most of the fatty acids in natural lipids contain evennumbered carbon chains

1
oxidation of fatty acids with odd-numbered carbon
chains
211

• most of the fatty acids in natural lipids contain
  even-numbered carbon chains
• BUT a small portion have odd-numbered carbon
  chains
• this is solved in the following way
• the substrate for the last cycle of ?-oxidation
  is the five-carbon homolog of acetoacetyl-CoA
• this five carbon compound is cleaved to yield
  acetyl-CoA and proprionyl-CoA
• proprionyl-CoA must be further metabolized before
  its carbon atoms can enter the citric acid cycle
  for complete oxidation to CO2
• this involves a biotin-dependent carboxylation
  and a coenzyme B12 dependent rearrangement to
  yield succinyl-CoA which can enter the citric
  acid cycle

2
The CoS CoA at C2 exchanged position with a
hydrogen atom at C-3
It does this without mixing with any hydrogen
atoms of the solvent H20 That is the hydrogen
at C-3 is the same one that ends up on C-2
212
3
The key to understanding how coenzyme B12
catalyzes hydrogen exchange isi n the properties
of the covalent bond between cobalt and C-5of
the deoxyadenosyl group This is a relatively
weak bond that can be broken by illuminating the
compound with visible light (this extreme
photolability probably accounts for the fact that
plants do not contain vitamin B12) Dissociation
produces a 5deoxyadenosyl radical and the Co2
form of the vitamin The free radical initiates
a series of transformations in which the
migrating hydrogen atom never exists as a free
species and is therefore never free to exchange
with the hydrogen of surrounding water molecules
213
4
The formation of coenzyme B12 occurs in one of
only 2 known reactions in which triphosphate is
cleaved from ATP
The other reaciton is the formation of S-
adenosyl methionine from ATP and methionine
214
5
Omega oxidation of fatty acids
A minor pathway that occurs in the endoplasmic
reticulumn of liver and kidney A hydroxyl is
introduced onto the omega carbon by a mixed
function oxidase Two more enzymes act on the
omega carbon alchohol dehydrogenase oxidizes
the hydroxyl group to an aldehyde aldehyde
dehydrogenase oxidizes the aldehyde group to a
carboxylic acid the product is a fatty acid
with a carboxyl group at each end Either end
can be attached to Co-A and as the molecule
undergoes beta oxidation Dicarboxylic acids are
produced
215
6
There is a subclass of enzymes that carry out
oxidation-reduction reactions in which molecular
oxygen is a participant Mixed function oxidases,
oxygenases   oxidases enzymes that catalyze
oxidation reactions in which molecular oxygen
serves as the electron acceptor, but neither of
the oxygen atoms in incorporated into the
product   oxygenases enzymes that catalyze
reactions in which oxygen atoms are directly
incorporated into the product, forming a new
hydroxyl or carboxyl group
1. dioxygenases both O atoms are incorporated
into the product   2. monooxygenase, only one of
the two O atoms is incorporated
the other is reduced
to H2O this require two substrates to serve
as reductants of the two oxygen atoms of O2 the
main substrate accepts one of the two oxygen
atoms and a cosubstrate furnishes hydrogen atoms
to reduce the other oxygen atom to H20
AH BH2 OO AOH B H2O
because the main substrate becomes hydroxylated
they are also called hydroxylases, mixed-fuction
oxidases or mixed-function oxygenases (to
indicate that they oxidize two different
substrates simultaneously)
We will see mixed function oxidases in Fatty
acyl-CoA desaturation, conversion of squalene to
cholesterol, steroid hormome synthesis,
prostaglandin and Leukotriene synthesis, omega
oxidation
216
7
Control of fatty acid oxidation
in most cells, fatty acid oxidation is controlled
by availability of substrates for oxidation (ie
the fatty acids themselves) in animals this
availability is controlled in turn by the
hormonal control of fat mobilization in
adipocytes the action of glucagon or epinephrine
causes fat breakdown and release which leads
ultimately to fatty acid accumulation in other
cells in liver, the level of malony-CoA provides
another regulatory mechanism controlling the
uptake of fatty acyl-CoAs from the cytosol into
mitochondria malonyl CoA is an inhibitor of
carnitine acyltransferase I (the first enzyme in
the acyl-carnitine shuttle) malonyl-CoA is the
committed intermediate in fatty acid
biosynthesis THEREFORE malonyl-CoA has the dual
effect of activating fatty acid biosynthesis (as
the product of the rate limiting reaction) and
inhibiting fatty acid oxidation by preventing
transport of fatty acid into mitochondria
217
8
metabolic fates of acetyl-CoA

• oxidation to CO2 in the citric acid cycle
• biosynthesis of fatty acids
• biosynthesis of cholesterol
• ketogenesis

• Ketogenesis
• When acetyl CoA accumulates beyond its capacity
  to be oxidized or used for fatty acid synthesis,
  ketone bodies are formed in the mitochondrial
  matrix
• acetone
• acetoacetate
• ?-hydroxybutarate

synthesized primarily in the liver they are
important sources of fuel for many peripheral
tissues, including brain, heart and skeletal
muscle (brain normally uses glucose as its source
for metabolic energy but during starvation,
ketone bodies may be the major energy
source) acetoacetate and b-hydroxybutarate are
the preferred and normal substrates for kidney
cortex and heart muscle acetone is exhaled
218
9
Reactions of ketogenesis
219

• Condensation of two molecules of acetyl-CoA to
  form acetoacetyl-CoA catalyzed by thiolase
• (this is the same enzyme that carries out the
  thiolase reaction in ?-oxidation but here it runs
  in reverse)
• Condensation of acetoacetyl-CoA to another
  molecule of acetyl-CoA to form ?-hydroxy-?-methygl
  utaryl-CoA (HGM-CoA)
• catalyzed by HMG-CoA synthase
• HGM-CoA is cleaved to acetoacetate and acetyl-CoA
• acetoacetate can undergo either NADH-dependent
  reduction to ?-hydroxybutarate or in very small
  amounts decarboxylation to acetone

10
Ketone bodies formed in the liver are exported to
other organs
acetoacetate and ?-hydroxybutarate are
transported through the blood from liver to
target organs and tissues where they are
converted to acetyl-CoA for energy
generation ketone bodies are easily
transportable forms of fatty acids that move
through the circulatory system without the need
for complexation with serum albumin and other
fatty acid binding proteins
2110
11
Ketone bodies and Diabetes Mellitus
2111

• Diabetes Mellitus is the most common endocrine
  disease and the third leading cause of death in
  North America
• characterized by an abnormally high level of
  glucose in the blood
• Type I diabetes (10) results from inadequate
  secretion of insulin by the pancreas
• Type II diabetes (90) results from an
  insensitivity to insulin
• (normal levels of insulin are produced but cells
  are not responsive to insulin)
• In both cases, transport of glucose into muscle,
  liver and adipose tissue is significantly reduced
  and even though there is abundant glucose in the
  blood, the cells are metabolically starved
• cells respond by increased gluconeogenesis and
  catabolism of fat and protein
• In type I diabetes, increased gluconeogenesis
  consumes most of the available oxaloacetate, but
  breakdown of fat (and protein) produce large
  amounts of acetyl-CoA

this increased acetyl-CoA would normally be
directed into the citric acid cycle, but with
oxaloacetate in short supply, it is used instead
for production of ketone bodies acetone can
often be detected on the breath of type 1
diabetes ketogenesis is also significant in
starvation
12
Fatty acid biosynthesis is not a simple reversal
of fatty acid degradation

• Degradation of fatty acids takes place in
  mitochondria
• Synthesis of fatty acids occurs in the
  cytosol
• 2. Intermediates in fatty acid breakdown are
  bound to the -SH group of coenzyme A
• Intermediates in fatty acid synthesis are linked
  covalently to the sulfhydryl groups of acyl
  carrier proteins
• 3. Degradative enzymes are not associated as a
  complex
• Enzymes of fatty acid synthesis (in
  animals) are components of one long polypeptide
  chain, the fatty acid synthase (plants and
  bacteria have separate enzymes that form the
  fatty acid synthase)
• 4. In fatty acid degradation the coenzyme for the
  oxidation-reduction reactions involves NAD/NADH
• In fatty acid synthesis the coenzyme for
  the oxidation-reduction reactions is NADP/NADPH

2112
13
The overall process of fatty acid synthesis is
similar in all prokaryotes and eukaryotes
Biosynthesis of palmitate from acetyl-CoA
overall strategy fatty acid chains are
constructed by the addition of two-carbon units
derived from acetyl-CoA the acetate units are
activated by formation of malonyl-CoA (at the
expense of ATP) the addition of two carbon units
to the growing chain is driven by decarboxylation
of malonyl-CoA the elongation reactions are
repeated until the growing chain reaches 16
carbons in length (palmitic acid) other enzymes
then add double bonds and additional carbon units
to the chain
2113
14
acetyl-CoA cannot penetrate the inner
mitochondrial membrane (nor can longer chain
acyl-CoAs) a shuttle system is used
which citrate is formed in mitochondrial matrix
from acetyl-CoA and oxaloacetate in the first
step of the citric acid cycle If citrate is
generated in excess of the amount needed for
oxidation in the cycle its transported through
the mitochondrial membrane to the cytosol In the
cytosol it is acted on by citrate lyase to
regenerate acetyl-CoA and oxaloacetate at the
expense of one ATP Oxaloacetate is reduced to
malate in order to return to the mitochondrial
matrix in exchange for citrate some of the
malate is oxidatively decarboxylated by malate
dehydrogenase (malic enzyme) to
pyruvate pyruvate is transported back into
mitochondria where it is reconverted to
oxaloacetate generates much of the NADPH needed
for the process
Synthesis of fatty acids occurs in the cytosol
How is acetyl CoA exported from mitochondrial
matrix to cytosol?
2114
15
Reaction 1 synthesis of malonyl-CoA by acetyl
CoA carboxylase
Acetyl CoA carboxylase has three functional
regions 1. Biotin carrier protein 2. Biotin
carboxylase which activates CO2 by attaching it
to a nitrogen in the biotin ring in an ATP
dependent reaction 3. Transcarboxylase which
transfers activated CO2 from biotin to acetyl-CoA
producing malonyl CoA
2115
16
Acetyl CoA carboxylase (in eukaryotes) is a
single protein with two identical polypeptide
chains of MW 230kD each
it polymerizes into a filamentous form which is
the active species the equilibrium between the
monomers and the filamentous form is
regulated the final product of fatty acid
biosynthesis (palmitoyl-CoA) shifts the
equilibrium toward the inactive monomers citrate
an allosteric activator of the enzyme shifts the
equilibrium toward the active filamentous
form the primary physiological regulator is
probably long chain fatty acyl-CoAs
2116
17
The next series of reactions in the synthesis of
fatty acids involve a multienzyme complex fatty
acid synthase complex
acyl carrier protein (ACP) is part of fatty acid
synthase
Step 1 an acetyl group from acetyl CoA is
transferred to ACP by acetyl-CoA-ACP transacylase
2117
18
Step 1 an acetyl group from acetyl CoA is
transferred to ACP by acetyl-CoA-ACP transacylase

• Step 2 the acetyl molecule is transferred to a
  Cys-SH of the
• -ketoacyl-ACP synthase portion of fatty acid
  synthase complex
• Note that the ACP is now empty

Step 3 malonyl-CoA is transferred to acyl
carrier protein (ACP) by malonyl-CoA-ACP
transferase Note that the malonyl and acyl
groups are now very close to each other on
thefatty acid synthase complex
2118
19
Step 4 Acetyl and Malonyl condense to form
?-ketoacyl-ACP bound to ACP reaction is catylzed
by ?-ketoacyl-ACP synthase in this reaction the
acetyl group is transferred to the malonyl group
simultaneously a molecule of CO2 is produced
The decarboxylation facilitates the nucleophilic
attack of the methylene carbon on the thioester
linking the acetyl group to ?-ketoacyl-ACP
synthase
Coupling the condensation to the decarboxylation
makes the overall reaction highly exergonic
2119
20
step 7 the synthesis of butyryl-ACP the double
bond generated in the last reaction is reduced
the enzyme is enoly-CoA reductase NADPH is the
electron donor
Step 6 dehydration of D-3-hyroxyacly-ACP to
trans-?2-enoyl-ACP water is removed from C-2 and
C-3 to yield a double bond in the product the
enzyme is 3-hydroxyacyl-ACP dehydrase
(dehyratase)
Step5 ?-ketoacyl-ACP is reduced (at the carbonyl
group at C-3) to D-3-hydroxyacyl-ACP the enzyme
is ?-keotacyl-ACP reductase the electron donor is
NADPH note that in fatty acid oxidation, the
3-hydroxyacyl-CoAs produced have the L
configuration
2120
21
One round of synthesis through the fatty acid
synthase complex is complete
A 4 carbon butyryl-ACP has been generated

• The butyryl-ACP is now transferred from ACP to a
  Cys-SH of the
• -ketoacyl-ACP synthase portion of fatty acid
  synthase complex
• (just like an acetyl group was transferred in the
  first round)

To start a second cycle (of the last four
reactions) in order to lengthen the chain by two
more carbons, butyryl-ACP condenses with another
molecule of malonyl-ACP butyryl acts just like
acetyl did in the first cycle, and CO2 is lossed
Now there would be six carbons total here, with
the first two from malonyl CoA and the next four
from butyryl
the product of this second round of reactions is
hexanoly-ACP the pattern continues till seven
cycles of condensation and reduction produce the
16 carbon saturated palmitoyl group (still bound
to ACP)
2121
22
for reasons not well understood, chain elongation
generally stops at this point and free palmitate
is released from the ACP molecule by the action
of a hydrolytic activity in the synthase
complex small amounts of longer fatty acids such
as stearate (180) are also formed in certain
plants (cococut and palm) chain termination
occurs earlier and up to 90 of the fatty acids
in the oils of these plants are between 8 and 14
carbons long
2122
23
2123
24
2124