Metabolism of Lipids
and Proteins
Chapter 35
Hein and Arena
Version 1.0
Colleen Kelley
Chemistry Department
1 College
Pima Community
© John Wiley and Sons, Inc.
Chapter Outline
35.1 Metabolic Energy
Sources
35.2 Fatty Acid Oxidation
(Beta Oxidation)
35.3 Fat Storage and
Utilization
35. 4 Biosynthesis of Fatty
Acids (Lipogenesis)
35.5 Amino Acid Metabolism
35.6 Metabolic Nitrogen
Fixation
35.7 Amino Acids and
Metabolic Nitrogen
Balance
35.8 Amino Acids and
Nitrogen Transfer
35.8 Nitrogen Excretion and
the Urea Cycle
35.9 Nitrogen Excretion and
the Urea Cycle
35.10 Acetyl-CoA, a Central
Molecule in Metabolism
2
Metabolic Energy
Sources
3
1) Lipids
- fatty acids are used most commonly for
cellular energy.
- Ex: palmitic acid, CH3(CH2)14COOH
2) Proteins (amino acids)
- Source of reduced carbon atoms that
can be catabolized to provide cellular
energy.
- Provide the major pool of usable
nitrogen for cells.
4
Fatty Acid
Oxidation
5
•
Fats are the most energy-rich
class of nutrients.
•
Most of the energy from fats is
derived from their constituent
fatty acids.
6
• Fats are broken down in a series of enzymecatalyzed reactions that also produce useful
potential energy in the form of ATP.
• In complete biochemical oxidation, the carbon
and hydrogen of a fat ultimately are combined
with oxygen (from respiration) to form carbon
dioxide and water.
7
Beta Oxidation
• Beta oxidation, or the two-carbon
chop, is accomplished in a series of
reactions whereby the first two carbon
atoms of the fatty acid chain become
the acetyl group in a molecule of
acetyl-CoA.
8
9
Figure 35.1 A comparison of ATPs produced from 18 carbons
of one stearic acid molecule and 18 carbons of three glucose
10
molecules.
Fat Storage and
Utilization
11
• Fats (triacylglycerols) are stored
primarily in adipose tissue, which
is widely distributed in the body.
• Fat is the major reserve of potential
energy.
12
Biosynthesis of Fatty
Acids (Lipogenesis)
13
• The biosynthesis of fatty acids from acetylCoA is called lipogenesis.
• Acetyl-CoA can be obtained from the
catabolism of carbohydrates, fats, or
proteins.
• After they are synthesized, fatty acids
combine with glycerol to form
triacylglycerols, which are stored in adipose
tissue.
• Consequently, lipogenesis is the pathway
by which all three of the major classes of
nutrients are ultimately converted to fat.
14
Lipogenesis vs. Beta Oxidation
1. Fatty acid catabolism occurs in the
mitochondria, but fatty acid anabolism
(lipogenesis) occurs in the cytoplasm.
2. Lipogenesis requires a set of enzymes that
are different from the enzymes used in the
catabolism of fats.
15
Lipogenesis vs. Beta Oxidation
3. In the anabolic pathway (lipogenesis), the
growing fatty acid chain bonds to a
special acyl carrier protein, ACP-SH,
which acts as a ‘handle’ to transfer the
growing chain from one enzyme to
another through the series of enzymecatalyzed reactions in the pathway.
Coenzyme A is the carrier in fatty acid
catabolism.
16
Lipogenesis vs. Beta Oxidation
4. A preliminary set of reactions,
involving malonyl-CoA, occurs for
each two-carbon addition cycle in the
synthesis. “Malonyl-” refers to a threecarbon group and has no counterpart in
the catabolic pathway.
17
Steps in
Lipogenesis
18
Amino Acid
Metabolism
19
•
Amino acids serve an important and unique
role in cellular metabolism; they are the
building blocks of proteins and also provide
most of the nitrogen for other nitrogencontaining compounds.
•
The metabolism of the carbon structures of
amino acids is very complex:
–
–
–
–
Metabolic nitrogen fixation
Nitrogen balance
Nitrogen transfer
Nitrogen excretion
20
Metabolic Nitrogen
Fixation
21
Nitrogen Fixation
•
The conversion of diatomic
nitrogen (N2) to a biochemically
useful form is termed nitrogen
fixation.
22
Nitrogen Fixation
•
Atmospheric nitrogen is fixed – that
is, converted into chemical
compounds that are useful in higher
forms of life – by three general
routes:
1. Bacterial action
2. High temperature
3. Chemical fixation
23
Nitrogen Cycle
The process by which nitrogen is
circulated and recirculated from the
atmosphere through living organisms and
back to the atmosphere is known as the
nitrogen cycle.
24
Figure 35.2 The nitrogen cycle. Arrows indicate the
movement of nitrogen through the cycle.
25
Amino Acids and
Metabolic Nitrogen
Balance
26
•
•
Protein is digested and absorbed to
provide the amino acid dietary
requirements.
Once absorbed, an amino acid can be:
1. Incorporated into a protein
2. Used to synthesize other nitrogenous
compounds such as nucleic acids, or
3. Deaminated to a keto acid, which can either
be used to synthesize other compounds or be
oxidized to carbon dioxide and water to
provide energy.
27
Amino Acid Pool
• Absorbed amino acids enter the amino
acid pool – the total supply of amino
acids available for use throughout the
body.
28
Figure 35.3 Major biological nitrogen pools related to
the central amino acid pool.
29
Nitrogen Balance
• Nitrogen balance occurs when the
nitrogen pools within the body remain
constant.
30
Amino Acids and
Nitrogen Transfer
31
Transamination
• In general, when an amino acid is used
for some purpose other than protein
synthesis, the amino acid carbon
skeleton is separated from the amino
acid nitrogen.
• A process called transamination is
responsible for most of the nitrogen
transfer to and from amino acids.
32
Transamination
• Transamination is the transfer of an amino group
from an -amino acid to an -keto acid:
33
34
Nitrogen Excretion and
the Urea Cycle
35
Figure 35.4 Urea Cycle.
The cycle begins with the
reaction between carbamoyl
phosphate and ornithine.
36
37
Acetyl-CoA, a Central
Molecule in Metabolism
38
Figure 35.5 Simplified diagram showing acetyl-CoA is at
the hub of protein, carbohydrate and fat metabolism. 39
40