Enzymes

Enzymes

Prepared by

Fahd Al-Dhabaan

Ph.D. student

Faculty of Science

King Saud University

Enzymes Mode of Action

Penetration and Infection of the Host

Pathogens are attached directly into surface of host cells by mucilaginous substances which present on the surface of pathogens.

Mucilaginous substances consisting of mixture of polysaccharides, glycoproteins, lipids, and fibrillar materials, which, when moistened, become sticky and help the pathogen adhere to the host.

After attachment the pathogen grows toward the host surface and pierces the cell wall through mechanical force and enzymatic activity.

For instance, phytopathogenic fungi can penetrate plant cell walls by secretion of degrading enzymes include pectinases, cellulases, proteases, and xylanases.

Pectinases are the most important in pathogenesis, as they are responsible for tissue maceration by degrading the pectic substances in the middle lamella and, indirectly, for cell death.

Pectinases are a group of enzymes that hydrolyze pectin by different mechanisms. They are divided into two classes: pectinesterases and depolymerases.

1.The pectinesterases remove methoxy groups from methylated galacturonides.

2.The depolymerases catalyze the cleavage of glycosidic bonds via hydrolysis (hydrolases) or via β-elimination (lyases).

We measure amount of pectinase by its activity assayed by unit of rate (IU).

IU is defined as the production of one μmole per minute under defined conditions.

Chemical Weapons of Pathogens

The chemical weapons are:

1.Enzymes

2.Toxins

3.Growth regulators

4.Polysaccharides

These substances vary in importance in pathogenicity and from one disease to another.

In soft rots, enzymes are important. Enzymes break down structural components of cells.

In crown gall its growth regulators. Growth regulators exert hormonal effects.

Helminthosporium of Victoria  Oats its toxins. Toxins act directly on protoplast components or interfere with their function

Pathogenic enzymes and virulence

The virulence of some bacteria is aided by the production of extracellular enzymes.
Coagulases are bacterial enzymes that coagulate (clot) the fibrinogen in blood.

Bacterial kinases are bacterial enzymes that break down fibrin and thus digest clots formed by the body to isolate the infection.

Constitutive and inducible enzymes

Constitutive enzymes are produced by the cell continually if their substrates are found or not such as phosphorylases.

Inducible enzymes are produced by the cell only if their substrates are found such as amylase to hydrolyze starch.

FACTORS AFFECTING ENZYME ACTIVITY
 Enzymes are large globular proteins

Some reaction kinetics

Effect of Temperature on Enzyme Activity

0°C

Low temperatures à low Kinetic Energy of enzymes and substrates.

No/Very few enzyme-substrate complexes are formed.

Enzymes are inactivated.

20°C (increasing temperature)

Increasing the temperature will lead to the increase in kinetic energy of enzyme and substrate molecules.

Enzyme and substrate molecules move with increasing speed and collide more frequently with each other.

This increases the rate of enzyme-substrate complex formation This increases the rate of enzyme-substrate complex formation and product formation.

37°C

As the temperature continues to increase, the rate of enzyme activity also increases until the optimal temperature is reached.

Optimal temperature is the temperature at which the enzyme works best.  Rate of product formation is highest!

Denaturation

Effect of pH on enzyme activity

Enzyme works best within a narrow pH range.

Each enzyme works best at particular pH, known as its optimum pH level.

At extreme pH levels, enzymes lose their shape and function and become denatured. 

Effect of Substrate on Enzyme Activity

Effect of Calcium on Enzyme Activity

Calcium is considered cofactor or coenzyme which activates the enzyme.

Calmodulin (calcium modulated protein). Binding of Ca++ to calmodulin changes its shape and allows binding and activation of certain enzymes.

Effect of Age on Enzyme Activity

Enzyme activity tends to decline substantially as we age.

The effect of pH on cell walls

-pH = -log [H+]

Acids and bases

Acid has a higher amount of H+ ions. pH lower than 7.

Bases contain OH- ions. pH above 7.

Water which is neutral has an equal amount of H+ and OH- ions (hydroxide). pH of 7.

Acidic and basic media leads to destruction of cell wall and cell membrane. Therefore leads to cell death.