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replaced by another metal without deactivation of the enzyme.
Examples are the iron complexes of the haeme proteins,
copper complexes of the haemocyanina,
the
the cobalt complex of
vitamin B,2, and the zinc-containing metallo-enzyme, carbonic
anhydrase.
In the second category are those enzymes in which
the metal ions are loosely bound to both enzyme and substrate,
and by bringing about steric or energy changes, serve to catalyze the rate of reaction.
In this type of association, the
metal ions are readily dialyzable from the proteins and the
metal ion specificity is much less than in the first group.
Lehninger
(1950) has discussed in detail the role of
metal ions in enzyme systems.
exclusive,
Three, not necessarily mutually
functions were described for the metallic components
of enzyme systems:
(1) the metal may serve as the catalytic
center of the enzyme,
(2) the metal may not be primarily
involved in catalysis but may be required as a binding group
to bring enzyme and substrate together,
and
(3) the metal ion
may function as a physiological control by antagonizing the
activating effect of some other metal on an enzyme system.
The physical basis for the specificity of metal ions in enzyme
systems lies in the fundamental parameters of ion structure:
(1) mass,
(2) ionic charge,
(3) ionic radius,
(4) potentiality
of reversible valence charge and the electrode potential of