Factors Influencing Enzymic Reactions

Dr.Geeta Jaiswal

Effect of pHMost Enzymes have characteristic pH

at which their activity is maximum

Above and below that pH, the enzyme activity decreases

If a curve is drawn between Enzyme activity and pH on a given substrate concentration maximum activity is revealed at a definite pH – known as Optimum pH

Suitable ionic formation is controlled by pH

At low pH proteins are protonated and the structure is destroyed or substrate may not be in the right ionic form

High pH may denature the protein molecule

Effect of Temperature

Each Enzyme is most active at a particular temperature.

Starting with zero the kinetic energy of molecules increases with the rise in temperature.

More molecules collide and undergo a reaction

After crossing the optimum temperature enzyme activity declines since the bonds in the protein molecules are disrupted.

At times high temperature results in denaturation of the enzyme molecule.

For most enzymes optimum temperature lies between 35

o to 400 C

At low temperatures enzyme reaction requires more time.

Effect of Substrate ConcentrationThe initial velocity of an enzyme catalyzed

reaction is proportional to the substrate concentration.

With the rise of substrate concentration the enzyme activity increases up to a certain point.

After this point a further increase in substrate concentration has no effect, since the enzyme is fully saturated.

Michaelis Menten’s theoryIt was Michaelis Menten who suggested

an explanation to these findingsHe postulated that :- At low substrate

concentrations the enzyme is not saturated with the substrate and the reaction is not proceeding at maximum velocity.

When enzyme is saturated with the substrate max velocity is observed.

Maximum Velocity or Vmax

When the enzyme is fully saturated with the substrate Maximum Velocity of an enzymic reaction is observed denoted as Vmax

The substrate concentration at which the velocity is half the maximum is called

Michaelis Constant and is termed as Km

Km and affinity of Substrate

Km indicates the affinity of the substrate towards the enzyme and is inversely proportional to the affinity.

Km α 1

affinity Higher affinity smaller Km

Lower affinity higher Km

The Michaelis Menten’s is given by the expression

Vmax [S] + [S]

V0 = Km + [S]

OR Vmax [S] V0 = [S] + Km

Importance of Km

Km is the concentration of substrate at which half of the active sites are filled , so half maximum velocity.

It indicates the degree of affinity of an enzymes for a particular substrate.

The more the Km the less the affinity of enzyme for substrate

Km is neither influenced by enzyme concentration nor by non-competetive inhibitors .

It is altered completely by competitive inhibitors; allosteric modulators, pH temperature and substrate concentration.

Determination of important physical constants of an enzyme such as Vmax and Km are difficult from a curve that would be

obtained by plotting [ v ] and [ s ]

Determination of important physical constants of an enzyme such as Vmax and km are difficult from a curve that would be

obtained by plotting [v] and [s]

So the Michaelis Menten equation can be transformed into the form which is useful in plotting experimental data

This equation is called Line weaver Burk and is an equation for a straight line Y=mx + c

1 1 Km. 1 = + . v Vmax Vmax [s]

Double Reciprocals or LineWeaverBurks Plot

When 1/v0 is plotted against 1/[s] ,a straight line is obtained

Slope of which is Km / Vmax

Intercept of 1/Vmax on Y axisIntercept of 1/Km on X axis

Slope = Km/Vmax

1/Vmax

-1/ Km

• M=slope of straight line• C=Intercept on Y axis• X=Intercept on X axis

Thank - You