ENZYMES

IMPORTANCE OF ENZYMES

  • All biochemical reactions are enzyme catalyzed in the living organism

  • Enzymes play an important role in metabolism, diagnosis, and therapeutics.

DEFINITION

Enzymes - are biological catalysts

  • They catalyze nearly all the chemical reactions taking place in the cells

    of the body.

  • They are proteins that increase the rate of reaction by looking for an

    alternative pathway that has a low energy of activation

CHARACTERISTICS

  • They are not altered or consumed during reaction.

  • Reusable

  • Enzymes show specificity to the reaction they control

  • Enzymes are sensitive to their environment so they can be controlled

    by adjusting the temperature, the pH or the substrate concentration

STRUCTURE OF ENZYMES

TERMS TO UNDERSTAND BIOCHEMICAL NATURE OF ENZYMES

Active Site - The area on the enzyme where the substrate or

substrates are attached to.

Substrate – the reactant in the chemical reaction

Apoenzyme – the enzyme without its non-protein moiety is termed as

apoenzyme and it is inactive.

Holoenzyme – the active enzyme with its non-protein component.

Cofactor - is a non-protein chemical compound that is bound (either

tightly or loosely) to an enzyme and is required for catalysis.\

Coenzymes - large organic molecule, loosely bound to apoenzyme

Prosthetic groups - Could be lipids metals or carbohydrates, usually tightly bound to apoenzyme

NAMING ENZYMES

  • The name of an enzyme in many cases end in –ase

    • For example, sucrase catalyzes the hydrolysis of sucrose

  • The name describes the function of the enzyme

    • For example, oxidases catalyze oxidation reactions

  • Sometimes common names are used, particularly for the digestion

    enzymes

    • For example, pepsin and trypsin

  • Some names describe both the substrate and the function

    • For example, alcohol dehydrogenase oxides ethanol

CLASSIFICATIONS OF ENZYMES

Enzymes Are Classified into six functional Classes (EC number Classification) by the International Union of Biochemists (I.U.B.) on the Basis of the Types of Reactions that they catalyze

Oxidoreductases - Catalyze oxidation-reduction reactions

Transferases - Catalyze group transfer reactions

Hydrolases - Catalyze hydrolysis reactions where water is the acceptor of the transferred group

Lyases - Cleave various bonds by means other than hydrolysis and oxidation.

Isomerase - Catalyze isomerization reactions

Ligases - Join two molecules with covalent bonds , Catalyze reactions in which two chemical groups are joined (or ligated) with the use of energy from ATP.

Enzymatic Action

THE ACTIVATION ENERGY - Chemical reactions need an initial input of energy

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

● Biological systems are very sensitive to temperature changes.

● Enzymes can increase the rate of reactions without increasing the temperature.

● They do this by lowering the activation energy. They look for a new reaction pathway “a short cut”

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

Enzyme-Substrate Binding

  • Lock and Key model

    • the active site has a rigid shape

    • only substrates with the matching shape can fit

    • the substrate is a key that fits the lock of the active site

  • Induced Fit Model

    • the active site is flexible, not rigid. It can change its conformation.

    • the shapes of the enzyme, active site, and substrate adjust to maximize the fit, which improves catalysis

    • there is a greater range of substrate specificity

    • this model is more consistent with a wider range of enzymes

Enzyme Specificity

Absolute -Catalyze only one reaction for single substrates

Group - Catalyze only one reaction for substrates with similar functional groups

Linkage - One type of reaction for a specific type of bond

Stereochemical - act on a particular stereoisomer. Chirality is inherent in an enzyme active site

Factors Affecting Enzyme Activity

  • Environmental Conditions

    • Temperature

    • pH

    • Substrate concentration

    • Enzyme Concentration

  • Cofactors and Coenzymes

  • Allosteric regulation

    • Positive regulation

    • Negative regulation

  • Enzyme Inhibitors

Effect of Temperature

  • Temperature increases enzyme activity up to its optimum temperature

  • Optimum temperature is the temperature at which enzymatic reaction

    occur fastest

  • For most enzymes the optimum temperature is about 40°C

  • Many are a lot lower, cold water fish will die at 30°C because their

    enzymes denature

  • A few bacteria have enzymes that can withstand very high temperatures

    up to 100°C

  • Most enzymes however are fully denatured at 70°C

Effect of pH

  • If pH slightly different from the enzyme’s optimum value, there will be small changes in the charges of the enzyme and it’s substrate’s molecule.

  • This change in ionization will affect the binding of the substrate with the active site.

  • different optimum pH where enzyme activity is at its highest

  • Extreme changes in pH can cause denaturation