Our knowledge about the mechanisms involved in the enzyme catalysis is very little. However, the view that has dominated all attempts to explain mechanisms of enzyme-action is that the enzyme forms an intermediate complex with the substrate or substrates.
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Before proceeding to a more detailed examination of this concept, it is desirable to summarize briefly some of the early experiments that led to this view.
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1. In 1880, Wurtz observed that, after addition of the soluble proteinase papain to the insoluble protein, fibrin, repeated washing of the fibrin did not stop the proteolysis.
He then concluded that the papain had formed a substance or complex with the fibrin.
2. In 1890, O’Sullivan and Thompson observed that the enzyme invertase could withstand higher temperatures in the presence of the substrate, sucrose, than in its absence.
They also concluded the Wurtz’s observation that the invertase had formed a complex with its substrate, sucrose.
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3. In 1890, Emil Fischer, after conducting so many experiments on different enzymes, quoted the following remark for the enzymes that “inasmuch as the enzymes are in all probability proteins it is probable that their molecules also have an asymmetrical structure, and one whose asymmetry is, on the whole, comparable to that of the hexoses.
Only if enzyme and fermentable substance have a similar geometrical shape can the two molecules approach each other close enough for the production of a chemical reaction. Metaphorically, we may say that enzyme and glucoside must fit into each other like lock and key.
Keeping all these observations in knowledge, Fischer, in 1894 suggested the “lock and key” hypothesis to explain that how do enzymes perform their activity.
This hypothesis is still acceptable but in a modified way as “induced fit” hypothesis. These theories suppose the existence of active-sites on the surface of the enzyme molecules.
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According to this hypothesis the substrate molecules are thought to fit into the active sites located on the surface of the enzyme molecules just as one particular kind of key fits into one particular kind of lock.
This results in the rapid formation of intermediate compounds the so called enzyme-substrate complexes by reversible reaction enzyme + substrate±?enzyme-substrate complex.
These intermediate compounds are believed to be much less stable than the original substrate and so they break down spontaneously, the enzyme being again liberated.
Recently in the year of 1958 Fruton and Simmonde have also demonstrated that the enzyme substrate complex is an essential first step in enzyme action.
According to some workers the amino-acid residues comprising the active site of the enzyme are remained to be in direct contact with substrate molecule.
Therefore, the size, shape and charge of the possible substrate are defined by the size, shape and charge of the active site, and it is these factors which determine the specificity of the enzyme towards certain substances only.
Moreover there are three groups or constellations of amino-acids (perhaps more in some instances) on the enzyme which are actually involved and cause chemical changes.
One group is responsible chiefly for binding the substrate to the enzyme, the second combining with another part of the substrate so as to reduce its stability and makes it more susceptible to reaction, while the third is responsible for specificity.
This view has been developed particularly in connection with peptidases, esterases and other transferases.