Enzymes - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 324

several million
daltons) that act as catalysts. Recently, it has been shown that some RNA molecules are also
catalytic, but the vast majority of cellular reactions are mediated by protein catalysts. RNA
molecules that have catalytic properties are called ribozymes. Enzymes are specific, versatile,
and very effective biological catalysts, resulting in much higher reaction rates as compared to
chemically catalyzed reactions under ambient conditions. More than 2000 enzymes are
known. Enzymes are named by adding the suffix -ase to the end of the substrate, such as
urease, or the reaction catalyzed, such as alcohol dehydrogenase. Some enzymes have a simple
structure, such as a folded polypeptide chain (typical of most hydrolytic enzymes). Many
enzymes have more than one subunit. Some protein enzymes require a nonprotein group
for their activity. This group is either a cofactor, such as metal ions, Mg, Zn, Mn, Fe, or a coen-
zyme, such as a complex organic molecule, NAD, FAD, CoA, or some vitamins. An enzyme
containing a nonprotein group is called a holoenzyme. The protein part of this enzyme is the
apoenzyme (holoenzyme

Enzymes are usually proteins of high molecular weight (15,000 < M <

cofactor). Enzymes that occur in several different
molecular forms, but catalyze the same reaction, are called isozymes. Some enzymes are
grouped together to form enzyme complexes. Enzymes are substrate specific and are classi-
fied according to the reaction they catalyze. Major classes of enzymes and their functions
are listed in Table 8.1 .
Enzymes have been classified into six main types, depending on the nature of the reaction
catalyzed. A four-digit coding (numbering scheme) for enzymes has been developed, in
which the classes are distinguished by the first of four digits. The second and third digits
describe the type of reaction catalyzed, and the fourth digit is employed to distinguish
between the types of the same function on the basis of the actual substrate in the reaction
catalyzed. This scheme has proven useful in clearly delineating many enzymes that have
similarities. It was developed by the Nomenclature Commission of the International Union
of Biochemistry and Molecular Biology, and the prefix EC (stands for Enzyme Code, or
Enzyme Commission as it was originally initiated) is generally employed with the numerical
scheme. This classification scheme is useful as it unambiguously identifies the enzyme in
question. Earlier nomenclature often resulted in one enzyme being identified by several
names if its activity was broad.

¼ apoenzyme þ

Example: Alcoholdehydrogenase (trivial name)
Systematic name: alcohol: NAD þ oxidoreductase (an alcohol is the electron donor and
NAD þ is the electron acceptor)

Enzyme code: EC 1. 1. 1. 1

Number for further identification
acceptor is NAD + or NADP +
donor group is CH-OH
oxidoreductases

Figure 8.1 shows the structure and active sites of one alcoholdehydrogenase. While
enzymes are specific in function, the degree of specificity varies. Some may act on closely
related substrates (eg. based on a functional group) and are said to exhibit group specificity;