Enzymes Enzymes are protein catalysts that

increase the rate of chemical reactions by providing an alternative pathway for the reaction .

This pathway involves increasing the free energy of activation of the reaction.

Enzymes are not changed in the overall catalytic process.

All enzymes are of a protein nature . Most enzymes have a globular structure. Enzymes are found in all tissues and fluids

of the body. They can be intracellular and extracellular.

Some enzymes require chemical groups or molecules other than proteins for enzyme activity , when the chemical needed is an ion or metal it is called a cofactor , when it is a small organic molecule it is called coenzyme such as vitamins

If the additional group needed for the reaction binds tightly and permanently it is called a prosthetic group.

Apo enzyme + coenzyme Holo enzyme

(Inactive enzyme) (active enzyme) form)

Property of enzymes Solubility: water soluble Molecular weights range from 10000-several hundred thousand Absorb light in the UV range. Maximum absorption at 280nm due

to aromatic a.a Enzymes are charged molecules. Charge on enzyme depends on

pH of the solution. Positively charged below pI, and negatively charged above pI.

They have distinctive pI . Enzymes are proteins thus when exposed to heat or other

denaturating agents they lose their native conformation and consequently lose their catalytic activity.

Denaturing agent include: Extreme change in pH 8M urea Heavy metals Radiations Detergents

Enzyme specificityEnzymes are very specific towards the reaction they

catalyse and the substrate they react upon.Enzyme specificity can be classified into the following;1-Absolute specificity; Here the enzyme reacts with one

substrate only , such as the glucokinase enzyme which acts on glucose only.

2- Group specificity; Herethe enzyme acts upon a group of substrates(reactants) that share a common functional group,

Such as the polyphenoloxidase enzymes which act on di and tri hydroxy phenols.

3- Bond specific; The enzyme identifies a specific bond and acts upon it .

4-sterio-isomer specific ; The enzyme identifies a specific isomer and acts only upon it. It Is capable of differentiating between L- and D- isomers of a compound.

Enzymes Enzyme Nomenclature; Enzymes are

named by adding “ase”to the end of substrates name for example,

glucosidase, urease, sucrase OR,the enzyme is named indicating the

substrate acted upon and the type of reaction that it catalyzes :

e.g: GlutamicOxaloaceticTransaminase (GOT), where Glutamic and Oxaloacetic acid are the substrates and the type of reaction is a transfer reaction which involves the transfer of an amino group.

How do enzymes increase the rate of chemical reactions

Enzymes are released unchanged and thus can be reused repeatedly which explains why only small amounts of enzyme is needed in biological systems.

For any chemical reaction to occur the reactants should reach a high energy state called the transition state.

The activation energy of a reaction is the energy needed to transfer all the reactant molecules in one mole to the top of the energy barrier (transition State) at standard

conditions. So the activation Energy is simply the Minimum energy neededFor the reaction to occur.

Enzymes Enzymes increase the rate of chemical reactions by

decreasing the activation energy of the reaction. Enzymes decrease the activation energy by forming the

ES complex , which has higher energy than the reactants thus it needs less energy to reach the transition state .

Free energy

Enzymes

Enzymes Specificity hypothesis;1- Lock and key hypothesis; The enzyme active site should

have a conformation complementary to that of the substrate, just like the key and lock are complementary to each other.

Enzymes Lock and Key Analogy: lock = enzyme, key = substrate.

Enzymes Lock and Key Analogy: lock = enzyme, key = substrate.

Enzymes

Induced Fit Theory

Enzyme is not rigid, changes shape with substrate.

Induced Fit Theory Enzyme is not rigid, changes shape with substrate.

Enzymes

Induced- fit theory: The enzyme changes shape on binding to the substrate, so that the conformation of substrate and enzyme active site is complementary only after binding. When the substrate binds to the enzyme it induces a change in the enzyme conformation the enzymes active site is then moulded into a precise conformation that is complementary to that of the substrate .

This explains how enzymes can react upon a group of substrates that share some chemical similarities.