Q.Name two amino acids in which sulphur is present.

Enzymes are the biological catalysts that make the chemistry of life happen fast enough to sustain it. Almost every reaction inside a cell needs an initial input of activation energy before it can proceed, and at the mild temperatures found in a living organism very few reactant molecules naturally carry that much energy. An enzyme lowers the activation energy barrier for its particular reaction, so the reaction proceeds rapidly under the gentle conditions of temperature and pH that exist inside the body, while the enzyme itself emerges from the reaction chemically unchanged.

This gives enzymes a distinctive set of properties. They are highly specific, generally acting on only one substrate or a small family of closely related substrates, which lets a cell run many different reactions side by side without them interfering with one another. They are also reusable: because an enzyme is not consumed by the reaction it catalyses, a small quantity of enzyme can convert a very large quantity of substrate over time, simply by being used again and again. Enzyme activity is, however, sensitive to conditions - it depends strongly on temperature, pH, and the concentrations of substrate and any inhibitors or co-factors present. …