Enzymes, inhibition

ENZYMES, CATALYSTS OF BIOLOGICAL SYSTEMS 1.Enzymes in general 2. Development of enzymes 3. General mechanisms of enzymes 4. Kinetic characteristics of enzymes 5. Inhibition of enzymes regulation/control of enzymes

1. Enzymes in general Keywords: catalyst, activation energy, enzyme, substrate, active centre, coenzyme, cofactor, prosthetic group, metalloenzyme, metal ion activated enzyme, inhibition The rate constant of several biochemical reactions and the half life of the reactants without a catalyst (pH = 7, t = 25 °C)

1. Enzymes in general Living systems need enzymes for normal functioning because of the following reasons: (i) The non-catalysed reactions are very slow under conditions (temperature, pressure, concentration, etc.) of life, and thus these rections have to be accelerated in living systems. (ii) Possibility for coupled reactions – an endothermic reaction may occur only enzymatically, when the necessary energy is covered by a parallel high energy reaction step, and the two reactions together becomes favoured energetically. Most cases the energy is provided by the hydrolysis of a high energy phosphoric acid esters. Among these, hydrolysis of ATP is the most important coupling reaction. (iii) The controlled function of the biological systems requires high specificity. (iv) The biochemical processes must proceed without side reactions.

1. Enzymes in general Names: The systhematic name consists of two parts: the first part relates to the name of the substrate (compound which reacts in the reaction) followed by the type of the reaction in which it reacts with the traditional „ase” ending. E.g. ribonucleotide reductase. Enzyme clasess: (i) Oxidoreductases (ii) Transferases (iii) Hydrolases (iv) Liases (v) Isomerases (vi) Ligases

1. Enzymes in general

Several prosthetic group and coenzyme and their role 1. Enzymes in general

2. Development of enzymes Examples for template reactions:

The RNA life hypothesis Ribozyme 2. Development of enzymes

Substrate mediated formation of amino acid based biocatalysts. The substrate may serve as a template for the formation of a specific biocatalyst, which will transfer further and further substrates. The reproduction is provided by the substrate itself. These hypothesis (RNA and substrate role) might be enough to understand for example the possibility of the prebiotic-biotic „big jump”. And as a results of these about 3.5 billion years ago the life occurred on the earth. 2. Development of enzymes

3. Mechanism of enzymes Lock and key model Induced fit hypothesis Transition state stabilisation

3. General mechanism of the enzymes

Schematic energy diagram of the enzymatic (full line) and non-catalysed (dashed line) reactions 3. General mechanism of the enzymes

The schematic mechanism of the adenosine-deaminase 3. General mechanism of the enzymes

Dependence of the initial rate of a simple enzymatic reaction on the concentration of the substrate 4. Enzyme kinetics

At the initial part of the reaction: In steady state: Michaelis constant 4. Enzyme kinetics

Considering that [E] 0 = [E] + [ES], The above equation can be modified: Expressing [ES]: Then substituting this into the V 0 = k 2 [ES] rate equation, the Michaelis-Menten equation is obtained: 4. Enzyme kinetics

By compairing the above equations the meaning of a and b are obtained. K M can be considered as the dissociation constant of complex ES, or it equals the substrate concentration where v 0 = V max /2. 4. Enzyme kinetics

The Lineweawer–Burk plot 4. Enzyme kinetics

5. Enzyme inhibition As it is an equilibrium system, at very high substrate concentrations the effect of the inhibitor is negligible, and thus the maximum reaction rate (V max ) does not change, while the K M increases with the increasing inhibitor concentration.

Increasing the substrate concentration, it is not able to displace the inhibitor and thus V max decreases. The substrate binding site remains the same and thus K M does not depend on the concentration of the inhibitor. 5. Enzyme inhibition

As the inhibitor does not compete with the substrate but does decrease the activity of the enzyme V max decreases and K M increases with the increase of the concentration of the inhibitor. 5. Enzyme inhibition