ert211 chp 1-4.pdf

CHAPTER 1: THE KINETICS OF ENZYME

CATALYZED REACTIONS

BY:

EN. MOHD. FAHRURRAZI TOMPANG

ERT 211/1 Biochemical Engineering

Michaelis-Menten Kinetics

Enzyme E and substrate S combine to form a complex ES, which then dissociates into product P and free or uncombined enzyme E:


Equilibrium Constant

Enzyme balance

Decomposition of the complex to product and free enzyme is irreversible.

Product formation Rate, v


Solving the equations

Substitute e in following equation esee o

mKk

k

es

se

1

1

sK

sese

m 0


From Product formation equation

sK

sek

m 0

2

sK

svv

m max

02max Where ekv

Quasi-steady-state Approximation

Briggs and Haldane first proposed Quasi-steady-state assumption

Applying mass balance for substrate and intermediate

esksekdt

dsv 11

eskksek

dt

esd211


In a batch reactor at closed system [E0] is considered very small compared S

Therefore, d(es)/dt ≈0

From equation

21

1

kk

sekes

eskksek

dt

esd211


Substituting e

Production formation kinetics

s

k

kk

sees

1

21

0

eskdt

dp

dt

dsv 2

skkk

sek

121

02

/


Substituting

There is difference between Michaelis-Menten and Quasi-steady-state constant.

02max ekv

1

21m and

k

kkK

sK

svv

m max

Evaluation of Parameters in Michaelis-MentenEquation

Lineweaver-Burk plots are convenient for determination of Km

Double reciprocal plot

plot v versus v/[S] gives a line of slope –Km and y-axis intercept of Vm

Eadie–Hofstee plot

Hanes–Woolf plot

Plot of [S]/v versus [S] gives line of slope I/Vm

and y-axis intercept of Km/Vm.

This plot is used to determine Vm more accurately.

Modulation and Regulation of Enzyme Activity

Chemical species other than the substrate can combine with enzymes to alter or modulate their catalytic activity.

Such substances are called modulators or effectors, may be normal constituents of the cell.

They enter from the cell's environment or act on isolated enzymes.


• The combination of an enzyme with an modulator is chemical reaction

• Modulator can be fully reversible, partially reversible, or essentially irreversible.

• Examples of irreversible inhibitors include poisons such as cyanide ions, which deactivate xanthineoxidase,

• Nerve gases, which deactivate cholinesterases

(enzymes which are part of nerve transmission).


Reversible modulation of enzyme activity is one control mechanism employed by the cell to achieve efficient use of nutrients.

The enzyme regulation involve interconnected networks of reactions with several control loops


Example, five-step sequence for the biosynthesis of the amino acid L-isoleucine.

Regulation of this sequence is achieved by feedback inhibition:


The final product, L-isoleucine, inhibits the activity of the first enzyme.

Thus, if the final product begins to build up, the biosynthesis process will be stopped


• enzyme-substrate inhibitors systems classify by their influence on the Michaelis-Menten equation parameters vmax and Km

• Reversible inhibitors are termed competitive if their presence increases the value of Km but does not alter vmax The effect of such inhibitors can be countered or reversed by increasing the substrate concentration.

• On the other hand, by rendering the enzyme or the enzyme-substrate complex inactive, a noncompetitiveinhibitor decreases the vmax of the enzyme but does not alter the Km value.

Mechanisms of Reversible Enzyme Modulation

Many competitive inhibitors bear close relationships to the normal substrates. This are called substrate analogs.

It is thought that these inhibitors have the key to fit into the enzyme active site, or lock,

But the key is not quite right so the lock does not work; i.e., no chemical reaction results.


For example, inhibition of succinic acid dehydrogenation by malonic acid:

The malonic acid can complex with succinic dehydrogenase, but it does not react


How the sulfa-drug act against bacteria?

The action of one of the sulfa drugs, sulfanilamide, is due to its effect as a competitive inhibitor.


Sulfanilamide is very similar in structure to p-aminobenzoic acid, an important vitamin for many bacteria.

By inhibiting the enzyme which causes p-aminobenzoic acid to react to give folic acid, the sulfa drug can block the biochemical machinery of the bacterium and kill it.

Some noncompetitive inhibition and is thought to be the dominant mechanism for noncompetitive inhibition and activation. These are called allostericcontrol

An enzyme which possesses sites for modulation as well as catalysis has consequently been named an allosteric enzyme.

ert211 chp 1-4.pdf

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