ENZYME Enzymes as organic catalysts. Factor affecting enzyme activity.

Enzymes as organic catalysts The lowering of activation energy. Mechanism of enzyme action: lock and key hypothesis. The properties of enzymes.

The lowering of activation energy Enzymes can accelerate reaction rate by lowering activation energy of the substrate. The amount of energy needs to bring one mole of a substance to the activated state at a given temperature termed activation energy.

Lock and key hypothesis The enzyme molecule has a specific region termed active site to combine with the substrate molecules, , substrate molecules which have a shape fitting the active site bind to the enzyme (seem a lock and key) in the enzyme-substrate complex. The substrate is activated and reactions occur readily between substrate molecules changing them into the product molecule which no longer fit into the active site and escape into the surrounded medium.

The properties of enzymes Enzymes keep unchanged after reactions. Catalytic function of enzymes Specific active site of enzyme for specific substrate molecule to combine with.

The model of lock and key hypothesis

Factor affecting enzyme activity Temperature pH Substrate concentration Enzyme concentration The presence of cofactor The presence of inhibitor

Factors affecting enzymatic activity

Temperature At high temperature, heat can denature the protein structure of enzyme and then change the active site , hence it will prevent the substrate from binding on the enzyme and decrease the reaction rate. And the low temperature will curb the activity of the enzyme.

The reaction rate vs the temperature

pH value Extreme pH change can affect the ionic state of functional groups within the active site.

The enzyme activity in different pH value

Substrate concentrate Increasing the substrate concentration at the suitable enzyme concentration, the reaction rate will be increased because of more enzyme-substrate complex formed. Over the saturation point, further increasing the substrate concentration has no effect on the reaction rate. The merely way to increase the reaction rate in this time is to raise the enzyme concentration.

The rate of reaction vs the substrate concentration

Enzyme concentration Increasing the enzyme concentration at the suitable substrate concentration, the reaction rate will be increased because of more enzyme-substrate complex formed. Over the saturation point, further increasing the enzyme concentration has no effect on the reaction rate. The merely way to increase the reaction rate in this time is to raise the substrate concentration.

Cofactor 1 The non-protein component of an enzyme which present for enzyme activity. The enzyme-cofactor complex is termed holoenzyme. The enzyme portion without cofactor is termed apoenzyme.

Cofactor 2 The cofactors may be simple inorganic ions or complex organic molecules. The cofactors can keep unchanged or be regenerated at the end of the reaction. The are stable at relatively high temperature. There are several kinds of cofactors.

The kinds of cofactors Enzyme activator. Coenzyme. Prosthetic group.

Enzyme activator It is an inorganic ion which certainly bound to an enzyme or an substrate to increase the chance to an enzyme-substrate complex forming. e.g. copper, iron, calcium 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. ATP, NADP etc.

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

Inhibitor They are substances interact with an enzyme and lead to a loss in a catalytic activity of enzyme. There are reversible inhibitor and irreversible inhibitor.

Reversible inhibitor The activity of enzyme is restored when the inhibitor is removed. There are including competitive inhibitors and non-competitive inhibitors.

Competitive inhibitors Substance with a similar molecular structure as the substrate, competing with the substrate for the active site on the enzyme. The degree of inhibition rely on the relative concentration of substrate and inhibitor. Competitive inhibition can be overcome by increasing the concentration of substrate due to more chance to react the active site.

Non-competitive inhibitors The inhibitor has no structural similarity to the substrate, it forms an enzyme-inhibitor complex in the next part or site of the enzyme other than the active site.

Irreversible inhibitor The inhibitor permanently attaches to the active site excluding the substrate molecule. The degree of inhibition depends completely on the inhibitor concentration and cannot be reversed by increasing substrate concentration.

The biological importance of inhibitors They can affect the catalytic function of enzymes. They can be the end product in some metabolic pathways.