Enzymes: Catalysis and Kinetics

Enzymes: Catalysis and Kinetics

General Properties of Enzymes:

1) Enzymes are biological catalysts they speed up reactions with-out being consumed.

2) Enzymes are highly specific for their substrates.

3) Enzymes display a high degree of reaction specificity which discourages wasteful byproducts.

4) Co-factors: organic coenzymes, and prosthetic groups (covalent) or inorganic (non-covalent)

5) In a metabolic pathway one reaction or one enzyme always represents the rate-limiting step, this determines the rate for the entire pathway.

Types of Enzyme Reactions:

Transferases-catalyze the transfer of groups from one molecule to another.

Hydrolases-cleave a substrate using water (hydrolysis).
Oxidoreductase-involved in oxidation-reduction, transfer of e-'s between molecules.
Lyase-catalyzes the lysis of a substrate w/out water or oxygen.
Ligases-catalyze the joining of two substrates, often by the elimination of water.
Isomerases-catalyze the rearrangement of a single substrate.

Energy of Enzyme Reactions:

Free Energy of an Enzyme Catalyzed Exergonic Reaction-

Exergonic (Exothermic):

Energy level of reactants higher than products.

The free energy change (DG) is negative (-).

Reaction will proceed spontaneously.

Free Energy of an Enzyme-Catalyzed Endergonic Reaction-

Endergonic (Endothermic):

Energy level of products higher than reactants.

The free energy change (DG) is positive (+).

Reaction will not proceed spontaneously.

Temperature:

Raising the temperature of a system will increase the energy of the interacting molecules and the frequency with which they collide.

Normally the body operated within a very narrow temperature range so this has little effect on enzyme rates.

However, when a fever develops the rate of all metabolic pathways will increase ------->raising the energy demands in the body, hence the phrase "feeding a fever".

Enzyme Kinetics:

Michaelis-Menten Equation-

V = V_max [S] / K_m + [S]

V = velocity (rate of reaction)

V_max = when all of the enzyme molecules contain bound substrate (saturated), maximal velocity

[S] = concentration of the substrate

K_m = concentration of the substrate needed to give half maximal velocity (1/2 V_max), a measure of affinity, enzyme for substrate

The Michaelis-Menten Equation Can be Re-Arranged:

V = V_max [S] / K_m + [S]

Invert:

1 / V = K_m + [S] / V_max [S]

Re-Arrange:

1 / V = K_{m / V}_max [S] + [S] / V_max [S]

1 / V = K_m / V_max ^. 1 / [S] + 1 / V_max

(Y = m x + b), thus we have the equation of a line and can now look at enzyme kinetics graphically using the Lineweaver-Burke plot

Lineweaver-Burke Plot:

Enzyme catalyzed reactions can be inhibited. There are two types of inhibitors, competitive and non-competitive.

Competitive Inhibition:

-A competitive inhibitor binds to the active site of the enzyme.

-A competitive inhibitor does not change the maximum rate for the reaction.

-A competitive inhibitor lowers the enzymes affinity for its substrate.

Non-Competitive Inhibition:

-A non-competitive inhibitor binds to a site other than the active site.

-A non-competitive inhibitor slows the maximum attainable rate of the reaction.

-A non-competitive inhibitor slightly decreases an enzymes affinity for its substrate by altering the shape of the active site.