LIPOLYSIS, BETA OXIDATION,

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BIOC 460 DR. TISCHLER LECTURE 31
 LIPOLYSIS, BETA OXIDATION, KETONES, LIPOGENESIS
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OBJECTIVES
1. For the lipolytic pathway (lipolysis)
describe the pathway, identify where it occurs,
name the principal enzyme involved, and explain
the role of albumin and fatty acid binding
protein in the transport and metabolism of free
fatty acids liberated by lipolysis 2. For the
degradation of fatty acyl CoAs describe the
roles of acyl CoA synthetase, carnitine-palmitoyl
transferases (CPT-I and CPT-II), and carnitine
acylcarnitine translocase (CAT) discuss the
relationship of products of the ?-oxidation
pathway to energy production. 3. For ketone
body metabolism identify where and when ketone
body formation (ketogenesis) occurs, state the
role of ketogenesis, identify where ketone
oxidation occurs and explain why normally
individuals do not develop ketoacidosis even
when producing ketone bodies.
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OBJECTIVES
4. Describe the reactions catalyzed by malic
enzyme and acetyl CoA carboxylase 5. For
the fatty acid synthase reaction list the
substrates and key products, identify the
sources of NADPH for the reaction, and describe
its general mechanism. 6. Describe how fatty
acids are stored as a source of fuel during
starvation or stress.
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PHYSIOLOGICAL PREMISE Would you believe that
diabetics having a ketotic crisis have actually
been arrested for DUIs even though they have
consumed no alcohol? Indeed a blood analysis
would show no alcohol. Why would this occur?
During a ketotic crisis a byproduct of the excess
ketone production is acetone. Having nowhere else
to go, it is expired through the lungs. It is the
acetone that arresting officers have smelled on
the breath of these individuals and despite their
protestations have innocently believed them to be
consuming alcohol.
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LIPOLYSIS OF STORED TRIACYLGLYCEROL

• fatty acids hydrolytically cleaved from
  triacylglycerol
• largely in adipose to release fatty acids as a
  fuel
• may also occur in muscle or liver - smaller
  amounts of fatty acids are stored
• hormone-sensitive (cyclic AMP-regulated) lipase
  initiates lipolysis cleaves first fatty acid
• this lipase and others remove remaining fatty
  acids
• fatty acids/glycerol released from adipose to
  the blood
• hydrophobic fatty acids bind to albumin, in the
  blood, for transport

Lipolysis
Triacylglycerol
Glycerol Fatty acids
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CAPILLARY
CYTOPLASM
cell membrane
FA fatty acid
LPL lipoprotein lipase
FABP fatty acid binding protein
ACS acyl CoA synthetase
Figure 1. Overview of fatty acid degradation
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Cytoplasm
OUTER MITOCHONDRIAL MEMBRANE
palmitoyl-CoA
Intermembrane Space
CPT-I defects cause severe muscle weakness
because fatty acids are an important muscle fuel
during exercise.
Figure 2 (top). Activation of palmitate to
palmitoyl CoA (step 4, Fig. 1) and conversion to
palmitoyl carnitine
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Intermembrane Space
INNER MITOCHONDRIAL MEMBRANE
Matrix
palmitoyl-carnitine
Figure 2 (bottom). Mitochondrial uptake via of
palmitoyl-carnitine via the carnitine-acylcarnitin
e translocase (CAT) (step 5 in Fig. 1).
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Cytoplasm
OUTER MITOCHONDRIAL MEMBRANE
ACS
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Intermembrane Space
palmitoyl-CoA
INNER MITOCHONDRIAL MEMBRANE
palmitoyl-carnitine
carnitine
Matrix
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CoA
palmitoyl-CoA
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inner mitochondrial membrane
Carnitine translocase
matrix side
2 ATP
3 ATP
Figure 3. Processing and ?-oxidation of
palmitoyl CoA
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(excess acetyl CoA)
MITOCHONDRION
Figure 4. Ketone body formation (ketogenesis) in
liver mitochondria from excess acetyl CoA derived
from the ?-oxidation of fatty acids
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KETONE BODY OXIDATION

• high rates of lipolysis (e.g., long-term
  starvation or in uncontrolled diabetes) produce
  sufficient ketones in the blood to be effective
  as a fuel
• ketones are the preferred fuel if glucose,
  ketones, fatty acids all available in the blood
• primary tissues using ketones, when
  available, are brain, muscle, kidney and
  intestine, but NOT the liver.
• ?-Hydroxybutyrate NAD ? acetoacetate NADH
  
• ?-hydroxybutyrate dehydrogenase in mitochondria
  reverse of ketogenesis

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KETOSIS Excessive build-up of ketone bodies
results in ketosis eventually leading to a fall
in blood pH due to the acidic ketone bodies.
Adipose Tissue
Liver
Free fatty acids
Ketone Bodies
Pancreas
Figure 5. Mechanism for prevention of ketosis
due to excess ketone body production that can
lead to ketoacidosis
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LIPOGENESIS
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LIPOGENESIS

• principally in adipose tissue and liver
• lipogenesis cytoplasm requires acetyl CoA
• adipose FA stored as triacylglycerols via
  esterification
• liver produces TAG packaged into VLDL and
  exported
• compounds metabolized to acetyl CoA can serve
  as a fat precursor
• glucose primary source of carbons for fat
  synthesis.

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CYTOPLASM
MITOCHONDRIAL MATRIX
Figure 6. Export of acetyl CoA as citrate for
fatty acid biosynthesis, generation of NADPH and
pathway of lipogenesis. (similar to diagram
discussed for cholesterol synthesis exxept this
involves PDH reaction)
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KEY MITOCHONDRIAL REACTIONS
PYRUVATE CARBOXYLASE pyruvate CO2 ATP ?
oxaloacetate ADP Pi
PYRUVATE DEHYDROGENASE pyruvate NAD coenzyme
A (CoA) ? acetyl CoA CO2 NADH
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KEY CYTOPLASMIC REACTIONS INDIRECTLY NEEDED FOR
LIPOGENESIS
Citrate Lyase citrate CoA ATP ? acetyl CoA
oxaloacetate ADP Pi Malate
dehydrogenase oxaloacetate NADH ? malate
NAD Malic Enzyme malate NADP ? pyruvate
NADPH
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KEY CYTOPLASMIC REACTIONS DIRECTLY NEEDED FOR
LIPOGENESIS AND FATTY ACID ACTIVATION
Acetyl CoA Carboxylase acetyl CoA HCO3-
ATP ? malonyl CoA ADP Pi Fatty 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
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A C P

CE acp
CO2
CO2
CO2
CO2
COO-
malonyl CoA
acetyl CoA
4-C unit
Figure 7. General mechanism for the fatty acid
synthase reaction. CE is condensing enzyme. ACP
is acyl carrier protein. This row represents the
initial steps for priming the reaction with
acetyl CoA and the addition of two carbons from
malonyl CoA.
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malonyl CoA
4-C unit
Figure 7. General mechanism for the fatty acid
synthase reaction. CE is condensing enzyme. ACP
is acyl carrier protein. This row depicts a
typical cycle of adding two more carbons to the
fatty acid chain.
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thioesterase cleavage
malonyl CoA
6-C unit
palmitate
Figure 7. General mechanism for the fatty acid
synthase reaction. CE is condensing enzyme. ACP
is acyl carrier protein. This row shows the
release of the finished product, palmitate,
through cleavage by thioesterase.
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Sources of NADPH for the Biosynthesis of Fatty
Acids.
malic enzyme Malate NADP ? Pyruvate CO2
NADPH pentose phosphate pathway Glucose-6-P
2 NADP ? Ribulose-5-P 2 NADPH CO2
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Lysophosphatidic acid
Phosphatidic acid
Diacylglycerol
Triacylglycerol
Figure 8. Formation of phosphatidic acid from
glycerol-3-P or DHAP, and its conversion to
triacylglycerol