Basics of ENZYMES

Jony MallikB. Pharmacy; M. Pharmacy

E-mail: jonymallik@ymail.com

DEFINITION

“Enzymes are the protein made biochemical that’s are synthesized, stored & released by/in/from exocrine gland of human body.”

“Enzymes are proteins that catalyze (i.e., increase the rates of reaction by decreasing activation energy) chemical reactions without consumed in it.”

“Enzyme are biological catalysts, that can catalyze more biological reaction(digestion, metabolism etc.) leads to a rapid ending of that reaction with suitable product.”

DEFINITION RELATED TO ENZYME

In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products.

The site on the enzyme to which substrate is bind & converted into product is called as active site of enzyme.

ENZYME-SUBSTRATE COMPLEX

Enzymes are organic catalyst produced by an organisms. The reactant in an enzyme-catalyzed reaction is called “substrate”.

HOLOENZYME

A completely active enzyme with all its quality to fit with a substrate & to accelerate a reaction rate is known as “Holoenzyme”

They contain a protein and non-protein part. Both parts must be present before the enzyme can function.

The protein part is called the “apoenzyme” and the non-protein (organic part) is called “coenzyme”.

HOLOENZYME = APOENZYME +

COENZYME

HOLOENZYME

COENZYME

Coenzymes are not proteins and so are not inactivated by heat. Examples of coenzymes are the vitamins or compounds derived from vitamins. The reaction involving a coenzyme can be written as follows:

coenzyme + apoenzyme = enzyme

Coenzyme A is essential in the metabolism of carbohydrates, lipids, and proteins in the body.

COFACTORSSome enzymes cannot work on their own, they need a

molecule called a cofactor in order to work properly. Cofactors modify the enzyme complex so that it has the chemical properties necessary to catalyze a reaction.

There are three kinds of cofactors:

a) Prosthetic group: Organic molecule that is permanently attached to an enzyme.

b) Coenzymes: Relatively small organic molecules are not permanently attached to the enzyme molecule.

c) Metal cofactors: Inorganic metal ions that are also known as enzyme activators.

NOMENCLATURE

Formerly enzyme were given names ending in “-in”. With no relation being an indicator between the enzyme and the substance it affects the substrate.

The current system for naming enzymes uses the name of the substrate or the type of reaction involved, with the ending “-ase”.

CLASSIFICATION

Oxidoreductases – are enzymes that catalyze oxidation-reduction reactions between two substrates. The enzymes of the oxidation-reduction reactions in the body are important because these reactions are responsible for the production of heat and energy.

Transferases – are enzymes that catalyze the transfer of a functional group between two substrates.

CLASSIFICATIONHydrolases – hydrolytic enzymes catalyze the

hydrolysis of carbohydrates, esters and proteins.

Lyases – are enzymes that catalyzes the removal of groups from substrates by means other than hydrolysis, usually with the formation of double bonds.

Isomerases – are enzymes that catalyze the interconversion of cis-trans isomers.

Ligases – or synthetases, are enzymes that catalyze the coupling of two compounds with breaking of pyrophosphate bonds.

The Chemical nature of enzymes

• Enzymes are globular proteins. They have a complex tertiary and quaternary structure in which polypeptides are folded around each other to form a roughly spherical or globular shape. The overall 3D shape of an enzyme molecule is very important: if it is altered, the enzyme cannot bind to its substrate and so cannot function. Enzyme shape is maintained by hydrogen bonds and ionic forces.

Enzymes have several important properties: Enzymes are specific: each enzyme usually catalyses only one

reaction. Enzymes combine with their substrates to form temporary

enzyme-substrate complex. Enzymes are not altered or used up by the reactions they

catalyze, so can be used again and again. Enzymes are sensitive to temperature and pH. Many enzymes need cofactors in order to function. Enzyme function may be slowed down or stopped by inhibitors.

The specificity of enzymesTwo models that may explain how enzymes work are:

1) The lock and key hypothesis

2) The induced fit hypothesis

1)The lock and key hypothesis Enzyme has a particular shape into which the

substrate or substrates fit exactly. This is often referred to as the ‘lock and key’ hypothesis where the substrate is imagined being like a key whose shape is complementary to the enzyme or lock. The site where the substrate bonds in the enzyme is known as the active site and it has a specific shape.

2) The induced fit hypothesis– The active site in many enzymes is not exactly

the same shape as the substrate, but moulds itself around the substrate as the enzyme substrate complex is formed.

– Only when the substrate binds to the enzyme is the active site, the correct shape to catalyze the reaction. As the products of the reaction from they fit the active site less well and fall away from it. Without the substrate, the enzyme reverts to its ‘relaxed’ state, until the next substrate comes along.

Factors affecting enzyme activity

The factors that affect enzyme activity also affect the functions of the cell and ultimately the organism. Enzymes are proteins and their function is therefore affected by:

Temperature pH Substrate concentration Enzyme concentration Cofactors Inhibitors

TemperatureFor a non-enzymatic chemical reaction, the general rule is:

the higher the temperature, the faster the reaction. This same rule holds true for a reaction catalyzed by an enzyme, but only up to about 40-450C. Above this temperature, enzyme molecules begin to vibrate so violently that the delicate bonds that maintain tertiary and quaternary structure are broken, irreversibly changing the shape of the molecule. When this happens, the enzyme can no longer function and it is said to be denatured.

pH Like other proteins, enzymes are stable over a limited range of pH.

Outside this range, at the extremes of pH, enzymes are denatured. Free hydrogen ions (H+) or hydroxyl ions (OH-) affect the changes on amino acid residues, distorting the 3D shape and causing an irreversible change in the proteins tertiary structure.

Enzymes are particularly sensitive to changes in pH because of the great sensitivity of their active site. Even if a slight change in pH is not enough to denature the molecule, it may upset the delicate chemical arrangement at the active site and so stop the enzyme working.

Substrate concentration

The rate of an enzyme-controlled reaction increases as the substrate concentration increases, until the enzyme is working at full capacity. At this point, the enzyme molecules reach their turnover number and assuming that all other conditions such as temperature are ideal, the only way to increase the speed of the reaction even more is to add more enzyme.

Enzyme concentration

In any reaction catalyzed by an enzyme, the number of enzyme molecules present is very much smaller than the number of substrate molecules. When an abundant supply of substrate is available, the rate of reaction is limited by the number of enzyme molecules present. In this situation, increasing the enzyme concentration increases the rate of reaction.

Inhibitor Inhibitors slow down or stop enzyme reaction. Usually,

enzyme inhibition is a natural process, a means of switching enzymes on or off when necessary.

Inhibition can be reversible and the enzyme returns to full activity once the inhibitor is removed. Drugs and poisons can inhibit particular enzymes, this type of inhibition is often non-reversible.

Reversible inhibitors are either competitive or non-competitive.

Competitive inhibitors

Compete with normal substrate molecules to occupy the active site. A competitive inhibitor fits into the active site of the enzyme preventing the real substrate from gaining access. The inhibitor cannot be converted to the products of the reaction and so the overall rate of reaction is slowed down.

Fig: Competitive inhibitors bind reversibly to the enzyme, preventing the binding of substrate. On the other hand, binding of substrate prevents binding of the inhibitor. Substrate and inhibitor

compete for the enzyme.

Non-competitive inhibitors

Non-competitive inhibitors bind to the enzyme away from the active site but change the overall shape of the molecule, modifying the active site so that it can no longer turn substrate molecules into product. Non-competitive inhibition has this name because there is no competition for the active site.

Examples • Cyanide combines with the Iron in the enzymes cytochrome oxidase.• Heavy metals, Ag or Hg, combine with –SH groups. These can be removed by using a chelating agent such as EDTA.

Irreversible inhibitorsIrreversible inhibitors bind permanently to

the enzyme, rendering it useless. For example, cyanide is an irreversible inhibitor.

Examples: nerve gases and pesticides, containing organophosphorus, combine with serine residues in the enzyme acetylcholine esterase.

Chemical reactions

• Chemical reactions need an initial input of energy = THE ACTIVATION ENERGY

• During this part of the reaction the molecules are said to be in a transition state.

Reaction Pathway

Enzyme controlled pathway

Enzyme controlled reactions proceed 108 to 1011 times faster than corresponding non-enzymatic reactions.