Page 189 - Color Atlas of Biochemistry

P. 189

180 Metabolism

Amino acid degradation and ketogenic. This group includes phenylala-
nine, tyrosine, tryptophan, and isoleucine.
A large number of metabolic pathways are Degradation of acetoacetate to acetyl CoA
available for amino acid degradation, and an takes place in two steps (not shown). First,
overview of these is presented here. Further acetoacetate and succinyl CoA are converted
details are given on pp. 414 and 415. into acetoacetyl CoA and succinate (enzyme:
3-oxoacid-CoA transferase 2.8.3.5). Acetoacetyl
CoAis then brokendownby β-oxidation into
A. Amino acid degradation : overview
two molecules of acetyl CoA (see p. 164),
During the degradation of most amino acids, while succinate can be further metabolized
the α-amino group is initially removed by via the tricarboxylic acid cycle.
transamination or deamination. Various
mechanisms are available for this, and these
are discussed in greater detail in B.The carbon B. Deamination
skeletons that are left over after deamination There are various ways of releasing ammonia
undergofurther degradationinvarious ways. (NH 3 ) from amino acids, and these are illus-
During degradation, the 20 proteinogenic trated here using the example of the amino
amino acids produce only seven different acids glutamine, glutamate, alanine, and ser-
degradation products (highlighted in pink ine.
and violet). Five of these metabolites (2-oxo- [1] In the branched-chain amino acids (Val,
glutarate, succinyl CoA, fumarate, oxaloace- Leu, Ile) and also tyrosine and ornithine, deg-
tate, and pyruvate) are precursors for gluco- radation starts with a transamination. For ala-
neogenesis and can therefore be converted nine and aspartate, this is actually the only
into glucose by the liver and kidneys (see degradation step. The mechanism of transa-
p. 154). Amino acids whose degradation sup- mination is discussed in detail on p. 178.
plies one of these five metabolites are there- [2] Oxidative deamination,withthe forma-
+
fore referred to as glucogenic amino acids. tion of NADH+H , only applies to glutamate in
The first four degradation products listed are animal metabolism. The reaction mainly takes
already intermediates in the tricarboxylic acid place in the liver and releases NH 3 for urea
cycle, while pyruvate can be converted into formation (see p. 178).
oxaloacetate by pyruvate carboxylase and thus [3] Two amino acids—asparagine and glu-
made available for gluconeogenesis (green tamine—contain acid–amide groups in the
arrow). side chains, from which NH 3 can be released
With two exceptions (lysine and leucine; by hydrolysis (hydrolytic deamination). In the
see below), all of the proteinogenic amino blood, glutamine is the most important trans-
acids are also glucogenic. Quantitatively, port molecule for amino nitrogen. Hydrolytic
they represent the most important precursors deamination of glutamine in the liver also
for gluconeogenesis. At the same time, they supplies the urea cycle with NH 3 .
also have an anaplerotic effect—i. e., they re- [4] Eliminating deamination takes place in
plenish the tricarboxylic acid cycle in order to the degradation of histidine and serine. H 2 Ois
feed the anabolic reactions that originate in it first eliminated here, yielding an unsaturated
(see p. 138). intermediate. In the case of serine, this inter-
Two additional degradation products (ace- mediate is first rearranged into an imine (not
toacetate and acetyl CoA) cannot be chan- shown), which is hydrolyzed in the second
neled into gluconeogenesis in animal metab- step into NH 3 and pyruvate, with H 2 Obeing
olism, as there is no means of converting taken up. H 2 O does not therefore appear in
them into precursors of gluconeogenesis. the reaction equation.
However, they can be used to synthesize ke-
tone bodies, fatty acids, and isoprenoids.
Amino acids that supply acetyl CoA or aceto-
acetate are therefore known as ketogenic
amino acids. Only leucine and lysine are
purely ketogenic. Several amino acids yield
degradation products that are both glucogenic

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