Chapter 5 Enzymes

Enzymes as organic catalysts Enzyme is for lowering of activation energy.Enzyme is for lowering of activation energy.

Energy level of substrate particles No. of substrate particles

Energy level of substrate particles No. of substrate particles E E’ E: Activation energy when enzyme is not added. E’:Activation energy when enzyme is added.

Lock and key hypothesis Enzyme Active site substrate Enzyme-substrate complex Products

Induced fit model The active site is induced by the substrate to change its shape to fit the shape of the substrate.

Enzyme can catalyze both forward and backward reactions. It does not alter the equilibrium position of a biochemical reaction. The normal progress of a biochemical reaction is due to the immediate removal of the end product.

Specificity The degree of specificity varies from one enzyme to another. Intracellular enzyme works on one particular substrate. Extracellular enzyme works on a range of related substrates.

Factors affecting enzymatic activity

Temperature affects the energy content and hence the mobility of the enzyme and substrate molecules. Higher temperature enables enzyme and substrate molecules to collide more frequently and therefore facilitates the substrate molecule to bind with the active site of the enzyme. If the temperature is too high (above optimum), the polypeptide chains made up the enzyme will vibrate too much so that the structure will be disrupted, and the enzyme is said to be denatured.

Temperature

Temperature coefficient Q 10 Q 10 = Reaction rate at (X+10) o C Reaction rate at X o C When the temperature is below the optimum temperature, Q 10 usually is 2. When the temperature is above the optimum temperature, Q 10 usually is less than 1.

Energy level of substrate particles No. of substrate particles T1T2 T2>T1

pH Changes in pH alter the ionic charge on the enzyme surface, thus causing change in shape of its active site, thus diminishing its catalytic activity. If extremes of pH is encountered, the enzyme is denatured and loses its catalytic function.

Reversible inhibitors: The effect is temporary. The inhibiting effect can be eliminated if the reversible inhibitor is removed. Irreversible inhibitors: This kind of inhibitor disrupts the bonds of the polypeptides which form the enzyme. Therefore the shape of the active site is altered permanently and loses its catalytic properties.

Reversible inhibitor(I): Competitive Inhibitor Normal substrate Competitive Inhibitor Enzyme

Usually the end product of a biochemical reaction acts as competitive inhibitor. This is known as end-product inhibition. Immediate removal of the end product facilitates enzyme reaction.

Non-competitive Inhibitor Normal substrate Allosteric site The shape of the enzyme changes so that the normal substrate cannot bind to it.

Example of competitive inhibitor Sulphonamides have very similar molecular shape to amino­benzoic acid, a compound essential for growth of many pathogenic bacteria. Therefore, sulphonamide acts as competitive inhibitor to check the growth of many bacteria in treatment of many diseases caused by bacteria.

Example of non-competitive inhibitor Diisopropylflurophosphate (DFP), a nerve gas used in warfare is a non ‑ competitive inhibitor. DFP acts on enzyme cholinesterase which is involved in synaptic transmission. DFP causes prolonged muscle contraction and death is the end result.

Enzyme concentration and substrate concentration affects the concentration of the enzyme-substrate complex and hence the rate of enzyme-catalysed biochemical reaction.

Enzyme cofactor It is non-protein substance which makes some enzyme to function more efficiently.

Enzyme activator It is an inorganic ion which loosely bound to an enzyme or an substrate to increase the chance to an enzyme-substrate complex forming. Some times, the inorganic ions are free and does not bind to the enzyme or substrate, e.g. copper, iron, calcium, Cl - etc.

Coenzyme It is an organic non-protein molecule firmly associated with the enzyme and is essential for enzyme activity. It acts a carrier for transferring chemical groups or atoms from one enzyme to another. Many coenzymes are synthesized from vitamins. e.g. NAD

Prosthetic group It is an organic non-protein group which is tightly bound to the enzyme or as an integral part with the enzyme. It can act as carriers of atoms or electrons while transferring one compound to another in an overall metabolic pathway. e.g. FAD, haemoglobin etc.

Application of enzymes Some washing powders are biologically active. They contain enzyme protease to digest the protein dirt. Meat tenderizer contains enzyme protease which break down and soften the texture of meat.