Page 412 - Elements of Chemical Reaction Engineering Ebook

Sec. 7.4 Enzymatic Reaction Fundamentals 383
For the free-radical polymerization in which termination is by transfer to
the moi,omer or a chain transfer agent and by addition, the kinetic chain length is
v -rP- kp MR* - kP
-
N----
r, k,,MR* + k,(R*)2 + k,, R*C k,,M + k, R* + k,,C

kP (7-71)
1 ‘N= kr,,,M+(2k,k,Z, f)”*+k,,C

For termination by combination

and Cor termination by disproportionation

Excellent examples that will reinforce and expand the principles dis-
cussed in this section can be.found in Holland and Anthony,12 and the rleader
is encouraged to consult this text as the next step in studying polymer reaction
engineering.

7.4 Enzymatic Reaction Fundamentals

7.4.1 Definitions and Mechanisms

Another class of reactions in which the PSSH is used is the enzymati-
cally catalyzed reaction, which is characteristic of most biological reactions.
An enzyme, E, is a protein or proteinlike substance with catalytic properties. A
substrate, S, is the substance that is chemically transformed at an accelerated
rate because of the action of the enzyme on it. An important propenty of
enzymes is that they are specific in that one enzyme can catalyze only one
reaction. Far example, a protease hydrolyzes onZy bonds specific between spe-
cific #amino acids in proteins, an amylase works on bonds between glucose
molecmles in starch, and liptase attacks fats, degrading them to fatty acids and
glycerol. Consequently, unwanted products are easily controlled. Enzymes are
produced only by living organisms, and commercial enzymes are generally
produced by bacteria. Enzymes usually work (i.e., catalyze reactions) under
mild conditions: pH 4 to 9 and temperatures 75 to 160°F.
Figure 7-10 shows the schematic of the enzyme chymotrypsin. In many
cases the enzyme’s active catalytic sites are found where the various loops
interact. For chymotrypsin the catalytic sites are noted by the numbers 57, 102,
and 195 in Figure 7-10. A number of structures of enzymes or pertinent infor-
mation can be found on the following WWW sites:

‘*C. D. Holland and R. G. Anthany, Fundamentals of Chemical Reaction Engineering,
2nd ed., Prentice Hail, Upper Saddle River, N.J., 1977, p. 457.