respiratory chain to form ATP
PRODUCTION OF LARGE QUANTITY OF ATP
FA degradation
Activation of FA
fatty acid
ATP
acyl-CoA-synthetase
acyl adenylate
pyrophosphate (PPi)
acyl-CoA-synthetase
pyrophosphatase
2Pi
acyl-CoA
AMP
fatty acid+ ATP + CoASH
PPi + H2O
acyl-CoA + AMP + PPi
2Pi
FA degradation
The role of carnitine in the transport of LCFA into mitochondrion
FA transfer across the inner mitochondrial membrane
by 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
Carnitine
3-hydroxy-4-N-trimethylaminobutyrate
Sources:
Exogenous: meat, dairy products
Endogenous: synthesis from lysine and methionine
Transported into the cell by specific transporter
Deficiency:
Decreased transport of acyl-CoA into mitochondria
lipids accumulation
myocardial damage
muscle weakness
Increased utilization of Glc
hypoglycemia
Similar symptoms are the genetically determined deficiency carnitinpalmitoyltransferase I or II
FA degradation
β-oxidation
Steps of cycle:
 dehydrogenation
acyl-CoA
acyl-CoA-dehydrogenase
oxidation by FAD
creation of unsaturated acid
trans-Δ2-enoyl-CoA
 hydration
enoyl-CoA-hydratase
addition of water on the β-carbon atom
creation of β-hydroxyacid
L-β-hydroxyacyl-CoA
L-β-hydroxyacyl-CoA-dehydrogenase
 dehydrogenation
oxidation by NAD+
creation of β-oxoacid
 cleavage at the presence of CoA
β-ketoacyl-CoA
β-ketoacyl-CoA-thiolase
formation of acetyl-CoA
formation of acyl-CoA (two carbons shorter)
acyl-CoA
acetyl-CoA
FA degradation
Oxidation of unsaturated FA
 the most common unsaturated FA in the diet:
β-oxidation of oleic acids
oleic acid, linoleic acid
 degradation of unsaturated FA
by β-oxidation to a double bond
 Unsaturated FA are cis isomers - aren´t
substrate for enoyl-coA hydratase
3 rounds of β-oxidation
 conversion of cis-isomer of FA
by specific isomerase to trans-isomer
 intramolecular transfer of double bond
from β- to - β position
 continuation of β-oxidation
Normal intermediates of β-oxidation
http://che1.lf1.cuni.cz/html/Odbouravani_MK_3sm.pdf
FA degradation
Oxidation of odd-chain FA
 shortening of FA to C5
propionyl-CoA
stopping of β-oxidation
HCO3- + ATP
 formation of acetyl-CoA and propionyl-CoA
propionyl-CoA carboxylase
(biotin)
methylmalonyl-CoA
 carboxylation of propionyl-CoA
 intramolecular rearrangement to form succinyl-CoA
ADP + Pi
methylmalonyl-CoA mutase
(B12)
succinyl-CoA
 entry of succinyl-CoA into the citric acid cycle
FA degradation
Peroxisomal oxidation of VLCFA
Very-long-chain FA (VLCFA, > C20)
 transport of acyl-CoA into the peroxisome without carnitine
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
FA degradation
Peroxisomal oxidation of VLCFA
Differences between β-oxidation in the mitochondrion and peroxisome:
4. step – cleavage at the presence of CoA
acetyl-CoA
mitochondrion: metabolization in the citric acid cycle
peroxisome:
export to the cytosol, to the mitochondrion (oxidation)
a precursor for the synthesis of cholesterol and bile acids
a precursor for the synthesis of fatty acids
of phospholipids
In peroxisome shortened FA bind to carnitine
transfer acylcarnitine into mitochondrion
acylcarnitine
β-oxidation
FA degradation
- oxidation
Oxidation  carbon
mixed function oxidase
ER liver, kidney
Substrates C10 a C12 FA
Products: dicarboxylic acids
Excreted in the urine
Comparison of FA biosynthesis and FA degradation
Ketone bodies
Ketogenesis
increased ketogenesis:
lipolysis
 starvation
 prolonged exercise
 diabetes mellitus
FA in plasma
 high-fat diet
 low-carbohydrate diet
β-oxidation
utilization of ketone bodies as an energy source
(skeletal muscle, intestinal mucose, adipocytes, brain, heart etc.)
excess of acetyl-CoA
to spare of glucose and muscle proteins
ketogenesis
Ketone bodies
Ketogenesis
 in the liver
localization:  mitochondrial matrix
substrate:
 acetyl-CoA
products:
 acetone
 acetoacetate
medium strength acids - ketoacidosis
 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
Utilization of ketone bodies
 water-soluble FA equivalents
 energy source for extrahepatic tissues
(especially heart and skeletal muscle)
 in starvation - the main source of energy
for the brain
citric acid cycle
energy
production
Bibliography and sources
Devlin, T. M. Textbook of biochemistry: with clinical correlations. 6th edition.
Wiley-Liss, 2006.
Marks, A.; Lieberman, M. Marks' basic medical biochemistry: a clinical approach.
3rd edition. Lippincott Williams & Wilkins, 2009.
Matouš a kol. Základy lékařské chemie a biochemie. Galén, 2010.
Meisenberg, G.; Simmons, W. H. Principles of medical biochemistry. 2nd edition.
Elsevier, 2006.
Murray et al. Harper's Biochemistry. 25th edition. Appleton & Lange, 2000.
http://www.hindawi.com/journals/jobes/2011/482021/fig2/