The formation of an enzyme-substrate complex increases the possibility for chemical reaction by

1. Lowering the energy of activation, and

2. Reducing the element of chance in the collisions of molecules or ions.

The formation of enzyme-substrate complexes is confined to relatively small areas of the enzyme molecule known as active sites, the precision with which an enzyme binds its substrate led to a rather stylized concept of the enzyme substrate complex which is based on the analogy of a lock and key. According to this hypothesis, the enzyme can only combine with a substrate which like the key, has the right shape to fit into the appropriate hole, or lock, on the enzyme surface.

It is now clear that the lock-and-key hypothesis provided too restrictive a view of the interactions between enzyme and substrates, for it has been observed that some enzymes appear to undergo small structural modifications when they form a complex with the substrate. Such observations led Koshland to propose the induced-fit hypothesis, which requires that combination of the substrate with the enzyme cause some distortion of the enzyme structure. Thus only certain types of substrate molecules would be able to establish a close fit with a given type of enzyme molecule.

Because hundreds of reactions are simultaneously carried out in the living cell, it becomes difficult to study a single reaction in an intact living cell (in vivo). However, it is possible to extract enzymes from cells and thus study enzyme-controlled reactions in a test tube (in vitro).

A single enzyme molecule, even though it can react over and over, is only capable of combining with a certain maximum number of substrate molecules per minute. This number, known as the turnover number, varies with the different kinds of enzymes. The enzyme carbonic anhydrase, required for excretion of CO₂ from the body has the highest turnover number of any known enzyme: the formation of 36 million molecules of H₂CO₃ per minute by one molecule of enzyme.

The protein component of enzymes is termed the apoenzyme. The non-protein portion found in some enzymes and which frequently can be separated from the apoenzyme, is called the coenzyme. Most vitamins (e.g., thiamin and niacin) have a coenzyme function in cells. Other coenzymes include nicotinamide adenine dinucleotide (NAD+), flavin adenine dinucleotide (FAD), and coenzyme A. There are many metallo-proteins in which the metal ion (e.g., Cu++, Mg++, Zn++) is bonded either to the apoenzyme or to the coenzyme. The metal ion is usually designated as the enzyme cofactor.

Most organisms have a preferred temperature range in which they survive, and their enzymes most likely function best within that temperature range. If the environment of the enzyme is too acidic or too basic, the enzyme may irreversibly denature, or unravel, until it no longer has the shape necessary for proper functioning.

Catalase and Peroxidase

Enzymes can be conveniently categorized according to the kind of chemical reaction they can catalyze. One such group are the Oxidoreductases, which are enzymes that catalyze oxidation-reduction reactions and two important subclasses include:

a. Dehydrogenases (reductases) which catalyze the removal (dehydrogenation) of hydrogen atoms (or electrons) from one substrate and their transfer to another substrate (which cannot be molecular oxygen).

AH₂ + B =dehydrogenaseÞ A + H₂O

Catalase and peroxidase belong in the oxidase subgroup.