I. Chemical Reactions need Energy to get started. A.That Energy is calls Activation Energy. Ex. Match: the energy from the friction of the match head on the match box is the activation energy. B.Chemical Reaction: Match Burning C.Heating a reaction generates activation energy. D.Enzymes speed up reactions in living things. E.How long would it take to digest your food without enzymes?

II. Characteristics of Enzymes 1. Enzymes are proteins that speed up reactions by reducing the activation energy. 2. Enzymes are Specific. They act on specific Substrates: Ex. Lactase acts on Lactose. 3. Enzymes are not used up during reactions.

reactants products energyenergy Intermediate – formed between enzyme and one or more reactant molecules Uncatalysed reaction Enzyme-catalysed reaction exergonic reaction Reaction profiles: uncatalysed and enzyme-catalysed Click to see how an enzyme is involved in an enzyme-catalysed reaction Course of reaction

How Enzymes Work

1. Lock and key hypothesis This is the simplest model to represent how an enzyme works. The substrate simply fits into the active site to form a reaction intermediate.

2. Induced fit hypothesis In this model the enzyme molecule changes shape as the substrate molecules gets close. The change in shape is 'induced' by the approaching substrate molecule.

Factors affecting catalytic activity of enzymes

Temperature As the temperature rises, reacting molecules have more and more kinetic energy. This increases the chances of a successful collision and so the rate increases. There is a certain temperature at which an enzyme's catalytic activity is at its greatest (see graph). This optimal temperature is usually around human body temperature (37 o C) for the enzymes in human cells. Above this temperature the enzyme structure begins to break down (denature).

Temperature Above this temperature the enzyme structure begins to break down (denature).

pH Each enzyme works within quite a small pH range. There is a pH at which its activity is greatest (the optimal pH). Ex. Pepsinase works best in your stomach (pH: 2)

Concentration of enzyme and substrate In General: The rate of an enzyme- catalysed reaction depends on the concentrations of enzyme and substrate. As the concentration of either is increased the rate of reaction increases (see graphs).

Concentration of enzyme and substrate Point of Saturation: This is because the active sites of the enzyme molecules at any given moment are virtually saturated with substrate. All of the active sites are full.

Concentration of enzyme and substrate Enzyme Concentration Provided that the substrate concentration is high and that temperature and pH are kept constant, the rate of reaction is proportional to the enzyme concentration. (See graph)

Inhibition of enzyme activity Some substances reduce or even stop the catalytic activity of enzymes in biochemical reactions. They block or distort the active site. These chemicals are called inhibitors, because they inhibit reaction. Competitive Inhibitors that occupy the active site and prevent a substrate molecule from binding to the enzyme. Non-Competitive Inhibitors that attach to other parts of the enzyme molecule, perhaps distorting its shape, so it doesn’t work anymore.

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