Enzymes - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 359

nonreactive-charged medium components. The intrinsic activity of the enzyme is altered by
the local changes in pH and ionic constituents. Further alterations in the apparent kinetics
are due to the repulsion or attraction of substrates or inhibitors.
The activity of an enzyme toward a high molecular weight substrate is usually reduced
upon immobilization to a much greater extent than for a low-molecular-weight substrate.
This is mainly because of steric hindrance by the support. Certain substrates, such as starch,
have molecular weights comparable to those of enzymes and may therefore not be able to
penetrate to the active sites of immobilized enzymes.
Immobilization also affects the thermal stability of enzymes. Thermal stability often
increases upon immobilization due to the presence of thermal diffusion barriers and the
constraints on protein unfolding. However, decreases in thermal stability have been noted
in a few cases. The pH stability of enzymes usually increases upon immobilization, too.

8.4. ANA LYSIS OF BIOPROCESS WITH ENZYMATIC REA CTIONS

Enzymatic reactions are normally carried out in batch reactors. They can be carried out in
flow reactors especially when enzymes are immobilized. Analysis with enzymatic reactions
can be performed in the same manner as we have learned in Chapters 4 and 5.

Example 8-2. Urease catalyzed urea decomposition reaction

NH 2 CONH 2 / 2 NH 3 þ CO 2

is to be carried out in a batch reactor. The Michaelis e Menten rate parameters determined
from experiment with an enzyme loading of 5 g/L are given by r max ¼ l 1 s 1 1.35 mol

and K m ¼ 0.0265 mol/L. Determine the time needed for 90% conversion of urea to ammonia

and carbon dioxide of a solution containing 0.2 mol/L of urea in a 2-l reactor. The enzyme
loading is 0.001 g/L.

Solution. The Michaelis e Menten rate law is given by

S
K m þ

r max

r ¼ (E8-2.1)

S

We know that K m ¼ 0.0265 mol/L. Since r max is proportional to the enzyme loading,

l 1 s 1 10 4 mol l 1 s 1 . we have r max ¼

¼

1.35 0.001/5 mol 2.7

Mole balance of substrate (urea) in the reactor gives

d n s
d t

00þ r s V ¼ (E8-2.2)

The reactor volume is constant (isothermal liquid/suspended phase reaction). Equation
(E8-2.2) is reduced to

dS
d t ¼

S
K m þ

r max

(E8-2.3)

S