Fat metabolism
Lipids: • • • • •
Triacylglycerol
Storage lipid (hydrophobic)
Phospholipids Sphingolipids Glycolipids Cholesterol
Membrane constituents (amphiphilic)
OH
O O O O O O
Structure of triacylglycerol
O CH2 H2 C CH2 H 2C
H2 C OH
H C OH
H2 C OH
CH2 H2 C CH2 H2 C CH2 H2 C CH2 H2 C
Glycerol
Palmitic acid HC (hexadecanoic acid) CH
2
CH2
3
Metabolism of triacylglycerol (TAG): Overview Sugars
TAG
Amino acids
ADP Acetyl-CoA
ATP
CO + H O 2 2
TAG
Cholesterol
Ketone bodies
Triacylglycerol metabolism (Overview) triacyl-glycerol
glucose
glycerol
fatty acids
pyruvate
acetyl-CoA ADP ATP
CO2 + H2O
ketone bodies
Digestion and uptake of triacylglycerol (TAG)
1. Gastric lipase: Limited effect because fat is not yet emulsified in the stomach. TAG reaches the duodenum largely unaltered. 2. TAG is solubilized by bile acids, which are secreted from liver / gall bladder. High CMC (critical micellar concentration) of bile acids ensures rapid action. 3. Solubilized TAG is degraded by pancreatic lipase (secreted into duodenum).
Structure of cholic acid
O H3C OH CH3 O
Cholic acid
CH3 H HO H
H3C CH3
OH
CH3 CH3
CH3 H HO H
Cholesterol
The critical micellar concentration (CMC) of a detergent determines its speed of fat solubilization (1)
CMC: The concentration of detergent monomers in equilibrium with micelles
The critical micellar concentration (CMC) of a detergent determines its speed of fat solubilization (1)
micellar
CMC
monomer
total detergent concentration
The critical micellar concentration determines the speed of fat solubilization by detergents (2)
Enzymatic hydrolysis of TAG yields fatty acids and diacylglycerol, monoacylglycerol, and free glycerol in turn
O O O O O OH OH
OH OH O O
O O
OH OH
O
OH OH O
OH OH O
OH OH
CH3
CH3 CH3
CH3 CH3 CH3
CH3
CH3
CH3
Free fatty acids are detergents and help solubilizing undigested TAG
OH O O OH
O
O
O
O
Fat is packaged into chylomicrons
Fatty acids, 2-MAG
lumen
Fatty acids, 2-MAG
Protein
ATP
ADP Triacylglycerol
intestinal epithelium
Apolipoproteins Chylomicrons
Triacylglycerol
Chylomicrons
lymphatics
Lungs
Some more anatomy...
Liver Portal vein
Systemic circulation
Chylomicrons are released from the intestine into the lymphatics, bypassing the liver
Lymphatics (thoracic duct)
Extraction of fat from chylomicrons by peripheral tissues
Fat cell Triacylglycerol
Endothelial cell
Fatty acids, glycerol
Chylomicron
Lipoprotein lipase
to Liver
Transport of lipids between organs
1. Lipoproteins. These basically are lipid droplets with a hydrophilic protein coat. Important examples are: • • • • Chylomicrons. Distribute triacylglycerol (TAG) from intestine to peripheral organs (bypassing the liver) VLDL = very low density lipoprotein. Moves TAG and some other lipids from liver to periphery LDL = low density lipoprotein. Moves lipids (particularly cholesterol) from liver to periphery HDL = high density lipoprotein. Moves excess cholesterol from peripheral organs to liver
2. Free fatty acids. These are bound to albumin. This is the major transport mechanism for release of fat from fat tissue
Utilization of fatty acids: -Oxidation
O HO
O
CoA
S
O
CoA
S
O OH
CoA
S
O CoA S
O
-Oxidation (2)
O CoA CoA S S O
O
’
CoA S CH3 CoA S O
O CoA S
O
Activation of fatty acids (1)
O
C
O HO P O O
O P O O
O P O O Adenosine
Mg++
O O C O P O
O HO P O OH
O
Adenosine
O HO P O O
O HO P O O
O P O O
Activation of fatty acids (2)
O O C O P O O Adenosine
CoA-SO O P O
O
O
Adenosine
S
CoA
Acyl-CoA
CoA is transiently replaced by carnitine during transport into the mitochondrion
OM IM
Acyl-CoA ADP ATP Fatty acid CAT I
Acyl-CoA
Acyl-CoA
CAT II
CoA
Carnitine
Carnitine
CoA
CoA Acyl-carnitine Acyl-carnitine
Acyl CoA
CH3 H H2 C O O P O O P O CH2 HC HC O O P O O O O HC N CH CH OH CH N N O NH2 C C C O N H C H2 C H2 O CH3 OH NH CH2 C H2 N S O C
CH3 H3C N
+
CH3
O HO
CH2
Carnitine
HC CH2
OH
COOH
CH3 H3C N
+
CH3 O
CH2
Acyl-carnitine
HC CH2
O
COOH
O CoA S
Acyl-CoA
Reactions in –oxidation: Comparison with the TCA
O CoA S
FAD
O
CoA
S
FADH2
O OH
HO
O
O OH
FAD
FADH2
HO
O
Reactions in –oxidation: Comparison with the TCA (2)
O CoA S
?
O OH
CoA
S
O OH
HO
O
?
O OH OH
HO
O
Reactions in –oxidation: Comparison with the TCA (3)
O CoA S OH
NAD+
O O
NADH +H+
CoA
S
O
OH OH
HO
O
NAD+
NADH +H+
OH
O
O
HO
O
The mechanism of the thiolase reaction
O CoA S BH
+
O
Enzyme
S H
+
B Enzyme O S
O CoA S
CoA O B Enzyme CoA S S
S
O
CoA
S
CH3 O S CH3 O S CH3 O S
-Oxidation of even- and odd-numbered fatty acids
CoA
CoA
Acetyl-CoA (C2)
CH3
CoA
O
CoA
S
CH3
O CH3
O CH3 CoA S C
CoA
S
Propionyl-CoA (C3)
Utilization of Propionyl-CoA (1)
O CoA S CH2 CH3 CoA S O CH2 CH2 COOH
ATP CO2 ADP
O H3C C COOH S CoA
H3C C COOH O S CoA
(S)-Methylmalonyl-CoA
(R)-Methylmalonyl-CoA
Propionyl-CoA
O CoA S CH2 CH3
Succinyl-CoA
O CoA S CH2 CH2 COOH
ATP CO2
1
ADP
O
O
3
H3C
2
C COOH S CoA
H3C
C COOH
S
CoA
(S)-Methylmalonyl-CoA
(R)-Methylmalonyl-CoA
Utilization of Propionyl-CoA (2): Activation of carbon dioxide
O Adenosine O P O O
O P O O
O P O OH
O C O
OH
O Adenosine O P O O
O P O O
HO
O OH P O O C O
Utilization of Propionyl-CoA (3): Activation of carbon dioxide with ATP and biotin
O HN NH
H N S
Enzyme
Biotin
O
O
O O HO P O C O OH
O C
O
HN
NH
O
N
NH
R S
Pi
R S
Fat utilization
TAG
glycerol + FA
FA
acetyl-CoA
ketogenesis
ketone bodies
acetyl-CoA
gluconeogenesis
glucose
glycolysis
Ketone bodies
OH
O OH
-hydroxybutyrate
O O
acetoacetate
OH
?
O
?
Overview of ketone body metabolism
TAG
FA + glycerol
Acetyl-CoA
Succinate
Acetoacetyl-CoA
TCA
FAD FADH2
FA
NAD+
Acetyl-CoA
NADH+H+
Succinyl-CoA
Acetoacetate
NADH+H+
NAD+
-HB
Acetoacetate
-HB
Ketogenesis: Synthesis of acetoacetyl-CoA
O H CoA B Enzyme S
+
O CoA-SH CH3 Enzyme B S CH3
S
H
CH2 O
S
CoA
O H3C
O S CoA
Ketogenesis: Synthesis of hydroxymethylglutaryl-CoA and of acetoacetate O O
CoA S CH3 CoA
Enzyme O S
B
+
H O CH3 CH2 S O H2O CoA
Enzyme-B
1
H
CH2 O
S
CoA CoA 1
O HO
O 2 CH3 H CH2 O S CoA S
O
OH CH3 CH2 OH O
CoA
Ketogenesis: Reduction or decarboxylation of acetoacetate
NADH+H+ NAD+
O HO C H2 O CH3
O HO C H2
OH CH3
-Hydroxybutyrate
CO2
O H3C CH3
Acetone
Stocking up for the winter: fatty acid synthesis
Fatty acid synthesis: • • Proceeds in the cytosol Uses one large multienzyme complex (fatty acid synthase)
Involves NADPH+H+ as the redox cosubstrate Is not reversible (no oxidation in the cytosol)
-oxidation • •
• •
Proceeds in the mitochondrion Uses a set of separate enzymes
Involves NAD+ and FAD as redox coenzymes Is reversible - mitochondria can synthesize their own fatty acids
• •
Mitochondrial matrix
Cytosol
Acetyl-CoA
Transport of acetyl-CoA to the cytosol (1)
ADP+Pi
OA
Acetyl-CoA ADP Citrate ATP
Citrate
OA NADH+H+
ATP
NAD+ malate NADP+
NADPH+H+ CO2
CO2
pyruvate
pyruvate
Mitochondrial matrix
acetyl-CoA
Cytosol
acetyl-CoA
Transport of acetyl-CoA to the cytosol(2)
ADP ATP citrate citrate oxaloacetate malate NAD+ NADH+H+
oxaloacetate malate NADH+H+ NAD+
Transport of acetyl-CoA (3)
Mitochondrial matrix Cytosol
2 acetyl-CoA
2 acetyl-CoA
CoA acetoacetyl-CoA
2 CoA
ADP CoA
ATP acetoacetate acetoacetate
Reactions of cytosolic fatty acid synthesis (1)
O CoA S CH3
ATP
ADP
O CoA S C
OH C O
CO2
Acetyl-CoA carboxylase
Fatty acid synthase is a large molecule with multiple active sites
*
*
Brink, Jacob et al. (2002) Proc. Natl. Acad. Sci. USA 99, 138-143
Features of mammalian fatty acid synthase
2. All active sites are on one single polypeptide chain, but two such chains need to form a dimer in order to be active 3. Only in a polymeric state (polymers of dimers) does the enzyme have activity 4. Polymerization depends on protein phosphorylation, which depends on hormonal control 5. FA synthesis only proceeds to palmitate (hexadecanoate); elongation and desaturation proceed in the endoplasmic reticulum
Substrate attachment to fatty acid synthase
O S
Cys-S-
The phosphopantetheine group occurs in both FA synthase and coenzyme A
O Enzyme Ser O P O O C H2 CH3 H C C C N H C H2 C H2 C O N H C H2 C H2 S
CH3 OH O
O Adenosine P O O
O P O O C H2
CH3 H C C C N H C H2 C H2 C O N H C H2 C H2 S
CH3 OH O
Coenzyme A
Reactions in fatty acid synthesis (1)
Pant Enzyme Cys
S
H3C O
S
CoA
Pant Enzyme Cys
S
S
S O
CH3
CoA S
Reactions in fatty acid synthesis (2)
Pant Enzyme Cys
S Malonyl-CoA CoA S
Pant Enzyme
S O S O CH3 O
O
S O
CH3
2
Cys
Reactions in fatty acid synthesis (3)
Pant Enzyme Cys
S O S O CH3 O
O
Pant Enzyme CO2 Cys
S O S O
CH3
Reactions in fatty acid synthesis (4)
Pant Enzyme Cys S S O CH3 Pant Enzyme Cys NADP+ 6 NADPH 4 NADP+ Pant Enzyme Cys S S O H2O CH3 5 Enzyme Cys S O OH Pant S CH3 NADPH S S O O CH3
Reactions in fatty acid synthesis (5)
Pant Pant Enzyme Cys S O Malonyl-CoA CoA S Pant Enzyme Cys S S O Pant Enzyme Cys S O S O O O S Enzyme O Cys S S
Cerulenin, an irreversible inhibitor of fatty acid synthase
OH O O
+
-ketoacyl-CoA
H O
S
NH2
Cys
Enzyme
cerulenin
O
O
OH
S
Cys
NH2
Enzyme
O
O