Page 103 - Color Atlas of Biochemistry

P. 103

94 Metabolism

Enzyme kinetics II B. Substrate specificity
Enzymes “recognize” their substrates in a
The catalytic properties of enzymes, and con-
sequently their activity (see p. 90), are influ- highly specific way (see p. 88). It is only the
marked substrate specificity of the enzymes
enced by numerous factors, which all have to
be optimized and controlled if activity meas- that makes a regulated metabolism possible.
This principle can be illustrated using the ex-
urements are to be carried out in a useful and
reproducible fashion. These factors include ample of the two closely related proteinases
trypsin and chymotrypsin.Both belong to the
physical quantities (temperature, pressure),
the chemical properties of the solution (pH group of serine proteinases and contain the
value, ionic strength), and the concentrations same “triad” of catalytically active residues
(Asp–His–Ser, shown here in green; see
of the relevant substrates, cofactors, and in-
hibitors. p.176). Trypsin selectively cleaves peptide
bonds on the C-terminal side of basic amino
acids (lysine and arginine), while chymotryp-
A. pH and temperature dependency of sin is specific for hydrophobic residues. The
enzyme activity substrate binding “pockets” of both enzymes
have a similar structure, but their amino acid
The effect of enzymes is strongly dependent
on the pH value (see p. 30). When the activity sequences differ slightly. In trypsin, a nega-
tively charged aspartate residue (Asp-189,
is plotted against pH, a bell-shaped curve is
usually obtained (1). With animal enzymes, red) is arranged in such a way that it can
bind and fix the basic group in the side chain
the pH optimum—i. e., the pH value at which
enzyme activity is at its maximum—is often of the substrate. In chymotrypsin, the “bind-
ing pocket” is slightly narrower, and it is lined
close to the pH value of the cells (i. e., pH 7). with neutral and hydrophobic residues that
However, there are also exceptions to this. For
example, the proteinase pepsin (see p. 270), stabilize the side chains of apolar substrate
amino acids through hydrophobic interac-
which is active in the acidic gastric lumen,
has a pH optimum of 2, while other enzymes tions (see p. 28).
(at least in the test tube) are at their most
active at pH values higher than 9. The bell C. Bisubstrate kinetics
shape of the activity–pH profile results from Almost all enzymes—in contrast to the sim-
the fact that amino acid residues with ioniz-
able groups in the side chain are essential for plified description given on p. 92—have more
catalysis. In example (1), these are a basic than one substrate or product. On the other
hand, it is rare for more than two substrates to
group B (pK a = 8), which has to be protonated
in order to become active, and a second acidic be bound simultaneously. In bisubstrate reac-
tions of the type A + B C+ D, anumber of
amino acid AH (pK a =6), which isonly active
in a dissociated state. At the optimum pH of 7, reaction sequences are possible. In addition to
around 90% of both groups are present in the the sequential mechanisms (see p. 90), in
which all substrates are bound in a specific
active form; at higher and lower values, one
or the other of the groups increasingly passes sequence before the product is released, there
are also mechanisms in which the first sub-
into the inactive state.
The temperature dependency of enzymatic strate A is bound and immediately cleaved. A
activity is usually asymmetric. With increas- part of this substrate remains bound to the
enzyme, and is then transferred to the second
ing temperature, the increased thermal
movement of the molecules initially leads to substrate B after the first product C has been
a rate acceleration (see p. 22). At a certain released. Thisisknown as the ping-pong
temperature, the enzyme then becomes un- mechanism, and it is used by transaminases,
stable, and its activity is lost within a narrow for example (see p.178). In the Lineweaver—
Burk plot (right; see p. 92), it can be recog-
temperature difference as a result of denatu-
ration (see p. 74). The optimal temperatures of nized in the parallel shifting of the lines when
[B] is varied.
the enzymes in higher organisms rarely ex-
ceed 50 °C, while enzymes from thermophilic
bacteria found in hot springs, for instance,
may still be active at 100 °C.

98 99 100 101 102 103 104 105 106 107 108