Cellular Structure, Proteins, and Metabolism
89
required for fatty acid synthesis—hydrogen-bound coenzymes
and ATP—are produced during carbohydrate catabolism.
Third,
α
-glycerol phosphate can be formed from a glucose
intermediate. It should not be surprising, therefore, that much
of the carbohydrate in food is converted into fat and stored in
adipose tissue shortly after its absorption from the gastroin-
testinal tract.
It is very important to note that fatty acids or, more spe-
cifi cally, the acetyl coenzyme A derived from fatty acid break-
down, cannot be used to synthesize
new
molecules of glucose.
We can see the reasons for this by examining the pathways for
glucose synthesis (see Figure 3–48). First, because the reac-
tion in which pyruvate is broken down to acetyl coenzyme
A and carbon dioxide is irreversible, acetyl coenzyme A can-
not be converted into pyruvate, a molecule that could lead to
the production of glucose. Second, the equivalents of the two
carbon atoms in acetyl coenzyme A are converted into two
molecules of carbon dioxide during their passage through
the Krebs cycle before reaching oxaloacetate, another takeoff
point for glucose synthesis, and therefore they cannot be used
to synthesize
net
amounts of oxaloacetate.
Thus,
glucose can readily be converted into fat, but the
fatty acid portion of fat cannot be converted to glucose
.
Protein and Amino Acid Metabolism
In contrast to the complexities of protein synthesis, protein
catabolism requires only a few enzymes, termed
proteases,
to
break the peptide bonds between amino acids (a process called
proteolysis
). Some of these enzymes split off one amino acid
at a time from the ends of the protein chain, whereas others
break peptide bonds between specifi c amino acids within the
chain, forming peptides rather than free amino acids.
Amino acids can be catabolized to provide energy for
ATP synthesis, and they can also provide intermediates for
the synthesis of a number of molecules other than proteins.
Because there are 20 different amino acids, a large number of
intermediates can be formed, and there are many pathways for
processing them. A few basic types of reactions common to
most of these pathways can provide an overview of amino acid
catabolism.
Unlike most carbohydrates and fats, amino acids contain
nitrogen atoms (in their amino groups) in addition to carbon,
hydrogen, and oxygen atoms. Once the nitrogen-containing
amino group is removed, the remainder of most amino acids
can be metabolized to intermediates capable of entering either
the glycolytic pathway or the Krebs cycle.
Figure 3–50
illustrates the two types of reactions by
which the amino group is removed. In the fi rst reaction,
oxi-
dative deamination,
the amino group gives rise to a molecule
of ammonia (NH
3
) and is replaced by an oxygen atom derived
from water to form a
keto acid,
a categorical name rather than
the name of a specifi c molecule. The second means of remov-
ing an amino group is known as
transamination
and involves
transfer of the amino group from an amino acid to a keto acid.
Note that the keto acid to which the amino group is trans-
ferred becomes an amino acid. Cells can also use the nitro-
gen derived from amino groups to synthesize other important
nitrogen-containing molecules, such as the purine and pyrim-
idine bases found in nucleic acids.
Figure 3–51
illustrates the oxidative deamination of
the amino acid glutamic acid and the transamination of the
amino acid alanine. Note that the keto acids formed are inter-
mediates either in the Krebs cycle (
α
-ketoglutaric acid) or gly-
colytic pathway (pyruvic acid). Once formed, these keto acids
can be metabolized to produce carbon dioxide and form ATP,
or they can be used as intermediates in the synthetic pathway
leading to the formation of glucose. As a third alternative, they
can be used to synthesize fatty acids after their conversion to
acetyl coenzyme A by way of pyruvic acid. Thus, amino acids
can be used as a source of energy, and some can be converted
into carbohydrate and fat.
The ammonia that oxidative deamination produces is
highly toxic to cells if allowed to accumulate. Fortunately, it
passes through cell membranes and enters the blood, which
carries it to the liver. The liver contains enzymes that can com-
bine two molecules of ammonia with carbon dioxide to form
NH
3
COOH
R
1
CH
Amino acid 2
coenzyme
2H
Oxidative deamination
Transamination
COOH
O
C
NH
2
coenzyme
Keto acid 1
Ammonia
COOH
R
CH
Amino acid
COOH
R
O
C
NH
2
Keto acid
H
2
O
R
2
COOH
R
2
O
C
Keto acid 2
CH
Amino acid 1
COOH
NH
2
R
1
+
+
+
+
+
+
Figure 3–50
Oxidative deamination and transamination of amino acids.
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