Page 20 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN005G-231 June 15, 2001 20:46

Enzyme Mechanisms

Stephen J. Benkovic
Ann M. Valentine
Pennsylvania State University

I. Introduction to Enzymes as Catalysts
II. Enzyme Kinetics
III. Illustrative Examples
IV. Origins of the Catalytic Efficiency of Enzymes

GLOSSARY noninclusive article describes a small number of enzyme-
catalyzed reactions from the viewpoint of protein struc-
Inhibitor A molecule that by binding to the enzyme low- ture, reaction kinetics, and probable chemical identity of
ers its activity (i.e., its ability to process the substrate). intermediates along the reaction pathway. It concludes
Intermediate A molecular species usually bound to the with our musings as to the chemical origins of the unusual
enzyme that exists transiently in the course of convert- catalytic properties of enzymes.
ing the substrate of the enzyme to its product.
Product Amoleculethatresultsfromachemicaltransfor-
mation of its precursor substrate at an enzyme’s active I. INTRODUCTION TO ENZYMES
site. AS CATALYSTS
Substrate A molecule that binds to an enzyme’s active
site and is chemically transformed. Enzymes are biological molecules which accelerate the
rate, and often direct the specificity, of a chemical reac-
tion. Like all catalysts, they are not themselves consumed
THE IMPETUS for understanding how enzymes function in the reactions in which they participate but are regen-
is inspired by their enormous catalytic efficiency and their erated to take part in multiple cycles. Transformations
exquisite substrate stereospecificity. With the advent of which are very slow, such as the breakdown of DNA, can
the determination of enzyme structure and the application be accelerated by many orders of magnitude by an appro-
of physical organic tools to examine the reaction coor- priate enzyme. Enzymes cannot catalyze reactions that are
dinate for the enzymatic transformation of the substrate, not thermodynamically favorable, but they can facilitate
penetrating insights have been gained as to the number and accelerate those that are favorable but slow and can
and magnitude of the kinetic steps in the catalytic cycle, couple unfavorable reactions to even more favorable ones.
the chemical nature of intermediates, and the function of Most enzymes are proteins and thus are made up of amino
the active site residues contributed by the enzyme. This acids. Recently it was discovered that RNA molecules can

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