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178 Metabolism
Transamination and deamination The ketimine (3) is hydrolyzed to yield the 2-
oxoacid and pyridoxamine phosphate (4).
Amino nitrogen accumulates during protein In the second part of the reaction (see A,
degradation. In contrast to carbon, amino ni- 1b), these steps take place in the opposite
trogen is not suitable for oxidative energy direction: pyridoxamine phosphate and the
production. If they are not being reused for second 2-oxoacid form a ketimine, which is
biosynthesis, the amino groups of amino acids isomerized into aldimine. Finally, the second
are therefore incorporated into urea (see amino acid is cleaved and the coenzyme is
p. 182) andexcretedin this form. regenerated.
A. Transamination and deamination C. NH 3 metabolism in the liver
Among the NH 2 transfer reactions, trans- In addition to urea synthesis itself (see
aminations (1) are particularly important. p. 182), the precursors NH 3 and aspartate are
They are catalyzed by transaminases,and oc- also mainly formed in the liver. Amino nitro-
cur in both catabolic and anabolic amino acid gen arising in tissue is transported to the liver
metabolism. During transamination, the by the blood, mainly in the form of glutamine
amino group of an amino acid (amino acid (Gln) and alanine (Ala; see p. 338). In the liver,
1) is transferred to a 2-oxoacid (oxoacid 2). Gln is hydrolytically deaminated by glutami-
From the amino acid, this produces a 2-oxo- nase [3] into glutamate (Glu) and NH 3 .The
acid (a), while from the original oxoacid, an amino group of the alanine is transferred by
amino acidisformed(b).The NH 2 group is alanine transaminase [1] to 2-oxoglutarate (2-
temporarily taken over by enzyme-bound OG; formerly known as α-ketoglutarate). This
pyridoxal phosphate (PLP; see p. 106), which transamination (A)produces another gluta-
thus becomes pyridoxamine phosphate. mate. NH 3 is finally released from glutamate
If the NH 2 is released as ammonia, the by oxidative deamination (A). This reaction is
process is referred to as deamination.There catalyzed by glutamate dehydrogenase [4], a
are different mechanisms for this (see p. 180). typical liver enzyme. Aspartate (Asp), the sec-
A particularly important one is oxidative ond amino group donor in the urea cycle, also
deamination (2). In this reaction, the α-amino arises from glutamate. The aspartate
group is initially oxidized into an imino group transaminase [2] responsible for this reaction
(2a), and the reducing equivalents are trans- is found with a high level of activity in the
+
+
ferred to NAD or NADP . In the second step, liver, as is alanine transaminase [1].
the imino group is then cleaved by hydrolysis. Transaminases are also found in other tis-
As in transamination, this produces a 2-oxo- sues, from which they leak from the cells into
acid (C). Oxidative deamination mainly takes the blood when injury occurs. Measurement
placein the liver, whereglutamate is broken of serum enzyme activity (serum enzyme di-
down in this way into 2-oxoglutarate and agnosis;see also p. 98)is animportant
ammonia, catalyzed by glutamate dehydro- method of recognizing and monitoring the
genase. The reverse reaction initiates biosyn- course of such injuries. Transaminase activity
thesis of the amino acids in the glutamate in the blood is for instance important for di-
family (see p. 184). agnosing liver disease (e. g., hepatitis) and
myocardial disease (cardiac infarction).
B. Mechanism of transamination
In the absence of substrates, the aldehyde
group of pyridoxal phosphate is covalently
bound to a lysine residue of the transaminase
(1). This type of compound is known as an
aldimine or “Schiff’s base.” During the reac-
tion, amino acid 1 (A, 1a) displaces the lysine
residue, and a new aldimine is formed (2). The
double bond is then shifted by isomerization.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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