2. Introduction
In most living organisms, the pathways by which
a compound is synthesized are usually different
from the pathways by which it is degraded; two
reasons are
1. Flexibility: If a normal biosynthetic pathway is
blocked, the organism can often use the reverse
of the degradation pathway for synthesis.
2. Overcoming Le Châtelier’s principle:
– We can illustrate by this reaction:
3. Introduction
– Phosphorylase catalyzes both the forward and reverse
reactions.
– A large excess of phosphate would drive the reaction to the
right; that is, drive the hydrolysis of glycogen.
– To provide an alternative pathway for the synthesis of
glycogen, even in the presence of excess phosphate:
Most synthetic pathways are different from the
degradation pathways. Most also differ in location and in
energy requirements.
4. Carbohydrate Biosynthesis
We discuss the biosynthesis of carbohydrates under two
headings:
– Synthesis of glucose in animals and humans.
– Conversion of glucose to other carbohydrates.
Conversion of CO2 to carbohydrates in plants
– Photosynthesis takes place in plants, green algae, and
cyanobacteria.
5. Synthesis of Glucose
Gluconeogenesis: The synthesis of glucose from
noncarbohydrate sources.
– These sources are most commonly pyruvate, citric acid
cycle intermediates, and glucogenic amino acids.
– Gluconeogenesis is not the exact reversal of glycolysis;
that is, pyruvate to glucose does not occur by reversing
the steps of glucose to pyruvate.
– There are three irreversible steps in glycolysis:
---Phosphoenolpyruvate to pyruvate + ATP.
---Fructose 6-phosphate to fructose 1,6-bisphosphate.
---Glucose to glucose 6-phosphate.
– These three steps are reversed in gluconeogenesis, but by
different reactions and using different enzymes.
6. gluconeogenesis
• a mechanism animals use to keep maintain
blood glucose levels
• Takes place mainly in liver
• Takes place in the cytosol
• degradation of glycogen (glycogenolysis) is
another mechanism
• Important precursors of glucose are 3 carbon
lactate, pyruvate, glycerol and certain a.a.
• glycolysis and gluconeogenesis share 7 of 10
steps
• gluconeogenesis has 3 bypass steps –
bypass the irrevesible steps of glycolysis
7. gluconeogenesis
• 1st bypass: conversion of pyruvate to PEP
• requires 2 reactions:
• Pyruvate converted to oxaloacetate by action of pyruvate
carboxylase using ATP
• oxaloacetate to PEP by action of PEP carboxykinase using GTP
10. Other Carbohydrates
Glucose is converted to other hexoses and to
di-, oligo-, and polysaccharides.
– The common step in all of these syntheses is
activation of glucose by uridine triphosphate
(UTP) to form uridine diphosphate glucose (UDP-
glucose) + Pi .
11. Other Carbohydrates
– glycogenesis: The synthesis of glycogen from
glucose.
– The biosynthesis of other di-, oligo-, and
polysaccharides also uses this common activation
step to form an appropriate UDP derivative.
12. The Cori Cycle
The Cori cycle.
Lactate from
glycolysis in muscle
is transported to
the liver, where
gluconeogensis
converts it back to
glucose.
14. 14
Lipid Biosynthesis
keystone concepts:
• Biosynthesis of fatty acids does not proceed as a simple reversal of fatty acid
oxidation
• These reactions are under tight control because the process is energetically
expensive
• Fatty acid synthesis and oxidation are coordinated and regulated together
• Synthesis of storage and membrane lipids from fatty acids is determined by the
metabolic needs of the organism
• Cholesterol is synthesized from acetyl CoA and has several fates
• Cholesterol and other lipids are transported through the blood as lipoproteins
15. Fatty Acid Biosynthesis
While degradation of fatty acids takes place in
mitochondria, the majority of fatty acid synthesis
takes place in the cytosol.
These two pathways have in common that they both
involve acetyl CoA.
– Acetyl CoA is the end product of each spiral of
b-oxidation.
– Fatty acids are synthesized two carbon atoms at a time
– The source of these two carbons is the acetyl group of
acetyl CoA.
The key to fatty acid synthesis is a multienzyme
complex called acyl carrier protein, ACP-SH.
16. 16
comparison to b-oxidation
• Different pathway
• Different enzymes
• Different parts of the cell
– b-oxidation is in the mitochondria
– Fatty acid synthesis is in the cytosol
17. Fatty Acid Biosynthesis
• Synthesis takes place in the cytosol
• Intermediates covalently linked to acyl carrier protein
• Activation of each acetyl CoA.
• acetyl CoA + CO2 Malonyl CoA
• Four-step repeating cycle, extension by 2-carbons /
cycle
– Condensation
– Reduction
– Dehydration
– reduction
18. Fatty Acid Biosynthesis
The biosynthesis of
fatty acids.
– ACP has a side
chain that carries
the growing fatty
acid
– ACP rotates
counterclockwise,
and its side chain
sweeps over the
multienzyme
system (empty
spheres).
21. Malonyl CoA
• Malonyl CoA is synthesized by the action of
acetylCoA carboxylase.
• Biotin is a required cofactor.
• This is an irreversible reaction.
• Acetyl CoA carboxylation is a rate-limiting step
of FA biosynthesis.
• AcetylCoA carboxylase is under allosteric
regulation. Palmitate is a negative effector.
22. 22
fatty acid synthase complex
• Multienzyme Complex
with 7 different active
sites
• 4 repeated steps
include: Condensation,
Reduction, Dehydration,
and Reduction (NADPH
electron carrier)
• Saturated acyl group
produced is the
substrate for additional
rounds of the pathway
23. Fatty Acid Synthase (FAS)
• FAS is a polypeptide chain with multiple domains, each
with distinct enzyme activities required for fatty acid
biosynthesis.
• ACP: Recall that CoA is used as an activator for β-oxidation.
For fatty acid biosynthesis, the activator is a protein called
the acyl carrier protein (ACP). It is part of the FAS complex.
The acyl groups get anchored to the CoA group of ACP by a
thioester linkage
• Condensing enzyme/β-ketoacyl synthase (KS). Also part of
FAS, has a cysteine SH that participates in thioester linkage
with the carboxylate group of the fatty acid.
• During FA biosynthesis, the growing FA chain alternates
between K-SH and ACP-SH
24. 24
saturated acyl group is the substrate for additional
rounds of the pathway
•Reducing agent is NADPH
25. Stepwise reaction
1. The acetyl group gets transferred from CoA to ACP
by malonyl/acetyl-CoA-ACP transferase.
2. The acetyl (acyl) group next gets transferred to the
β-ketoacyl-ACP synthase (KS) of FAS complex.
3. Next, the malonyl group gets transferred from CoA
to ACP by malonyl/acetyl CoA ACP transferase.
• This results in both arms of FAS occupied
4. The COO group of malonyl ACP is removed as CO2,
the acetyl group gets transferred to the alpha carbon of
malonyl ACP. This results in acetoacetyl-ACP
27. Repeat cycles for elongation
• The result of the first cycle of fatty acid biosynthesis is a four carbon
chain associated to the ACP arm.
• This chain gets transferred to the KS.
• A new malonyl CoA is introduced on the ACP arm.
• The reactions proceed as before. For each cycle the acyl group
transferred to the malonyl CoA is 2-carbons longer the previous
cycle.
• At the end of 7 cycles a 16 carbon chain is attached to the
ACP arm (palmitoyl ACP).
• The C16 unit is hydrolyzed from ACP yielding free palmitate
Net reaction: Acetyl CoA + 7 malonyl CoA + 14 NADPH + 14 H+
Palmitate + 7 CO2 + 8 CoA + 14 NADP+ + 6H2O
28. Fatty Acid Biosynthesis
– Higher fatty acids, for example C18 (stearic acid), are
obtained by addition of one or more additional C2
fragments by a different enzyme system.
– Unsaturated fatty acids are synthesized from
saturated fatty acids by enzyme-catalyzed oxidation
at the appropriate point on the hydrocarbon chain.
29. 29
long chain saturated FA’s
are made from palmitate
• In the sER and mitochondria
• CoA is the acyl carrier
• Similar mechanism to FAS
30. 30
desaturation of FA’s requires a
mixed-function oxidase
• Mammalian liver cells desaturate fatty acids on sER
• Mammals can only make ω9 or higher fatty acids
• Plants can make ω6 and ω3 fatty acids in their sER and chloroplasts
31. Essential Fatty Acids
• Mammals lack the enzymes to introduce double bonds
at carbon atoms beyond C9.
• Hence, all fatty acids containing a double bond at
positions beyond C9 have to be supplied in the diet.
• These are called Essential fatty acids (EFA).
• Linoleate (18:2 Δ 9,12) and Linolenate (18:3 Δ 9,12,15)
are the two essential fatty acids in mammals.
• Other unsaturated fatty acids such as arachidonic acid
(20:4 Δ 5,8,11,14) are derived from these two EFA.
• Eicosanoids are derivatives of arachidonic acid and
have hormonal and signaling properties. Classified into
prostaglandins, thromaoxanes and leukotrienes
32. Membrane Lipids
The two building blocks for the synthesis of
membrane lipids are:
– Activated fatty acids in the form of their acyl CoA
derivatives.
– Glycerol 1-phosphate, which is obtained by
reduction of dihydroxyacetone phosphate (from
glycolysis):
CH2-OH
C=O
CH2-OPO3
2-
NADH + H+
CH2-OH
CH
CH2-OPO3
2-
HO NAD+
Dihydroxyacetone
phosphate
Glycerol
1-phosphate
+ +
33. Membrane Lipids
– Glycerol 1-phosphate combines with two acyl CoA
molecules, which may be the same or different:
– To complete the synthesis of a phospholipid, an
activated serine, choline, or ethanolamine is added
to the phosphatidate by formation of a phosphoric
ester.
– Sphingolipids and glycolipids are assembled in
similar fashion from the appropriate building blocks.
CH2-OH
CH
CH2-OPO3
2-
HO 2RC-S-CoA
O CH2-OCR
CH
CH2-OPO3
2-
RCO
O
O
2CoA-SH+ +
Acyl CoA A phosphatidateGlycerol
1-phosphate
34. Cholesterol
All carbon atoms of cholesterol and of all
steroids synthesized from it are derived from
the two-carbon acetyl group of acetyl CoA.
• Synthesis starts with reaction of three molecules
of acetyl CoA to form the six-carbon compound
3-hydroxy-3-methylglutaryl CoA (HMG-CoA).
• The enzyme HMG-CoA reductase then catalyzes
the reduction of the thioester group to a primary
alcohol.
36. 36
fates of cholesterol
• Synthesis in the liver
• Exported as: bile acids,
cholesteryl esters
• Needed for membrane
synthesis, hormone
precursors, Vitamin D
• Insoluble in water
• Cholesteryl esters (CE’s) are
transported in lipoprotein
particles or stored in the
liver.
39. 39
Major Classes of Lipoproteins
• Chylomicrons – movement of dietary TG from intestine to other tissues where
Apo CII activates LPL and uptake
• VLDL – Excess FA and glucose is converted into hepatic TG and transported as
VLDL to muscle and adipose
• LDL – VLDL remnant after unloading TG (rich in cholesterol and CE’s) delivered to
extrahepatic tissues
• HDL – protein rich and low in cholesterol and CE’s. Has LCAT activity to remove
cholesterol from chylomicrons and VLDL and return them to the liver
41. Amino Acids
Most nonessential amino acids are synthesized
from intermediates of either glycolysis or the
citric acid cycle.
• Glutamate, for example, is synthesized by amination
and reduction of a-ketoglutarate, a citric acid cycle
intermediate:
42. Amino Acids
• Glutamate in turn serves as an intermediate in
the synthesis of several amino acids by the
transfer of its amino group by transamination.
43. Amino Acids
• A summary
of
anabolism
showing the
role of the
central
metabolic
pathway.
44. citric acid cycle intermediates and some amino acids
are gluconeogenic
