Page 107 - Color Atlas of Biochemistry

98 Metabolism
Lactate dehydrogenase: structure
Lactate dehydrogenase (LDH, EC 1.1.1.27)is
discussed in some detail here and on the
next page as an example of the structure
and function of an enzyme.
A. Lactate dehydrogenase: structure
The active form of lactate dehydrogenase
(mass 144 kDa) is a tetramer consisting of
four subunits (1). Each monomer is formed
by a peptide chain of 334 amino acids
(36 kDa). In the tetramer, the subunits
occupy equivalent positions (1); each mono-
mer has an active center. Depending on met-
abolic conditions, LDH catalyzes NADH-de-
pendent reduction of pyruvate to lactate, or
+
NAD -dependent oxidation of lactate to pyru-
vate (see p.18).
The active center of an LDH subunit is
shown schematically in Fig. 2. The peptide
backbone is shown as a light blue tube. Also
shown are the substrate lactate (red), the
+
coenzyme NAD (yellow), and three amino
acid side chains (Arg-109,Arg-171,and His-
195; green), which are directly involved in the of the site of cellular injury, since each of the
catalysis. A peptide loop (pink) formed by
amino acid residues 98–111 is also shown. In
the absence of substrate and coenzyme, this
partial structure is open and allows access to
the substrate binding site (not shown). In the
+
enzyme lactate NAD complex shown, the
peptideloop closes the activecenter. Thecat-
alytic cycle of lactate dehydrogenase is dis-
cussed on the next page.
B. Isoenzymes
There are two different LDH subunits in the
organism—M and H—which have a slightly
different amino acid sequence and conse-
quently different catalytic properties. As these
two subunits can associate to form tetramers
randomly, a total of five different isoenzymes
of LDH are found in the body.
Fig. 1 shows sections from the amino acid
sequences of the two subunits, using the sin-
gle-letter notation (see p. 60). A common pre-
cursor gene was probably duplicated at some
point in evolution. The two genes then con-
tinued to develop further independently of
each other through mutation and selection.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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The differences in sequence between the M
and H subunits are mainly conservative—i. e.,
both residues are of the same type, e. g. gly-
cine (G) and alanine (A), or arginine (R) and
lysine (K). Non-conservative exchanges are
less frequent—e. g., lysine (K) for glutamine
(Q), or threonine (T) for glutamic acid (E).
Overall, the H subunit contains more acidic
and fewer basic residues than the M form, and
it therefore has a more strongly negative
charge. This fact is exploited to separate the
isoenzymes using electrophoresis (2;see
pp. 78, 276). The isoenzyme LDH-1, consisting
of four H subunits, migrates fastest, and the
M 4 isoenzyme is slowest.
The separation and analysis of isoenzymes
in blood samples is important in the diagnosis
of certain diseases. Normally, only small
amounts of enzyme activity are found in se-
rum. When an organ is damaged, intracellular
enzymes enter the blood and can be demon-
strated in it (serum enzyme diagnosis). The
total activity of an enzyme reflects the se-
verity ofthe damage,while thetypeof iso-
enzyme foundin the bloodprovidesevidence
genes is expressed in the various organs at
different levels. For example, the liver and
skeletal muscles mainly produce M subunits
of lactate dehydrogenase (M for muscle),
while the brain and cardiac muscle mainly
express H subunits (H for heart). In conse-
quence, each organ has a characteristic isoen-
zyme pattern (3). Following cardiac infarction,
forexample,there is a strongincrease inthe
amount of LDH-1 in the blood, while the con-
centration of LDH-5 hardly changes. The iso-
enzymes of creatine kinase (see p. 336) are
also of diagnostic importance.