Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

82 Chapter 5
O
=
C
NHR
peptidase
H 2 O
transpeptidase
O
=
C
OH
+ RNH 2
O
R'NH 2 C NHR' + RNH 2
=
Figure 5.1 Peptidase versus transpeptidase.
endo-
exo-
Figure 5.2 Exo- versus endo-cleavage
the polypeptides are hydrolysed by peptidase (or protease) enzymes; polysaccharides
by glycosidases; and nucleic acids by nucleases.
The group transferases are related in function to the hydrolases, but carry
out quite distinct reactions. In each of the above classes of hydrolases a group is
being cleaved and transferred to the hydroxyl group of water: for polypeptides
an acyl group; for polysaccharides a glycosyl group; and for nucleic acids a
phosphoryl group. Transferases simply transfer this group to an acceptor other
than water. For example, there are a few transpeptidases which cleave an amide
bond and transfer the acyl group to another amino group, forming a new amide
bond (see Figure 5.1). In the same way glycosyl transferases and phosphoryl
transferases transfer glycosyl and phosphoryl groups to acceptor substrates.
One Wnal piece of terminology regards the position of cleavage of a very
long polymeric biological macromolecule. Enzymes which cleave such biological
polymers either cleave progressively from the end of the chain, which
is known as exo-cleavage, or they cleave at speciWc points in the middle of the
chain, which is known as endo-cleavage (see Figure 5.2). In the case of exocleavage
the end which is cleaved is speciWed, for example 5 0 ! 3 0 -exonuclease.
This chapter will deal with each of the major classes of peptidases, glycosidases,
and nucleases, and we will focus on the human immunodeWciency virus
1 (HIV-1) protease as a topical example to examine in more detail. We will also
examine other examples of acyl group transfer and methyl group transfer which
are of considerable biological signiWcance.
5.2 The peptidases
Peptidases are responsible for hydrolysing the amide bonds found in the polypeptide
structures of proteins, hence they are often known as proteases or
proteinases. They have a very important role in the digestive systems of all