Enzymes are specific proteins produced by living cells of the body that allow the course of thousands of chemical reactions with speed, efficiency and specificity difficult to achieve in artificial systems. They act as catalysts for the reaction. Because they speed up reactions at least a million times, in their absence transformation in the cell occurs so slowly that they are unnoticeable.
In terms of chemical structure, the enzymes are proteins and for the sake of construction we divide them into
1) Enzymes occurring as a simple protein, and thus built exclusively from polypeptide chains, e.g. pepsin, urease, amylases.
2) Enzymes that are complex proteins, and therefore have a non-protein part – a small molecule called a cofactor. We distinguish in this type of enzymes
–enzymes, which both protein and non-protein parts are permanently connected to each other. These non-protein molecules are called prosthetic groups. Strongly embedded prosthetic groups are sugars, phosphoric acid residues, nucleotides, metalloporphyrin compounds (ironoporphyrin system in catalase and peroxidase)
–enzymes that are complex proteins whose non-protein active groups are loosely bound to the enzyme protein. Both components can be reversibly separated – after their recombination, the enzyme activity returns. In this system, the non-protein part is called coenzyme. And the protein apoenzyme, and the whole after their combination with a holoenzyme (e.g., niacyl coenzymes like NAD and NADP).
The specificity of the enzymes (i.e., the ability to act on specific substrates and the ability to catalyze specific reactions) depends on the type and sequence of amino acids in the protein chain as well as on the spatial conformation of the polypeptide chain. In the protein part of the enzyme, a fragment of the polypeptide chain is isolated in which the proper act of catalase takes place – it is the active center. The active center is a fragment of the polypeptide chain (produced by amino acid residues) directly connecting the substrate during the reaction.
Mechanism of enzymes action
In the first stage of catalase, the compound undergoing transformation (the substrate) is combined with the enzyme via the active site, forming a transient, unstable enzyme-substrate complex. In the further part of the catalase process, the enzyme-substrate complex breaks down, accompanied by the formation of reaction products and the regeneration of the enzyme to its original form.
Due to the protein nature, the enzymes are very susceptible to the influence of some external factors, which affects the rate of the catalyzed reactions. Thus, the activity and speed of enzymatic reactions depends, inter alia, on from
enzyme and substrate concentration
-activities of activators and inhibitors
Effect of enzyme and substrate concentration
As the substrate concentration increases, the reaction speed increases, reaching maximum efficiency when all enzyme molecules are combined with the substrate. Thus, as the substrate concentration increases, the saturation of the active enzyme centers increases steadily and at full saturation the speed reaches its maximum. Further increasing the amount of substrate does not increase the reaction rate, it may even decrease it slightly.
The influence of temperature
As the temperature rises, the rate of the enzymatic reaction increases. However, once the optimum is reached, the further increase causes a decrease in the reaction rate. High temperature destroys the enzyme irreversibly because it is a protein substance and an increase above its optimal temperature causes gradual denaturation and loss of catalytic properties. The optimal temperature for enzymes depends on their origin for animal enzymes, it is close to body temperature (36-40 °), for plant enzymes its range is 20-30 ° C. At low temperatures, enzyme activity is inhibited, but the process is reversible.
The effect of reactions (pH)
Each enzyme is characterized by an optimum pH at which it has the highest activity. A strongly acidic or alkaline environment (extreme pH values) usually denaturalises enzymes that are proteins, destroying their activity irreversibly. A slight deviation from the optimal value does not denature but decreases the rate of the catalyzed reaction. The influence of pH is related to the change of the degree of dissociation of the enzyme itself (dissociation of -NH2 and -COOH groups present in the polypeptide chain and mainly in the active center) – which negatively affects the formation of the enzyme-substrate complex. The pH optimum for most enzymes occurs at values close to neutral or slightly acidic. However, there are enzymes that show activity only in an acidic environment (pepsin pH 1.5-2.2) or alkaline environment (trypsin pH 8-9).
The influence of activators and inhibitors
Most enzymes require a full activity of various chemical agents to accelerate and even allow their action. These factors are called activators. The activators work by facilitating the formation of the enzyme – substrate system. Enzyme activators may be metal ions or anions interacting with the enzyme protein, compounds that regulate the redox potential of the environment, which determine the construction of active centers, or compounds that cleave certain chemical groups, blocking active enzyme centers, eg α-amylase requires Cl- ions, whereas polyphenol oxidase is activated by Cu2 +, numerous peptidases by Mn2 +, Co2 +, Zn2 +. The enzyme inhibitors are called inhibitors. The mechanism of action of inhibitors is different, most often it consists in joining them with the active center of the enzyme or with a coenzyme or prosthetic group causing their inactivation.
Classification of enzymes
Depending on the type of catalytic reaction, the enzymes were divided into 6 classes
Each enzyme is marked with a four-membered digital member, preceded by the letters EC. The four digits determine exactly the position of the enzyme in the accepted international classification system. The first digit – specifies the main class, the second – the subclass, the third – the subclass, the fourth – the enzyme numbers within the subclass. In modern enzymology, individual enzymes bear twofold colloquial and systematic names. The systematic names of enzymes consist of two first parts created from the name of the main class to which the enzyme belongs, adding the ending -aza, the second part consists of the name of the substrate undergoing enzymatic reaction.