6
Specificity
The majority of enzymes catalyze only one type of reaction
and act on only one compound or group of closely related
compounds. The enzyme and its substrate must have a close fit,
or complementarity, similar to that of a lock and key. In many
cases, a small structural change, even in a part of the molecule
remote from that altered by the enzymatic reaction, abolishes
the ability of a compound to serve as a substrate.
A remarkable property of many enzymes is their high degree
of stereospecificity, that is, their ability to discriminate between
asymmetric molecules of the right-handed and left-handed
configurations. For example, only the l isomer of malic acid is
formed by the enzyme fumarase from fumaric acid and water,
as shown in reaction (9).
An enzyme also may act asymmetrically on a symmetrical
substrate to yield a symmetrical product. Alcohol dehydroge-
nase, which catalyzes the conversion of ethanol to acetalde-
hyde, removes a specific hydrogen atom from carbon atom 1 of
ethanol. This hydrogen atom is transferred to the nicotinamide
ring of nicotinamide adenine dinucleotide (NAD). If reduced
NAD containing deuterium (D) is used in the reversal of this
reaction, asymmetrically labeled ethanol is produced, as shown
in reaction (10).
Enzyme (continued)
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