This statement states that catalysts do not initiate chemical reactions but only speed up reactions already proceeding at a slow rate.
For example, hydrogen peroxide gas undergoes a very slow spontaneous decomposition at room temperature with the formation of water and oxygen.
The addition of a little part of platinum or of an enzyme called “catalase” enormously increases the rate of this decomposition. It, thus, may be assumed that the reaction is actually proceeding but at an immeasurably slow rate. From practical stand point, however, the presence of a catalyst or enzyme in this reaction leads to the formation of products which would not be found in its absence.
Specificity of enzymes:
Enzymes are amazingly specific in their action. The degree of specificity varies with respect to the types of substrates and chemical reactions which the enzymes catalyze.
Some enzymes are so highly specific that they catalyze only one chemical reaction involving a particular reactant or substrate and therefore, exhibit a absolute specificity; others appear to catalyze a number of related reactions involving a wider range of reactants or substrates and exhibit relative specificity.
Proteolytic enzymes hydrolyze the peptide bonds of polypeptided and, therefore, exhibit relative specificity.
Likewise some esterases such as lipases act upon the esters of different fatty acids with a variety of alcohols and split the ester bonds.
Nevertheless, the esterases are specific in their esterase action; they do not catalyze other hydrolytic reactions, nor do they function as oxidases, decarboxylases, etc.
Examples of absolute specificity are more restricted: urease acts only on urea; carbonic anhydrase acts only on carbonic acid; and fumarase acts only on fumeric acid.
In addition, many enzymes exhibit a high degree of stereochemical specificity, e.g., arginase acts only on L-arginine, not on the ? D-isomer. Likewise glucose oxidase acts only on the anomer of glucose, not on the ?-D-anomer.
The specific combination that occurs between enzyme and substrate to form the enzyme-substrate complex is believed to reflect a particularly suitable spatial relationship between the substrate and certain active sites on the enzymes analogous to a lock and key or jigsaw arrangement.
It states that the structural configuration of the enzyme can only accomodate a particular type of substrate, as a key fits into its lock.
According to induced fit hypothesis proposed by Kashland that some enzymes have “flexible” active sites because of which the protein may not have a proper proximity of reactive groups unit the substrate binds to the enzyme.
Only the appropriate substrate can cause the precise alignment of calalytic groups needed for enzyme action.