Page 422 - Elements of Chemical Reaction Engineering Ebook

Sec. 7.5 Bioreactors 393

glucose(Sr) ( G.O*(Eo) (H,O,(P,)
6-lactone(P,) G.O.H,(Er) O,(S,)

Derivation of the rate laws for this reaction sequence is given on the CD-R.OM.

Enzyme Cofactors. In many enzymatic reactions, and in particular biological
reactions, a second substrate (i.e., species) must be introduced to activate the
enzyme. This substrate, which is referred to as a cofactor or coenzyme even
though it is not. an enzyme as such, attaches to the enzyme and is most (often
either reduced or oxidized during the course of the reaction. The enzyme-cofac-
tor complex is referred to as a holoenzyme. The inactive form of‘ the
enzynne-cofactor complex for a specific reaction and reaction direction is called
an apoenzyme. An example of the type of system in which a cofactor is used is
the formation of ethanol from acetaldehyde in the presence of the enzyme alco-
hol dehydrogenase (ADH) and the cofactor nicotinamide adenine dinucleotide
(NAD):
alcohol dehydrogenase
ethanol (PI) IC..
acetaldehyde (S 1) NADH (S,)

NAD+
(S;)
Derivation of the rate laws for this reaction sequence is given on the CD-E!OM.

7.5 Bioreactors

Because enzymatic reactions are involved in the growth of microorganisms, we
now proceed to study microbial growth and bioreactors. Not surprisingly, the
Monod equation, which describes the growth law for a number of bacteria, is
similar to the Michaelis-Menton equation. Consequently, even though bioreac-
tors are not truly homogeneous because of the presence of living cells, we
include them in this chapter as a logical progression from enzymatic reactiions.
The use of living cells to produce marketable chemical products is
becorning increasingly important. By the year 2000, chemicals, agricultural
products, and food pro cts produced by biosynthesis will have risen from the
‘llion
The growth of 1990 market of $275 2 to around $17 bi1li0n.l~ Both microorganisms and
bio@chnologY mammalian cells are being used to produce a variety of products, such as insu-
lin, most antibiotics, and polymers. It is expected that in the future a number
of organic chemicals currently derived from petroleum will be produced by
living cells. ‘The advantages of bioconversions are mild reaction conditions,
high yields (e.g., 100% conversion of glucose to gluconic acid with Aspergillus
niger), that organisms contain several enzymes that can catalyze successive
steps in a reaction, and most important, that organisms act as stereospecific
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l5 Frontiers in Chemical Engineering, National Academy Press, Washington, D.C.,
1988.