Lecture – 5 The Kinetics of Enzyme-Catalyzed Reactions

Dr. AKM Shafiqul IslamSchool of Bioprocess Engineering

University Malaysia Perlis12.01.10

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,

• They are may be normal constituents of the cell.

• Other wise they enter from the cell's environment or act on isolated enzymes.

Modulation and Regulation of Enzyme Activity

• The combination of an enzyme with a 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 xanthine oxidase,

• Nerve gases, which deactivate cholinesterases (enzymes which are part of nerve transmission).

Modulation and Regulation of Enzyme Activity

• 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

Modulation and Regulation of Enzyme Activity

• Example, five-step sequence for the biosynthesis of the amino acid L-isoleucine. Regulation of this sequence is achieved by feedback inhibition:

Modulation and Regulation of Enzyme Activity

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 stoppedIf L-isoleucine is depleted by its use in protein synthesis, inhibition of enzyme E1 must be quickly relaxed so that the required supply of the amino acid is restored.

Modulation and Regulation of Enzyme Activity

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

• competitive 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.

• competitive 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.

Modulation and Regulation of Enzyme Activity

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

Competitive inhibitor

• In this case binding of substrate and inhibitor to enzyme are mutually exclusive. Because some enzyme is bound in the EI complex, not all the enzyme is available for catalyzing substrate conversion, so the reaction rate is lowered by the inhibitor.

Competitive inhibitor

• Total enzyme balance

• Product formation

• Michaelis constant

Competitive inhibitor

• Comparing this with the original Michaelis-Menten form shows that vmax is unaffected

Noncompetitive inhibitor

• We can add two reaction steps

• Ks and Ki are identical to the corresponding dissociation constants. Also, it is assumed here that the EIS complex does not react to give product P

Noncompetitive inhibitor

• Product formation

• Now the Michaelis constant is unaffected, but the maximum reaction velocity which can be obtained is reduced to

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.

Mechanisms of Reversible Enzyme Modulation

• For example, inhibition of succinic acid dehydrogenation by malonic acid:The malonic acid can complex with succinic dehydrogenase, but it does not react

Mechanisms of Reversible Enzyme Modulation

• 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.

Mechanisms of Reversible Enzyme Modulation

• 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 allosteric control

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

The effect of pH on Enzyme

• amino acids from which all proteins are constructed.

• These biochemical units possess basic, neutral, or acidic groups.

• Enzyme may contain both positively or negatively charged groups at any given pH.

The effect of pH on Enzyme

• simple model of the active site ionization state:

• Where, acid-base reactions, E- denotes the active enzyme form while E and E2- are inactive forms obtained by protonation and deprotonation of the active site of E, respectively. K1 and K2 are equilibrium constants for the indicated reactions.

The effect of pH on Enzyme

• From equilibrium relationship

• Form Enzyme balance

The effect of pH on Enzyme

• The active fraction y- = e-/e0

• This is one of the Michaelis pH functions. The other two, y and y2-, give the fraction of enzyme in the acid and base forms, respectively.

The effect of pH on Enzyme

• The y- function depends upon pH in a manner quite similar to the enzyme activity-pH

• where pKi is defined as —log Ki. The y- function declines as pH is varied away from the optimum pH.

The effect of pH on Enzyme

• The influence on the maximum reaction velocity vmax is obtained by replacing the total enzyme concentration eo with the total active form concentration e0y-