THEME:

STRUCTURE AND PROPERTIES OF ENZYMES. THE MECHANISM OF ENZYMES ACTION. CLASSIFICATION OF ENZYMES. ISOENZYMES.

DefinitionDefinition

Enzymes are protein catalysts for biochemical reactions in living cells

They are among the most remarkable biomolecules known because of their extraordinary specificity and catalytic power, which are far greater than those of man-made catalysts.

NamingNaming

The name enzyme (from Greek word "in yeast")was not used until 1877, but much earlier it was suspected that biological catalysts are involved in the fermentation of sugar to form alcohol (hence the earlier name "ferments").

Naming and Classification of Naming and Classification of EnzymesEnzymes

Many enzymes have been named by adding the suffix -ase to the name of the substrate, i.e., the molecule on which the enzyme exerts catalytic action.

For example, urease catalyzes hydrolysis of urea to ammonia and CO2, arginase catalyzes the hydrolysis of arginine to ornithine and urea, and phosphatase the hydrolysis of phosphate esters.

Classification of enzymesClassification of enzymes

Oxido-reductases (oxidation-reduction reaction).

Transferases (transfer of functional groups). Hydrolases (hydrolysis reaction). Lyases (addition to double bonds). Isomerases (izomerization reactions). Ligases (formation of bonds with ATP

cleavage).

The structure of enzymesThe structure of enzymes

Protein part + Non- protein part Apoenzyme + Cofactor = Holoenzyme

Function of apoenzyme: It is responsible for the reaction Function of cofactor: It is responsible for the bonds formation between enzyme

and substrate Transfer of functional groups Takes plase in the formation of tertiary structure of protein

part

CofactorCofactor

1. Prosthetic group (when cofactor is very tightly bound to the apoenzyme and has small size )

2. Metal ion3. Coenzyme(organic molecule derived

from the B vitamin which participate directly in enzymatic reactions)

Prosthetic groupProsthetic group

1. Heme group of cytochromes

2. Biothin group of acetyl-CoA carboxylase

Metal ionsMetal ionsFe - cytochrome oxidase, catalaseCu - cytochrome oxidase, catalaseZn - alcohol dehydrogenaseMg - hexokinase, glucose-6-phosphataseK, Mg - pyruvate kinaseNa, K – ATP-ase

CoenzymeCoenzyme B1 TPP- Thiamine Pyro Phosphate B2 FAD- Flavin Adenine Dinucleotide FMN- Flavin Mono Nucleotide Pantothenic acid Coenzyme A (CoA) B5 NAD – Nicotinamide Adenine Dinucleotide NADP- Nicotinamide Adenine Dinucleotide

Phosphate

Chemical KineticsChemical Kinetics

The Michaelis-Menten EquationThe Michaelis-Menten Equation

In 1913 a general theory of enzyme action and kinetics was developed by Leonor Michaelis and Maud Menten.

1. Point А.

2. Point В.

3. Point С.

Mechanism of enzyme reactionMechanism of enzyme reaction

1. Formation of enzyme – substrate complexE + S → ES2. Conversion of the substrate to the productES→ EP3. Release of the product from the enzymeEP → E+P

The Free Energy of The Free Energy of ActivationActivation

Before a chemical reaction can take place, the reactants must become activated.

This needs a certain amount of energy which is termed the energy of activation.

It is defined as the minimum amount of energy which is required of a molecule to take part in a reaction.

The Free Energy of The Free Energy of ActivationActivation

For example,decomposition of hydrogen peroxide without a catalyst has an energy activation about 18 000. When the enzyme catalase is added, it is less than 2000.

The Free Energy of The Free Energy of ActivationActivation

The rate of the reaction is proportional to the energy of activation:

Greater the energy of activationSlower will be the reactionWhile if the energy of activation is less,The reaction will be faster

Energy of ActivationEnergy of Activation

Effect of pH on Enzymatic Effect of pH on Enzymatic ActivityActivity

Most enzymes have a characteristic pH at which their activity is maximal (pH- optimum);

above or below this pH the activity declines. Although the pH-activity profiles of many enzymes are bell-shaped, they may be very considerably in form.

Effect of pH on Enzymatic Effect of pH on Enzymatic ActivityActivity

Effect of Temperature on Effect of Temperature on Enzymatic ReactionsEnzymatic Reactions

.The rate of enzyme catalysed reaction generally increases with temperature range in which the enzyme is stable. The rate of most enzymatic reactions doubles for each 100 C rise in temperature. This is true only up to about 500 C. Above this temperature, we observe heat inactivation of enzymes.

The optimum temperature of an enzyme is that temperature at which the greatest amount of substrate is changed in unit time.

Effect of Temperature on Effect of Temperature on Enzymatic ReactionsEnzymatic Reactions

Enzyme InhibitionEnzyme Inhibition1. Reversible inhibition

A. Competitive

B. Non-competitive

C. Uncompetitive

2. Irreversible inhibition

Competitive InhibitionCompetitive Inhibition

Usage competitive inhibition in Usage competitive inhibition in medicinemedicine

The antibacterial effects of sulfanilamides are also explained by their close resemblance to para-amino-benzoic acid which is a part of folic acid, an essential normal constituent of bacterial cells. The sulfanilamides inhibit the formation of folic acid by bacterial cells and thus the bacterial multiplication is prevented and they soon die.

Non-competitive InhibitionNon-competitive Inhibition

In this case, there is no structural resemblance between the inhibitor and the substrate. The inhibitor does not combine with the enzyme at its active site but combines at some other site.

E + S +I =ESI (INACTIVE COMPLEX)E + S = ESES + I = ESI

Uncompetitive inhibitionUncompetitive inhibition

E + S +I =ESI (No active complex)

Irreversible InhibitionIrreversible Inhibition

The inhibitor is covalently linked to the enzyme.

The example:Action of nerve gas poisons on

acetylcholinesterase,an enzyme that has an important role in the transmission of nerve impulse.

These are the enzymes from the same These are the enzymes from the same organism which catalyse the same reaction organism which catalyse the same reaction

but are chemically and physically distinct but are chemically and physically distinct from each other.from each other.

Isoenzymes

Lactate dehydrogenaseLactate dehydrogenase

It occurs in 5 possible forms in the blood serum:

LDH1

LDH2

LDH3

LDH4

LDH5

Structure of LDHStructure of LDH

Each contains 4 polypeptide chains which are of 2 types: A and B which are usually called M (muscle) and H (heart).

LDH1 –H H H H LDH2 – H H H M LDH3 – H H M M LDH4 – H M M M LDH5 – M M M M

Clinical importance of LDHClinical importance of LDH

Acute myocardial infarctionLDH1 and LDH2

Acute liver damageLDH4 and LDH5

Creatine kinaseCreatine kinase

It has 3 isoenzymes: CK1

CK2

CK3

Clinical importance: When patient have acute myocardial infarction CK

appears in the blood 4 to 8 hours after onset of infarction and reaches a peak in activity after 24 hours.

Enzyme-Activity UnitsEnzyme-Activity Units

The most widely used unit of enzyme activity is international unit defined as that amount which causes transformation of 1.0 mkmol of substrate per minute at 25°C under

The specific activity is the number of enzyme units per milligram of protein.

Enzyme-Activity UnitsEnzyme-Activity Units

The molar or molecular activity, is the number of substrate molecules transformed per minute by a single enzyme molecule

The katal (abbreviated kat), defined as the

amount of enzyme that transforms 1 mol of substrate per 1 sec.