NEET Biology Notes Biomolecules and Enzyme Enzymes
Enzymes
Enzymes
The term ‘enzyme’ was coined by Kuhne (1878). There are approximately 3000 enzymes present in a cell. The molecular weight of enzymes ranges from 10,000 to more than 100000 daltons. Zymase was discovered by Buchner. Approximately, all enzymes are proteins (ribozymes are exception). Being proteins, they are coded for by DNA.
Enzymes are biocatalysts having prominent active sites. These are very efficient, i.e. a very small amount of catalyst brings about the change of a large amount of substrate. Metabolic reactions are catalysed reacton. There is no uncatalysed, metabolic conversion in living systems. The catalysts which fasten the rate of a given metabolic conversion are some protein compounds called enzymes.
These are highly specific that is an enzyme will generally catalyse only a single reaction.The constant making and breaking of biomolecules in a living cell through chemical reactions is called metabolism. Each of the metabolic reactions results in transformation of. biomolecules. The flow of metabolites through the metabolic pathway has a definite rate and direction. This metabolic flow is called the dynamic state of body constituents. Enzymes are also used for therapeutic means to teat disease like streptokinase is used in cleaning blood clots inside blood vessels. Peroxidase is the smallest enzyme. Diastase is the earliest known enzyme. Antiageing enzyme is catalase (non-porphyrin, enzyme)
Cofactors
Enzymes are composed of one or several polypeptide chains. The cofactors are bound to the enzyme to make the enzyme catalytically active. The protein portion of the enzyme is called apoenzyme. Three kinds of cofactors may be identified as prosthetic groups, coenzymes and metal ions. A complete enzyme is called holoenzyme. Holoenzyme consists of apoenzyme and prosthetic group. Enzymes are thermolabile, amphoteric, colloidal and substrate specific. Enzymes working in the cell, in which, they are produced are called endoenzymes. Enzymes secreted outside the cell and act on external medium are called exoenzymes. Most human enzymes function best within a relatively narrow temperature range between 35 and 40°C (close to body temperature). Below this temperature range, the bonds that determine enzyme shape are not flexible enough to permit the induced-fit change sometimes necessary for catalysis. Above this temperature range, the bonds are too weak to hold the enzyme’s peptide chains in the proper position.
In contrast, bacteria that live in hot springs have enzymes with stronger bonding between their peptide chains and therefore, can function at temperatures of 70°C or higher. The temperature coefficient (0-10) of enzyme is 2-3 within optimum range that is rate of reaction increases from 2-3 times for 10°C increment.
Mostly enzymes are named by adding a suffix-ase to the root word of the substrate, on which that enzymes act, e.g. lipase (fat hydrolysing enzymes sucrase breaking down sucrose). Sometimes the enzymes are named on the basis of the reaction that they catalyse, e.g. polymerase (aids in polymerisation), dehydrogenase (removal of H-atoms). Some enzymes have been named on the basis of source from which they were first identified, e.g., papay in from papaya. The names of some enzymes ends with an ‘in’ indicating that they are basically proteins, e.g., pepsin, trypsin, etc. Thomas Cech and Sydney Altmann were awarded Nobel Prize for the discovery of enzymatic activity of ribonuclease. (ribozyme). Ribozymes are non protein enzymes. Mainly enzymes are classified into six classes.
Mechanism of Enzyme Action
Enzymes possess active sites, where the reaction takes place. These have specific shapes. Enzymes remain unaltered upto the end of chemical reactions therefore, it can be used again and again.
Enzyme works by lowering the activation energy (energy to start a reaction). An enzyme combines with its substrate (S) to form a short lived Enzyme Substrate (ES) complex, which breaks up into products and enzyme.
Of the far large enzyme molecule only a small portion (4-12 amimo acids) comes in direct contact with the substrate, this portion is called active site.
Fisher (1980) suggested Lock and key hypothesis (template theory) for enzyme action on the basis of specificity. According this the enzyme has a particular shape (lock) into which particular substrate (key) fits.
Koshland (1959) proposed induced fit hypothesis. This states that combination of a substrate with enzyme induces changes in the enzyme structure, which enables the enzyme to perform its catalytic function effectively. On increasing the subsrate concentration, the catalytic activity of given concentration of an enzyme will increase to approach maximum rate Vmax. The substrate concentration at which the chemical reaction attain half its maximum velocity is called Michaelis Menten constant (Km).
EC number is called Enzymes Commission number. It gives a code number to an enzyme, which is in 4 digits. First digit of EC number denotes class, second digit denotes sub-class, third digit denotes sub-sub-class and the fourth one denotes enzyme number in sub-sub-class.
Specificity of an enzyme is due to apoenzyme position. Apoenzyme is a protein composed of a amino acid units. Tertiary structure of enzymatic protein is folded in such a way as to create a region called active site that has correct molecular dimension and topology to accommodate and bind with a specific substance.
Enzymes useful in hydrolysing fats and lipids are known as esterases. Building blocks of enzymes are amino acids.
There are some enzymes, which have slightly different molecular structure but exert similar catalytic action. Such enzymes are called isoenzymes or isozymes. More than 100 isozymes have been identified.
The enzyme lactic dehydrogenase (LDH) in human skeletal muscle has five isozymes. Enzymes get denatured at very high temperature (i.e., about 65°C). This denaturing occurs due to break down of the protein molecule.
Enzyme Inhibition
Enzyme action can be
- Competitive inhibition, some substances (inhibitors) have structural similarity with the substrate and compete for the active site of enzyme. In competitive inhibition, any chemical substance, which has a molecular structure that closely resembles a substrate can reduce or inhibit the activity of an enzyme. Such an inhibitor is called competitive inhibitor. This situation is comparable to a lock jammed by a key almost similar to the original one.
- Non-competitive inhibiton some inhibitor (poison) bind to enzymes other than active sites and disturb the structure of enzyme. In non-competitive inhibition, the chemical substances such as cyanides or phosphides can inhibit the action. of respiratory enzymes. They are not similar to the substrate molecule and as such do not complete with it. However, such inhibitor may become* attached to any site on the enzyme, other than the substrate binding site.
- Allosteric modulation or feed back inhibition the accumulation of end product causes inhibition in the activity of the first enzyme.Factors Affecting Enzyme Activity
- The activity of an enzyme is affected by pH, temperature, substrate concentration and enzyme concentration. Enzymes lower the activation energy of the reactions.
- Temperature An enzyme change only rate of reactions not the direction of the equilibrium. All enzymes are thermolabile.
- Optimum activity of an enzyme is between 25°C to 45°C. At temperature below freezing point, an enzyme is inactivated, whereas at high temperature (above 60°C) enzymes may be destroyed or denatured.
- pH Every enzyme has an optimum pH when it is most effective. A rise or fall in pH reduces the enzyme activity.
- Substrate concentration Increase in substrate concentration increases the rate of reaction. The rise of reaction velocity is high in begining but decreases progressively with increase in substrate concentration.
- Enzyme concentration The rate of biochemical reaction rise with increase in enzyme concentration up to a point called limiting or saturation point.