96
chapter
6
Enzymes
I:
General Properties, Kinetics, and Inhibition
low solubility in biological fluids and tends to crystallize
in derangements of purine metabolism that result in
hyperuricemia. The crystalline deposits of sodium urate
are responsible for recurrent attacks of acute arthritis or of
renal colic (pain in kidney due to either stone formation
or acute inflammation; see also discussion of purine
catabolism in Chapter 27).
(2)
In two-substrate enzyme-catalyzed reactions
with a double-displacement reaction sequence,
high concentrations of the second substrate may
compete with the first substrate for binding. For
example, in the reaction catalyzed by aspartate
aminotransferase,
L-Aspartate + a-ketoglularate — L-glutamate
+ oxaloacetate,
this enzyme is inhibited by excess concentrations of
a-ketoglutarate. The inhibition is competitive with
respect to L-aspartate.
(3) Competitive inhibition can occur in freely reversible
reactions owing to accumulation of products. Even
in reactions that are not readily reversible, the
product can function as an inhibitor. In the alkaline
phosphatase reaction, in which hydrolysis of a wide
variety of organic monophosphate esters into the
corresponding alcohols (or phenols) and inorganic
phosphates occurs, the inorganic phosphate acts
as a competitive inhibitor. Both the inhibitor and
the substrate have similar enzyme binding
affinities (i.e.,
Km
and
K,
are of the same order of
magnitude).
(4)
In reactions that require metal ions as cofactors,
similar metal ions can compete for the same binding
site on the enzyme. For example, Ca2+ inhibits some
enzymes that require Mg2+ for catalytic function.
Pyruvate kinase catalyzes the reaction
Phosphoenolpyruvate + ADP—>ATP + pyruvate
for which K+ is an obligatory activator, whereas
Na+ and Li+ are potent competitive inhibitors.
Competitive Substrates in Treatment of
Some Intoxications
Competitive inhibition is the basis for the treatment of
some intoxications (e.g., methyl alcohol, ethylene gly-
col). Methanol, which is widely used industrially as a
solvent, is added to ethanol (ethyl alcohol) to make it un-
suitable for human consumption. Such adulterated alcohol
is commonly known as denatured alcohol. Methanol is
metabolized primarily in the liver and kidney by oxida-
tion to formaldehyde and formic acid:
Alcohol
CH
3
OH
dehydrogenase
HCHO
-►
HCOOH
Methanol mhiJeji,,, Formaldehyde
Formic acid
ethanol
Major toxic effects are caused by formaldehyde and formic
acid. The former is responsible for damage to retinal cells
that may cause blindness, while the latter produces severe
acidosis that may eventually lead to death. A minor effect
of methanol is depression of the central nervous system
(CNS). Retardation of the first step in the oxidation of
methanol is accomplished by administration of ethanol,
the oxidation products of which are not as toxic as those
of methanol. Other therapeutic modalities include removal
of methanol by gastric lavage (to prevent further absorp-
tion), hemodialysis (to remove absorbed methanol), and
administration of exogenous bicarbonate (for treatment of
severe acidosis).
Ethylene glycol, which is widely used as an antifreeze
for automobile radiators, upon ingestion causes depression
of the CNS, metabolic acidosis, and severe renal damage.
Its oxidation in the body requires the action of alcohol
dehydrogenase:
Ethylene glycol
OH OH
I
I
H -C - C - H
I
I
H
H
Inhibited by ethanol
or 4-methylpyrazole
NAD+
Alcohol dehydrogenase
NADH + H*
Glycoaldehyde
OH
O
I
II
H -C - C - H
H
1
Glycolic acid
OH
O
I
II
H -C -C -O H
I
H
i
Glyoxylic acid
Formic acid
HCOOH
co
2
o
o
Il
II
-H -C -C -O H
i
Oxalic acid
O
O
Il
II
HO-C-C-OH
Kidney damage results from precipitation of oxalate
crystals in the convoluted tubules. The elevated anion-gap
metabolic acidosis is caused by glycolic acid and lactic
acid. The latter is formed from pyruvate due to a shift in
the redox potential favoring the production of lactate. The
treatment is the same as that for methanol intoxication.
