ENZYMES

BYDR. MARYJANE

INTRODUCTION

• Enzymes are biological, organic catalysts produced by the body.

• Catalysts are substances that increase the speed of a chemical reaction.

• Although a catalyst influences a chemical reaction, it is not itself permanently changed, nor does it cause the reaction to occur.

Enzymes• Most enzymes are proteins and so can be denatured by

heat, strong acids etc• They contain a protein and non protein part.• Both parts must be present before the enzyme can

function.

NOMENCLATURE

• Most commonly used enzyme names have the suffix “-ase” attached to the substrate of the reaction. E.g., glucosidase and urease

CLASSIFICATION OF ENZYMES• Oxidoreductases: catalyze oxidation-reduction reactions e.g.,

oxidases, dehydrogenases • Transferases: catalyzes the transfer of C-, N-, or P- containing

groups e.g., kinases, phosphotransferases• Hydrolases: catalyze cleavage of bonds by addition of water

e.g., peptidases• Lyases: catalyze cleavage of C-C, C-S and certain C-N bonds

e.g., decarboxylases• Isomerases: this group of enzymes catalyzes the

interconversion of one isomer into the other. E.g., isomerases, epimerases

• Ligases: catalyze formation of bonds between carbon and O, S, N coupled hydrolysis of high energy phosphates

ENZYME TERMINOLOGY• Kinase: uses ATP to add phosphate group onto

substrate e.g PFK• Phosphorylase: add high energy phosphates (pi)

onto substrate without using ATP• Phosphatase: removes phosphate group from the

substrate e.g., fructose-1,6-bisphosphatase, alkaline phosphatase

• Dehyrogenase: oxidize and reduce substrate e.g., pyruvate dehydrogenase

• Carboxylase: adds 1 carbon with the help of biotin e.g., pyruvate carboxylase

DEFINITION OF CERTAIN TERMS

• Rate of chemical reaction: it is the change in the amount of starting material (substrate or reactant) into products per unit time.

• Substrate: it is the substance upon which enzyme acts OR is the reactant in an enzyme catalyzed reaction

• Activation energy: is the energy that must be overcome for a chemical reaction to occur.

• Enzyme provide a chemical pathway that has a lower activation energy than that without the enzyme.

•

PROPERTIES OF ENZYMES

• A. Active sites: enzyme molecules contain a special pocket or cleft called the active site. The enzyme binds the substrate forming an enzyme-substrate (ES) complex.

• Binding is thought to cause a conformational change in the enzyme.

• ES is converted to an enzyme-product (EP) complex that subsequently dissociates to enzyme and product

• B. Catalytic efficiency: enzyme catalyzed reactions are highly efficient proceeding from 10³-10⁸ times faster than uncatalyzed reactions.

• C. Specificity: enzymes are highly specific, interacting with one or a few substrate and catalyzing only one type of chemical reaction.

• D. Holoenzymes: refers to the active enzyme with its nonprotein component, whereas the enzyme without its nonprotein moiety is termed apoenzyme and is inactive.

• If the nonprotein moiety is a metal ion such as Zn²⁺ or Fe²⁺ , it is called a cofactor.

• If it is a small organic molecule, it is termed a coenzyme. Coenzymes are frequently derived from vitamins.e .g., NAD⁺ contains niacin and FAD contains riboflavin, biotin

• E. regulation: enzyme activity can be regulated, that is, increased or decreased in response to the need of the cell.

Regulation

• Temperature: ↑ temp,↓ rate of enzyme catalyzed reaction

• pH: extremes of pH, ↓ rate of enzyme catalyzed reaction

• Effects of substrate concentration: ↑substrate conc, ↑ rate of enzyme catalyzed reaction

THEORIES OF ENZYME ACTION

• LOCK AND KEY THEORY:• According to this theory, the substrate must

‘’FIT” into the active site of the enzyme.

INDUCED FIT MODEL

• A more recent version of the activity of an enzyme ..the induced fit model suggests that the active site is not rigid , as in the lock and key model but flexible.

• That is the site changes in conformation upon binding to a substrate in order to yield an enzyme-substrate fit.

Activators and inhibitors• Inorganic substances that tend to increase the activity of

an enzyme are called activators.• An enzyme inhibitor is any substance that will make an

enzyme less active or render it inactive.• Inhibitor can be irreversible and reversible• Irreversible inhibitors bind to the enzyme active site

through covalent bonds• Reversible inhibitors : binds to the enzyme active site

through noncovalent bonds and can be competitive or noncompetitive

• Enzyme inhibitors that bind reversibly to the active site are called reversible inhibitors and some block access by the substrate are called competitive inhibitors.

Activators and inhibitors

• The effect of this competitive inhibitor can be overcome by increasing the concentration of the substrate.

• Other inhibitors that bind to another site on the enzyme to render it less active or inactive are called non competitive inhibitors.

Isoenzymes• Also known as isozymes.• The reason for their existence is not known but they are

used clinically in diagnoses of clinical conditions.• Lactate dehydrogenase (LDH), creatine kinase (CK), occur

in isoenzyme form and are of diagnostic value in myocardial infarction.

• Creatine kinase can exist in 3 forms:• CK-BB – found in the brain• CK-MM- found in skeletal muscle• CK-MB- found in cardiac muscle.• AST and ALT are elevated in liver disorders. • Alkaline phosphatase is elevated in bone disorders and

liver disorders.