Page 125 - Color Atlas of Biochemistry

P. 125

116 Metabolism

Allosteric regulation causes a left shift; it reduces both [A] 0.5 and
h (curve III). This type of allosteric effect was
The regulation of aspartate carbamoyltrans- first observed in hemoglobin (see p. 280),
ferase (ACTase), a key enzyme of pyrimidine which can be regarded as an “honorary” en-
biosynthesis (see p. 188) is discussed here as zyme.
an example of allosteric regulation of enzyme
activity. Allosteric effects are mediated by the
C. R and T states
substrate itself or by inhibitors and activators
(allosteric effectors, seep. 114). Thelatterbind Allosteric enzymes are almost always oligo-
at special sites outside the active center, pro- mers with 2–12 subunits. ACTase consists of
ducing a conformational change in the en- six catalytic subunits (blue) and six regulatory
zyme protein and thus indirectly lead to an subunits (yellow). The latter bind the allo-
alteration in its activity. steric effectors CTP and ATP. Like hemoglobin,
ACTase can also be present in two conforma-
tions—the less active Tstate (for “tense”) and
A. Aspartate carbamoyltransferase:
the more active Rstate (for “relaxed”). Sub-
reaction
strates and effectors influence the equili-
ACTase catalyzes the transfer of a carbamoyl brium between the two states, and thereby
residue from carbamoyl phosphate to the give rise to sigmoidal saturation behavior.
amino group of L-aspartate. The N-carbamoyl With increasing aspartate concentration, the
L-aspartate formed in this way already con- equilibrium is shifted more and more toward
tains all of the atoms of the later pyrimidine the R form. ATP also stabilizes the R confor-
ring (see p. 188). The ACTase of the bacterium mation by binding to the regulatory subunits.
Escherichia coli is inhibited by cytidine tri- By contrast, binding of CTP to the same sites
phosphate (CTP), an end product of the ana- promotes a transition to the T state. In the
bolic metabolism of pyrimidines, and is ac- case of ACTase, the structural differences be-
tivated by the precursor ATP. tween the R and T conformations are partic-
ularly dramatic. In T Rconversion, the cat-
alytic subunits separate from one another by
B. Kinetics
1.2 nm, and the subunits also rotate around
In contrast to the kinetics of isosteric (normal) the axis of symmetry. The conformations of
enzymes, allosteric enzymes such as ACTase the subunits themselves change only slightly,
have sigmoidal (S-shaped) substrate satura- however.
tion curves (see p. 92). In allosteric systems,
the enzyme’s af nity to the substrate is not
constant, but depends on the substrate con- D. Structure of a dimer
centration [A]. Instead of the Michaelis con- The subunits of ACTase each consist of two
stant K m (see p. 92), the substrate concentra- domains—i. e., independently folded partial
tion at half-maximal rate ([A] 0.5 )is given. The structures. The N-terminal domain of the reg-
sigmoidal character of the curve is described ulatory subunit (right) mediates interaction
2+
by the Hill coef•cient h. In isosteric systems, with CTPorATP (green). A second, Zn -con-
h = 1, and h increases with increasing sig- taining domain (Zn 2+ showninlight blue)
moidicity. establishes contact with the neighboring cat-
Depending on the enzyme, allosteric effec- alytic subunit. Between the two domains of
tors can influence the maximum rate V max , the catalytic subunit lies the active center,
the semi-saturation concentration [A] 0.5 ,and which is occupied here by two substrate ana-
the Hill coef cient h. If it is mainly V max that is logs (red).
changed, the term “Vsystem”is used. Much
more common are “Ksystems”, in which al-
losteric effects only influence [A] 0.5 and h.
The K type also includes ACTase. The inhib-
itor CTP in this case leads to right-shifting of
thecurve,withanincreasein [A] 0.5 and h
(curve II). By contrast, the activator ATP

120 121 122 123 124 125 126 127 128 129 130