Controlling and regulating biochemical pathways


Hint - use SAC 1 for Outcome 2 as revision

Enzymes- a specific protein that acts as a catalyst to increase that rate of a particular chemical reaction in living organisms - enzymes are protein catalysts - They increase that rate of reactions between substrates without being consumed in the reactions - They reduce the amount of energy required to begin the reactions they catalyze - Without enzymes, metabolism would be so slow at body temp that insufficient energy would be available to maintain life.

STRUCTURE OF ENZYMES: Enzymes are made up of proteins (amino acids). Many enzymes are pure proteins, folded so that they have a specific active site. Many enzymes require the presence of other factors as well as the protein part before they act. These non-protein parts are called cofactors and include- metallic ions such as iron, calcium, copper, zinc, potassium and magnesium. If the cofactor is an organic molecule, such as a vitamin, it is called a coenzyme.

There are as many enzymes in living organisms as there are types of chemical reactions. They are divided into two broad groups: - Intracellular enzymes- occurs inside cells, where they speed up and control metabolic reactions - Extracellular enzymes- are produces by cells but act as catalysts out side the cell; they include your digestive enzymes which break down food in the small intestine.

ENZYMES AND THEIR SUBSTRATES: The compound acted on by an enzyme is called a substrate. The enzymes are large globular protein molecules. As part of the enzymes tertiary configuration there is a characteristic group of amino acids (as part of a polypeptide chain) that form a reactive surface. This shape of the enzyme is at a region known as the active site and fits with apart of the substrate molecule. This complementary fitting of shapes is often called the ‘lock-and-key’ theory of enzyme actions. Intermolecular bonding interactions between the protein enzyme and the substrate can modify the shape of the active sire to ensure that the substrate if fully accommodated, this is called ‘induced-fit model of enzyme action’.


Temperature- High temps (fast reaction, gain more kinetic energy)- An increase in temperature above a human optimum temp of about 37C would change the shape of the protein molecule. The denaturation would cause the tertiary structure of the enzyme to change, and the active site destroyed. If the tertiary structure has been denatured (tertiary structure) then that enzyme cannot bind to specific substrate (cannot ‘lock-and-key’ or ‘induce fit&rsquoWink. When exposed to high temps the enzymes are permanently changed and they remain inactive even when the temp returns to normal.

Low temps (slow reaction) - At lower temps, the enzyme is reacting with the substrate, just a lot slower. Enzymes that is inactive or slower because of low temperatures become active again when temperatures are returned to normal.

pH (Hydrogen ions)- enzymes are sensitive to pH. Every enzyme has its own range of pH in which it functions best. Most intracellular enzymes function optimally around neutral (ph 7). Excessive acidity (less than pH 7) or alkalinity (more that pH 7) denatures them and makes them inactive. Digestive enzymes behave differently. Pepsin functions more effectively in an acidic environment (stomach), while trypsin functions effectively in an alkaline environment (duodenum).

Enzyme Concentration - only a very small number of enzyme molecules are usually involved in a reaction and these produce a given amount of product per unit time. If the amount of enzyme is increased, the amount of product made per unit time increases.

Substrate Concentration- the addition of more substrate to an enzyme solution will initially increase the rate of the reactions if not all active sites of the enzyme present are occupied. However, the enzyme solution contains a set amount of enzyme, and if no more is added, the rate of the reaction tapers off as all the active sites of the enzyme molecules become occupied

Inhibition- Other molecules may compete with the normal substrate for the active site of an enzyme. This other compound may combine permanently with the active site of this enzyme, interfering with the normal substrate-enzyme reactions and inhibiting the formation of the normal product.

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