Lipid metabolism
• Degradation and biosynthesis of fattyacids
• Ketone bodies
Fatty acids (FA)
• primary fuel molecules in the fat category• main use is for long-term energy storage• high level of energy storage: fats - 38 kJ/g,
carbohydrates 16 kJ/g
Primary sources of FA: - dietary triacylglycerols- triacylglycerols synthesized in the liver- triacylglycerols stored in adipocytes
FA must be delivered to cells where β-oxidation occurs:
FA cell β-oxidation ATP
• liver• heart• skeletal muscles
Fatty acid degradation - β-oxidation
ββββ-oxidation: two-carbon fragments successively removedfrom the carboxyl end of activated FA producing
acetyl-CoA entry into TCA cycle ATP
lokalized in the mitochondrial matrix (>18 C perixosomes)
• FA are delivered to cells by diffusion from blood capillaries
• upon entry into the cell FA are immediately activatedby linking them as thioesters to CoASH
• activation is coupled with FA transport throughthe mitochondrial membrane
Fatty acid degradation
C
O
OH
O
~SCoAC
ATP AMP + PPi
CoASH
FA entry into the cell (cytosol) acyl-CoA
transport across the inner mmitochondrial membrane (carnitine shuttle)
acyl-CoA β-oxidace acetyl-CoA TCA cycle
difusion activation
ATP
fatty acyl coenzyme A-synthetase
Fatty acid activation – formation of acyl–CoA:
carnitine
Carnitine shuttle – transport of long-chain FA (>12C)
into the mitochondrial matrix
CAT I
CAT II
CAT – carnitine acyltransferase(carnitine palmitoyltransferase)
Carnitine-acylcarnitinetranslocase
FA activation and transport across inner mitochondrial membrane
β-Oxidation of fatty acids – spiral pathway
Dehydrogenation/oxidation
Hydration
Dehydrogenation/oxidation
Thiolyticcleavage
ET chain2 ATP
ET chain3 ATP
Combination of FA activation, transport into themitochondrial matrix, β-oxidation and TCA cycle
Energy yield from β-oxidation of saturated FA
example: stearic acid , 18 C 9 acetyl-Coa,
8 turns of β-oxidation
ATP/unit ATP produced
activation -1(-2) -1(-2)
9 acetyl CoA TCA 12 108
8 FADH2 2 16
8 (NADH + H+) 3 24
total ATP 147 (146)
β-oxidation of unsaturated fatty acids
requires additional enzymes – isomerase, NADPH-dependentreductase (2,4-dienoyl-CoA reductase), epimerase
β-oxidation of unsaturated fatty acids
requires additional enzymes – isomerase, NADPH-dependentreductase (2,4-dienoyl-CoA reductase), epimerase
O
SCoA
O
SCoA
O
SCoA
O
SCoA
Enoyl-CoAisomerase
2,4-Dienoyl-CoAreductase
NADP+
NADPH + H+
Oleic acid
isomerase∆3 cis ∆2 trans
~SCoA
O
C
α ~SCoAC
O
3 2 1
βγγγγ
12
βO
~SCoACα
3
cis-double bond 3 turns of β-oxidation
Example:
Unsaturated FA provide less energy than saturated FA –less reducing equivalents (FADH2) are produced
hydration dehydrogenation ….
H3C C C
H
H SCoA
O
H3C C C
COO-
H
O
SCoA
H3C C C
H
COO-
O
SCoA
C C C
H
H
O
SCoA
H
H
-OOC
propionyl-CoA D-methylmalonyl-CoA
L-methylmalonyl-CoAsuccinyl-CoA
propionyl-CoAcarboxylase
methylmalonyl-CoA epimerase
methylmalonyl-CoA mutase
ATP +HCO3
-
AMP +PPi
β-oxidation of FA with an odd number of carbon
They can be handled normally until the last step where propionyl CoAis produced.Three reactions are required to convert propionyl CoA to succinyl CoAwhich can enter to the TCA cycle.
• After degradation of a fatty acid, acetyl CoA is further oxidized in the citric acid cycle.
• First step in the citric acid cycle
• acetyl CoA + oxaloacetate citrate
• If too much acetyl CoA is produced from β-oxidation, some is converted to ketone bodies.
Ketone bodiesKetone bodies
synthesis = ketogenesis
localization: liver, mitochondrial matrix
initial substrate: acetyl-CoA
function: energy source
physiological conditions - myocard
starvation - skeletal muscles
brain
Ketone bodies
β-hydroxybutyrateacetacetate acetone
CH3-C-CH2-COO-
O
CH3-CH-CH2-COO-
OH CH3-C-CH3
O
Formation and utilization of ketone bodies
enters into TCA cycle ATP
Ketogenesisliver(mitochondrial matrix) CH3-C- S-CoA
O
CH3-C-CH2-C-S-CoA
O O
CH3-C-S-CoA
O
CH3-C-CH2-C-O-
O O
CH3-C-S-CoA
O
CH3-C-CH3
O CH3-CH-CH2-COO-
OH
acetoacetyl-CoA
ββββ-hydroxy-ββββ-methylglutaryl-CoA (HMG-CoA)
acetoacetate
acetone
ββββ-hydroxybutyrate
2
CoASHCoASH
acetoacetyl-CoA thiolase
CoASH
CO2NADH+H+
NAD+
HMG-CoA synthase
HMG-CoA lyase
αβ
135-O-C-CH2-C--CH2-C-S-CoA
O OOH
CH3
Starvation and ketone bodiesStarvation and ketone bodies
0
1
2
3
4
5
6
7
0 1 2 3 4
hydroxybutyrate acetoacetate
Weeks of Starvation
Blo
od
Co
nce
ntr
atio
n(m
illim
ola
r)
Fate of ketone bodiesFate of ketone bodies
• Acetoacetate and β-hydroxybutyrateProduced in the liver, diffuse in blood to other tissues.
Primarily transported to muscles and brain for use as energy sources after reconverion to acetyl CoA
• Acetone
Only produced in small amounts, eliminated in urine or breath.
Spontaneous loss of CO2 from acetoacetate.It can be detected in the breath - acetone breathSymptom of untreated diabetes mellitus or starvation conditions.
acetoacetateacetoacetateacetoneacetone
H+
CO2
Utilization of ketone bodies in extrahepatic tissues
CH3-CH-CH2-COO-
OH
acetacetate
acetoacetyl-CoA
acetyl-CoA
ββββ-hydroxybutyrate
NADH+H+
NAD+
sukcinyl-CoA CH3-C-CH2-C-O-
O O
sukcinát
CH3-C-S-CoA
O
2
CH3-C-CH2-C-S-CoA
O O
CoASHCoASH
β-hydroxybutyrate dehydrogenase
β-ketoacyl-CoA transferase
acetoacetyl-CoA thiolase
CC
ATP
extrahepatic tissues – energy source
•myokard at physiological conditions•skeletal muscles, brain in starvation
Utilization of ketone bodies produced in the liver
, HEART, (BRAIN)
Abnormal production of ketone bodies: starvation, I. type diabetes mellitus, alcoholism
secretion of glucagon activation of hormone sensitive lipase
in adipocyte increased lipolysis in adipose tissue
increased entry of FA into the liver ß-oxidation
high production of acetyl-CoAketogenesis
+ lack of oxaloacetate
ketogenesis ketosis ketoacidosis
ketonemia, ketonuria
Ketone bodies – excretion from the organism
acetacetate
acetone
acetacetate20%
β-hydroxybutyrate
β-hydroxybutyrate78%
acetone
aceton2%
ketoacidosis
β-hydroxybutyrate +
acetacetate +
• increased water loss• loss of Na+
Na+
H2O
-
Na+
H2O
-
breathbreath močmoč urineurine
plasmaplasma
CH3-CH-CH2-COO- + H+
OH
CH3-C-CH2-COO- + H+
O
plasmaplasma
Biosynthesis of fatty acids
initial substrate: acetyl-CoA
tissues:liveradipose tissuelactating mammary gland
intracellular localization: cytosol - palmitic acid (C16)ER, mitochondria - elongation (C18 - C24) ER - desaturation – palmitooleic acid (C16)
oleic acid (C18)arachidonic acid (C20)
Reactions of FA biosynthesis- reversal course of FA ß-oxidation
ß-Oxidation BiosynthesisR- CH2 - CH2 - CO- S -
R- CH = CH2 - CO - S -
R- CH(OH) - CH2 - CO - S -
R - CO - CH2 - CO - S -
R - CO - S - + CH3- CO - SCoA
dehydrogenation
hydration
dehydrogenation
thiolytic cleavage
hydrogenation
dehydration
hydrogenation
condensation
+ H2O - H2O
CoASH
β α
Differences between FA synthesis and degradation
• Intracellular localizationdegradation (ß-oxidation) - mitochondrial matrixbiosynthesis - cytosol
• Activation of an acyldegradation (ß-oxidation) - CoA-SHbiosynthesis - ACP-SH (acyl carrier protein, prosthetic group
phosphopantheteine)
• Oxidoreduction cofactorsdegradation (ß-oxidation) - NAD+, FADbiosynthesis - NADPH + H+
• Biosynthesisgrowth of an FA chain catalyzed by 1 multifunctional enzyme (containsaktive sites for all reactions of the synthesis) – fatty acid synthase
1. reaction of synthesis distinct from a reversal reaction of degradation
malonyl CoA + acetyl CoA × acetyl CoA + acetyl CoAdonor of two-carbon unit in each turn
Production of cytosolic acetyl-CoATransport of acetyl-CoA from mitochondrial matrix to cytosol in the form of citrate
cytosol
citrate
CoA
citrate lyaseADP + Pi
ATP
oxalacetate acetyl-CoA
inner mitochondrialmembrane
oxalacetate acetyl-CoA
citrate
CoA
citrate synthase
mitochondrial
matrix
Sources of cytosolic NADPH- pentose phosphate pathway – oxidative phase- oxidation of malate – malate dehydrogenase dekarboxylating (malic enzyme):
oxalacetate malate pyruvate + CO2
NADP+ NADPH+ H+
at high concntration of mitochondrial citrate, isocitrate dehydrogenase inhibited by ATP
Tvorba malonyl-CoATwo-carbon units are incorporated into a growing chain in the form of malonyl-CoA
Acetyl CoA carboxylase - kee enzyme of biosynhesis (multienzyme complex)
CH3-CO~CoA-OOC-CH2 -CO~S-CoA
acetyl-CoA malonyl-CoA
enzyme-biotin-COO- enzyme-biotin
ATP + CO2 + enzyme-biotin
ADP+Pi
regulation: allosteric
citrate
palmitoyl-CoA(product)
hormonal
insulin
glucagon
+
-
+
-
acetyl CoA carboxylase
FA biosynthesis - synthesis of palmitate by thefatty acid synthase complex
1. two-carbon unit
source of other two-carbon unitsfor chain growth
Regulation of fatty acid metabolism
• Supply of fatty acids - liver FA - ß-oxidationat oxalacetate - citrate - ketogenesis
adipocyte FA - TAG synthesis
• ATP citrate (glucose supply – after meal, inhibited isocitratedehydrogenase in TCA cycle) – after translocation
to cytosol citrate activates acetyl CoA carboxylase - FA synthesis
• Malonyl-CoA - inhibits carnitine acyltransferase I – long-chain FA cannot enterinto mitochondria - ß-oxidation- FA accumulate in cytosol - TAG synthesis
• Palmitoyl-CoA - inhibits mitochndrial translocase for citrate – citratedo not enter into cytosol - FA synthesis
Regulation of fatty acid metabolism
Hormonal regulation:
• Insulin - dephosphorylation of acetyl CoA carboxylase(= activation) – FA synthesis
• Glukagon - phosphorylation of acetyl CoA carboxylase(= inaktivace) - FA synthesis
- imcreased lipolysis in adipose tissue - entry of FA into liver –
ß-oxidation
• Adaptive control - changes in enzyme expresionfood supply after fasting - expression of acetyl CoAcarboxylase, fatty acid synthase - FA synthesis
FA
Liver TAG VLDL chylomicrones TAG GIT
TAG - adipose tissue
acetyl-CoA
TCA cycle
respirarory chain
NADH+H+, FADH2
TAG stored in tissues
complex lipids
cellular membranes
catabolism of carbohydratesand amino acids
HMG CoA
ketone bodies
steroids
eicosanoids
LPL LPL
HSL
ßß--oxidaceoxidacebiosyntézabiosyntéza
Overview of fatty acid metabolism
ATP