Page 280 - Introduction to chemical reaction engineering and kinetics

Chapter 10

Biochemical Reactions:

Enzyme Kinetics

The subject of biochemical reactions is very broad, covering both cellular and enzymatic
processes. While there are some similarities between enzyme kinetics and the kinetics
of cell growth, cell-growth kinetics tend to be much more complex, and are subject to
regulation by a wide variety of external agents. The enzymatic production of a species
via enzymes in cells is inherently a complex, coupled process, affected by the activity
of the enzyme, the quantity of the enzyme, and the quantity and viability of the avail-
able cells. In this chapter, we focus solely on the kinetics of enzyme reactions, without
considering the source of the enzyme or other cellular processes. For our purpose, we
consider the enzyme to be readily available in a relatively pure form, “off the shelf,” as
many enzymes are.
Reactions with soluble enzymes are generally conducted in batch reactors (Chapter
12) to avoid loss of the catalyst (enzyme), which is usually expensive. If steps are taken
to prevent the loss of enzyme, or facilitate its reuse (by entrapment or immobilization
onto a support), flow reactors may be used (e.g., CSTR, Chapter 14). More compre-
hensive treatments of biochemical reactions, from the point of view of both kinetics
and reactors, may be found in books by Bailey and Ollis (1986) and by Atkinson and
Mavituna (1983).

10.1 ENZYME CATALYSIS

10.1.1 Nature and Examples of Enzyme Catalysis
Enzymes are proteins that catalyze many reactions, particularly biochemical reactions,
including many necessary for the maintenance of life. The catalytic action is usually very
specific, and may be affected by the presence of other substances both as inhibitors and
as c0enzymes.l

‘A coenzyme is an organic compound that activates the primary enzyme to a catalytically active form. A coen-
zyme may act as a cofactor (see footnote 2), but the converse is not necessarily true. For example, the coenzyme
nicotinamide adenine dinucleotide, in either its oxidized or reduced forms (NAD+ or NADH), often participates
as a cofactor in enzyme reactions.
261