102
chapter
6
Enzymes I: General Properties, Kinetics, and Inhibition
H
OH
H
OH
Amygdalin (D-mandelonitrile- ß-D-glucosido-6-ß-D-glucoside)
COOH
H
OH
Laetrile (1-mandelonitrile- p-glucuronic acid)
Oxyhemoglobin [Hb20 2(Fe2+)]
- Nitrites (N07)
Methemoglobin
Reduced form of nitrites
MetHbOH(Fe3+)]
Cyanide complex of cytochrome oxidase
[Cytaa3(Fe3+)CN]
Cytochrome oxidase
[Cytaa^Fe’ y]
Cyanomethemoglobin
[MetHbOH(Fe )CN]
^Thiosulfate (Sp3_)
Rhodanese
^S u lfite (SO3-)
Methemoglobin + thiocyanate (SON - )
FIG U R E 6-12
Reactivation of cytochrome oxidase and inactivation of cyanide.
CH2= CHCN
Acrylonitrile
NCCHPHPN
FIG U R E 6-11
Structures of some cyanogenic compounds.
Rhodanese is present in the mitochondria, particularly
of liver and kidney cells. A double-displacement mech-
anism has been proposed for its biochemical action.
The steps are as follows: The free enzyme reacts with
a sulfane sulfur-containing compound (a sulfane sulfur
is one that is divalent and covalently bonded only to
other sulfurs), cleaving the S-S bond of the donor sub-
strate (e.g.,
SSO
3
) to form the sulfur-substituted en-
zyme. The latter reacts with the cyanide (a thiophilic ac-
ceptor) to form thiocyanate in an essentially irreversible
reaction.
Nitrite administration has been augmented by thiosul-
fate administration (intravenously) in the treatment of
cyanide poisoning. Cobalt-containing compounds (e.g.,
cobalt chloride and cobalt ethylenediaminetetraacetate)
have also been used to form complexes with cyanide, in
order to decrease the amount of cyanide available for bind-
ing with cytochrome oxidase.
Proteinase Inhibitors and
Their Clinical Significance
In the body, enzymes are compartmentalized and func-
tion under highly restricted conditions. Some enzymes
(e.g., proteinases) are not substrate-specific. When present
in active form in an inappropriate part of the body,
they act indiscriminately and cause considerable dam-
age to the tissue. Inhibitors inactivate these enzymes at
sites where their action is not desired. Proteinase in-
hibitors, which are themselves proteins, are widely dis-
tributed in intracellular and extracellular fluids. Protein
inhibitors of enzymes other than proteinases are rela-
tively rare. Such inhibitors are available for a-amylases,
deoxyribonuclease
I,
phospholipase
A,
and
protein
kinases.
Many proteinase inhibitors are present in blood plasma
and participate in the control of blood coagulation
(Chapter 36), dissolution of blood clots (Chapter 36), ac-
tivation of the complement cascade (Chapter 35), forma-
tion and destruction of some peptide hormones, and in-
activation of proteinases released from phagocytic cells
(Table 6-3). The proteinase inhibitors rapidly combine
with their target enzymes to form stable complexes that
are practically nondissociable. Inhibition occurs through
binding of the reactive site (a substrate-like region) of the
inhibitor to the active site (the substrate binding region) of
the proteinase. At or near the center of the reactive site of
the inhibitor, a specific amino acid residue recognizes the
primary binding site of the target proteinase. Some of the
