Download Objectives 30 - u.arizona.edu

Summary of key points:
• for fat synthesis, glucose is converted to pyruvate that in turn produces acetyl
CoA in mitochondria;
acetyl CoA carbons in the mitochondria are transported to the cytoplasm via
citrate formed by
condensing oxaloacetate and acetyl CoA; citrate is cleaved in the cytoplasm to
liberate acetyl CoA for lipogenesis
• NADPH for lipogenesis is derived from malic enzyme and the pentose
phosphate pathway
• acetyl CoA carboxylase converts acetyl CoA to malonyl CoA in a biotinrequiring reaction
• fatty acid synthase progressively adds two carbon units, from malonyl CoA, to a
growing acyl chain
to form the 16-carbon palmitic acid in a process that requires 2 NADPH for each
addition
• fatty acid synthase uses acetyl CoA to prime the start of the formation of
palmitic acid
• in liver fat is stored by esterification of glycerol phosphate from glycerol or
dihydroxyacetone
phosphate, as carbon backbones, with long chain fatty acids, primarily palmitate
while other tissues
can only use dihydroxyacetone phosphate
Glycerol
Fatty Acids
Lipolysis
Phospholipids
A
C
S
Triacylglycerol
Phosphatidic Acid
Glycerol-P
Esterification
Fatty Acyl-CoA
-Oxidation
Glucose
Pyruvate
Acetyl-CoA
Glycolysis
L
i
p
o
g
e
n
e
s
i
s
Steroids
Steroidogenesis
Cholesterol
Cholesterogenesis
Ketogenesis
2 CO2
CITRIC
ACID
CYCLE
Ketone
Bodies
Figure 1. Interactions of lipid metabolic pathways
Objectives:
1. Describe the process by which carbons from glucose are used for the
synthesis of fatty acids including the intracellular compartments involved in the
process of supplying these carbons.
Glucose CYTOPLASM
Fatty
Acids
PPP
NAD+, CoA
Glycolysis
CO2
NADPH
NADP+
Malate
NAD+
MDH
NADH
Oxaloacetate
ADP+Pi
CL
PDH
Pyruvate
ATP, CO2
Pyruvate
ME
FAS
Malonyl CoA
ADP, Pi
ACC
CO2, ATP
Acetyl CoA
MITOCHONDRIAL MATRIX
NADH, CO2
Acetyl CoA
PC
ADP, Pi
Oxaloacetate
CS
ATP, CoA
Citrate
Citrate
Figure 2. Export of acetyl CoA as citrate for fatty acid biosynthesis, generation of
NADPH and pathway of lipogenesis.

Any compounds that are metabolized to acetyl CoA can be precursors for
fat synthesis

Acetyl CoA is required for fatty acid synthesis it is produced in the
mitochondria from the metabolism of pyruvate via PDH

Acetyl CoA is not transported directly across the mitochondria

Citrate carries these in from the mitochondria to the cytoplasm

Pyruvate transported into mitochondrial matrix and converted to acetyl
CoA or Oxaloacetate via PC in = amounts

Combine to form citrate in Citric Acid Cylce

↑[ ] in the matrix it is translocated via tricarboxylate translocase

In cyctoplasm it is cleaved by citrate lyase(CL) producing acetyl CoA for
lipogenesis & oxalacetate as products

To recycle carrier carbons oxaloactetate is reduced to malate via a
cytoplasmic malate dehydrogenase(MDH) with NADH providing reducing
power

NADP oxidizes the malate to pyruvate and CO2 in the reaction catalyzed
by malic enzyme

Oxaloacetate from the citric acid cycle is never consumed in the overall
carbon transport
2. Identify the sources of NADPH for lipogenesis.

NADPH is derived from malic enzyme & the pentose pathway
3. Describe the acetyl CoA carboxylase and fatty acid synthase reactions,
identifying their substrates, products and cofactors as well as the vitamin
precursors of the cofactors.
Figure 3. Key enzymes in the synthesis of palmitoyl CoA via lipogenesis
and acyl CoA synthetase.
Acetyl CoA Carboxylase:
acetyl CoA + HCO3- + ATP4-  malonyl CoA + ADP3- + Pi2Fatty Acid Synthase:
acetyl CoA + 7 malonyl CoA + 14 NADPH + 14 H+ 
palmitate + 7 CO2 + 8 CoA + 14 NADP+
Acyl CoA Synthetase: (also used for fatty acids other than palmitate)
palmitate + ATP + CoA  palmitoyl CoA + AMP + PPi

Acetyl CoA Carboxylase committed step for fat biosynthesis. Highly
regulated. Catalyzes a biotin dependant reaction(prosthetic group)

Fatty Acid Synthase progressively adds 2 carbon units from malonyl CoA
to a growing acyl chain to form the 16-carbon palmitic acid in process that
requires 2NADPH for each addition
4. Discuss the general mechanism of the fatty acid synthase reaction.
C
E
A
C
P
ACP = acyl carrier protein
CE = condensing enzyme
acetyl CoA
malonyl CoA
SH SH
Malonyl CoA
CoA
2CoA
C
E
A
C
P
S
CO2
COO[2]
S
C=O C=O
C
E
A
C
P
C
E
[3]
COO-
A
C
P
[4]
SH S
SH S
C=O
CH3 CH2
C=OC=O
COOCH3 CH
3
[5]
2NADP+
2NADPH
CH2
C=O
C
E
A
C
P
C
E
A
C
P
S
SH
S
S
C=O
C=O
C=O C=O
CH2
CH2
CH2 CH2
CH2
CH2
CH2 COO-
CH3
CH3
CH3
CH3
Figure 4. General mechanism for FAS. Steps 1 - 4 represent the first cycle that generates
a 4-carbon intermediate. Step 5 prepares for further chain growth
A
C C
E P
S S
C=OC=O
CH2 CH2
[9]
A
C C
E P
S SH
Palmitate
C=O
[8]
A
C C
E P
[7]
A
C C
E P
SH S
SH S 2NADP+
2NADPH
C=O
C=O
CH2
CH2
CH2
CH2
CH2
CH2
C=O
CH2
CH2
CH2
CH2
CH2
CH3
CH3
CH3
[6]
CH2 COOCH3
CO2
Figure 4. General mechanism for FAS. Addition of 2 carbons continues until palmitate is
formed and then released for activation
Fatty Acid synthase

2 components of the fatty acid complex are the condensing enzyme (CE)
and acyl carrier protein (ACP) both contain a free sulfhydryl group (SH)

SH group is derived from phosphopanthetheine

ACP & CE form a heterodimer therefore the SH groups are in close
proximity
1. Attachment of the acetyl group from acetyl CoA to the CE & the
malonyl group from malonyl CoA to ACP
2. Acetyl group from CE condenses with the malonyl residue on the
ACP causing the release of CO2 & leaving a 4 carbon intermediate
covalently bound to ACP
3. 2 molecules of NADPH from niacin a 4 carbon fatty acid is formed
4. That is transferred to the CE site
5. To continue adding 2 carbon units to growing acyl chain a second
molecule of malonyl CoA is attached to the vacated site on ACP
6. 6 additional cycles that include condensation of the growing acyl
chain from CE with the new malonyl group on ACP
7. Multiple reduction of this product to form a new acyl chain that is 2
carbons longer
8. Transfer of this acyl chain back to the CE site
9. 6 thru 8 continue until palmitate is formed & is released from the
ACP site to the CE site

Total of 7 cycles are required to form palmitate with oxidation of 14
molecules of NADPH 2 per cycle

NOTE → product of lipogenesis is the free fatty acid NOT fatty acyl CoA
form

Coenzyme A derivative of palmitic acid is formed by a separate enzyme
acyl CoA synthetase . 2 high energy bonds are consumed for activation

Palmitoyl CoA can undergo elongation & desaturation

Elongation → is a microsomal process (ER) requiring malonyl CoA and 2
NADPH in a process similar to that occurring on fatty acid synthase

Desaturation → microsomal process reducing power is derived from
NADH or NADPH & molecular oxygen is required as well
5. Outline the general process of esterification of glycerol-3-phosphate or
dihydroxyacetone phosphate with long chain fatty acids to form triacylglycerols in
liver.
Glycerol
ATP
glycerol
kinase
+
(liver/kidney) NAD
ADP
Glycerol-3-P
fatty acyl
CoA
CoA
NADH
glycerol-3-phosphate dehydrogenase
fatty acyl
CoA
Lysophosphatidic acid
Dihydroxyacetone
phosphate
(from glycolysis – all tissues)
CoA
Phosphatidic
acid
Diacylglycerol
Pi
fatty acyl
CoA
CoA
Triacylglycerol
Figure 6. Formation of phosphatidic acid from glycerol-3-phosphate or
dihydroxyacetone phosphate, and its conversion to triacylglycerol

Palmitoyl CoA is incorporated into acylglycerols via esterfication

Adipose tissue cells lack glycerol kinase so that DHAP is the only source
of carbon backbones in this tissue

Phospholipids are formed by addition of polar head groups to the
phosphate moiety