Enzymes are catalysts that, within the mild conditions of temperature, pH, and pressure of the cells, carry out chemical reactions at amazing high rate. They are characterized by a remarkable efficiency and specificity.

Substrates are the substances on which enzymes act.

Enzymes are named by adding the suffix -ase to the name of the substrate that they modify (i.e., urease and tyrosinase), or the type of reaction they catalyze (dehydrogenase, decarboxylase). Some have arbitrary names (pepsin and trypsin). The International Union of Biochemistry and Molecular Biology assigns each enzyme a name and a number to identify them.

Enzymes are classified into six categories according to the type of reaction catalyzed:

Oxidoreductases, transferases, hydrolases, lyases, ligases, and isomerases.

Structurally, the vast majority of enzymes are proteins. Also RNA molecules have catalytic activity (ribozymes).

Coenzymes are small nonprotein molecules that are associated to some enzymes. Many coenzymes are related to vitamins. Coenzymes and the protein portion with catalytic activity or apoenzyme form the holoenzyme. The apoenzyme is responsible for the enzyme’s substrate specificity. Coenzymes undergo changes to compensate for the transformations occurring in the substrate.

Metalloenzymes are enzymes that contain metal ions.

The mechanism of action of enzymes depends on the ability of enzymes to accelerate the reaction rate by decreasing the activation energy. During the course of the reaction, the enzyme (E) binds to the substrate/s (S) and forms a transient enzyme–substrate complex (ES). At the end of the reaction, the product/s are formed, the enzyme remains unchanged, can bind another substrate and can be reused many times.

Active site or catalytic site is the specific place in the enzyme where the substrate binds. The structural complementarity between E and S allows an exact reciprocal fit. The enzyme adapts to the substrate via a conformational change known as induced fit. The presence in the active site of amino acids that bind functional groups in the substrate ensures adequate location of the substrate and formation of the transition intermediary, which will be subjected to catalysis.

Zymogens or proenzymes are inactive precursors of enzymes. They acquire activity after hydrolysis of a portion of their molecule.

Cellular location of enzymes varies, the majority being in different compartments of the cell, while others are extracellular.

Multienzyme systems are those composed of a series of enzymes or enzyme complexes. There are also multifunctional enzymes with several different catalytic sites in the same molecule.

Enzyme activity is determined by measuring the amount of product formed, or substrate consumed in a reaction in a given time. Initial velocity corresponds to the activity measured when the amount of consumed substrate is less than 20% of the total substrate originally present. One IU of enzyme catalyzes the conversion of 1 μmol of substrate per second under defined conditions of pH and temperature. Specific activity is the units of enzyme per milligram of protein present in the sample. Molar activity or turnover number are the substrate molecules converted into product per unit time per enzyme molecule, under conditions of substrate saturation.

The rate of the enzymatic reaction is directly proportional to the amount of enzyme rate present in the sample.

Also, at low [S] and under constant conditions of the medium, enzyme activity rapidly increases with the raise in [S]. At higher substrate levels, the activity increases slowly and tends to reach a maximum. The effect follows a hyperbolic function; at low [S] the reaction is first order; at high [S] the reaction is zero order with respect to the substrate.

Km or Michaelis constant is the [S] at which the reaction rate reaches a value equal to half the maximum.

Under given conditions of pH and temperature, the Km value is distinctive for each enzyme and is used to characterize it. For most enzymes, the Km value is inversely related to the affinity of the enzyme for the substrate, the higher the affinity, the lower the Km.

Temperature affects enzyme activity, increasing it to reach a peak, which corresponds to the optimal enzyme activity. Beyond this maximum, enzyme activity rapidly drops. The optimal temperature for most mammalian enzymes is around 37°C. The inactivating effect of temperatures above 40°C is due to protein denaturation.

pH affects enzyme activity, by influencing the state of dissociation of functional groups involved in the ES complex. Enzymes have an optimum pH and extreme values of pH cause enzyme denaturation.

Enzyme inhibitors can be classified as:

Irreversible, which permanently inactivate the enzyme, and

Reversible, which consist of the following inhibitors:

Competitive: increase the Km but not the Vmax, its action is reversed by increasing [S]. Some have structural similarity to the substrate and compete with it for the active site.

Noncompetitive: bind to the enzyme in a site different to the catalytic center. They decrease Vmax, leave Km unaffected, and are not influenced by [S].

Anticompetitive: reduce Km and Vmax.

Enzymes are subjected to regulation, to adapt to the requirements of different cells. When the [S] in the cell is below the Km, changes in [S] modify the activity. Allosteric enzymes are those modulated by agents that bind to them at a site different to the active center. The curve of initial velocity versus [S] for allosteric enzymes is not hyperbolic, but sigmoid. Enzyme activity is also changed by covalent modification, such as phosphorylation.

Constitutive enzymes are those whose levels remain constant throughout the life of the cell. Inducible enzymes, are those whose synthesis is activated as required.

Isozymes are different proteins that have the same enzyme activity.