CHAPTER
Enzymes I: General Properties,
Kinetics, and Inhibition
This chapter deals with enzyme catalysis, kinetics, in-
hibition, and mechanisms. Enzymes are catalysts and
are functional units of cellular metabolism. Enzymes
are usually proteins but may include RNA molecules as
well. This chapter and the following two chapters dis-
cuss protein enzymes. RNA molecules are ribonucle-
ases (ribozymes) that recognize specific nucleotide se-
quences in the target RNA and hydrolyze phosphodiester
bonds. Ribosomal RNA functions as a peptidyl trans-
ferase in protein biosynthesis (Chapter 25). Some RNA
molecules also undergo a self-splicing process. One ex-
ample is the transformation of pre-mRNA to mature,
functional mRNA by the splicing out of intervening se-
quences (
introns
) and ligation of the coding sequences
(exons).
The splicing-ligation process requires
small nu-
clear ribonucleoprotein particles
(snRNPs) and other
proteins. Enzymatic RNA also can act on other RNA
molecules. An example is ribonuclease P (RNase P),
which is involved in the conversion of precursor tRNA
to functional tRNA by generating 5'-phosphate and 3'-
hydroxyl termini. RNase P contains both a catalytic
RNA moiety and an associated protein (Chapter 25).
Many enzymes are bound either covalently or noncova-
lently to nonprotein components essential for enzymatic
activity. The term
prosthetic group
is generally re-
served for those nonprotein components that are bound
tightly, whereas the term
coenzyme
applies to less
tightly bound nonprotein components. The coenzymes
are complex organic compounds,
frequently derived
from vitamins. The term
cofactor is
used for metal ions and
simple organic compounds that partipate in enzyme cata-
lysis. The protein portion of an enzyme is the
apoenzyme,
and the fully functional enzyme with its attached non-
protein component is the
holoenzyme
(i.e., apoenzyme +
coenzyme = holoenzyme).
6.1
Nomenclature
Enzymes are generally named for the
substrate
or chem-
ical group on which they act, and the name takes the
suffix
-ase.
Thus, the enzyme that hydrolyzes urea is
named urease. Examples of exceptions to this terminol-
ogy are trypsin, pepsin, and papain, which are trivial
names. Systematic nomenclature for the enzymes has been
developed by the Enzyme Commission of the Interna-
tional Union of Biochemistry. This system provides a
rational and practical basis for identification of all en-
zymes currently known as well as for new enzymes.
The systematic name describes the substrate, the nature
of the reaction catalyzed, and other characteristics. A
unique numerical code consisting of four numbers sep-
arated by periods (e.g., EC. 4.2.1.1) is designated. The
prefix “EC” denotes “Enzyme Commission.” The first
number in this designation specifies the class to which
the enzyme belongs. All enzymes are assigned to one
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