Biomolecules Test - 3

Most reactions do not start automatically because the reactions have an energy barrier which may be due to hydrogen bonding, absence of precise collisions due to small reactive site, mutual repulsion, etc. External supply of energy needed to overcome this barrier is known as activation energy. The enzyme lowers the activation energy of the reaction and allows a large number of molecules to react at a time.

Fig. Lowering of activation energy by enzyme in the energy relations of a chemical reaction

Every enzyme posses an optimum pH value where it is most effective. Most enzymes show maximum activity in a pH rang of about 6.0 to 7.5 i.e, near neutra pH. Some digestive enzymes have their optimum pH in the acidic or alkaline range. E.g., pepsin of gastic juice has its optimum at pH 2 (acidic) whereas typsin of pancreatic juice shows maximum activity at pH 8.8 (alkaline). A rise and fall in pH reduces enzyme activity by changing the degree of ionisation of its side chain.

Lyases enzyme catalyse the breakage of specific covalent bonds and removal of groups without hydrolysis producing double bonds or removal of double bonds by adding groups, e.g., histidine decarboxylase that splits C-C bond of histidine, forming CO₂ and histamine.

Ribozymes are a group of ribonucleic acids that function as biological catalysts and are regarded as non-protein enzymes.

Low temperature preserves the enzymes in their inactive state. Therefore it is used in preservation of foods inside cold storage. Low temperature present inside cold storage prevents spoilage of food.

High temperature destroys enzymes by causing their denaturation, This occurs at 50^oC or so. In between the minimum and maximum temperature the reaction velocity doubles for every 10^oC in temperature (general rule of thumb).

The peak in the graph refers to optimum pH or temperature. Enzyme activity is maximum at this point.

In feedback inhibition, product of a reaction inhibits the enzyme catalysing that reaction. It is a type of control mechanism at the enzyme level. If the product is produced in sufficient amount, in inhibits the enzyme to stop the further production.

In non-competitve inhibition, the inhibitor binds at a site other than the active site ob the enzyme surface. This binding impairs the enzyme function. The inhibitor has no structural resemblance with the sunstrate. It does not interfere with the enzyme-substrate binding but the catalysis is prevented, possibly due to a distortion in the enzyme conformation. Non-competitive inhibition is usually irreversible because it cannot be overcome by increasing the substrate concentration. The inhibitor (l) generally binds with the enzyme as well as the ES complex. The overall relation in non.competitve inhibition is represented as :

K_m  or the Michaelis-Menten constant is defined as the substrate concentration (expressed in moles/l) at which half-maximum velocity in an enzyme catalysed reaction is achieved. It indicates that half of the enzyme molecules (i.e. 50%) are bound with the substrate molecules when the substrate concentration equals the Km value. It was given by Leonor Michaelis and Maud Menten (1913). K_m value is a characteristic feature of a given enzyme. It is a representative for measuring the strength of ES complex. A low Km value indicates a strong affinity between enzyme and substrate, whereas a high K_m value reflects a weak affinity between them. For majority of enzymes, the K_m values are in the range of 10⁻⁵ to 10⁻² moles.

Isomerases catalyse the change of a substrate into a related isomeric form by rearrangement of molecules.

Enzyme may be broadly classified into two types depending on their chemical composition-simple enzymes and conjugated enzymes are wholly made up of proteins and any additional substance or group is absent, e.g., pepsin, trypsin, etc. Conjugated enzymes (or holoenzymes) are formed of two parts -a protein part called apoenzyme and a non-protein part named co-factor. The complete conjugated enzyme consisting of an apoenzyme and a co-factor is called holoenzyme. Holoenzyme is the functional unit of enzyme.

Co-factor may be inorgainc or roganic in nature. Catalytic activity is lost when co-factor is removed from the enzyme which indicates that it plays a crucial role in catalytic activity of enzymes.

Increase in substrate concentration increases the rate of reaction due to two factors: (i) occupation of more and more active sites by the substrate molecules, (ii) higher number of collisions between substrate molecules. The rise in velocity is quite high in the beginning but it decreases progressively with the increase in substate concentration. If a graph is plotted for substrate concentration versus reaction velocity, it appears as a hyperbolic curve. A stage is reached where velocity is maximum. It does not increase further by increasing the substrate concentration. At this stage the anzyme molecule becomes fully saturated and no active site is left free to bind additional substrate molecules.

Enzymes work precisely at certain narrowly optimum conditions, such as appropriate temperature, pH, and ion concentration. Deviation from the optimal conditions adversely affects enzyme activity. Most enzymes have an optimal temperature, at which the rate of reaction is fastest. For human enzymes, the temperature optima are near the human body temperature (35-40°C). Enzymatic reactions occur slowly or not at all at low temperatures. As the temperature increases, molecular motion increases, resulting in more molecular collisions. The rates of most enzyme-controlled reactions, therefore, increase as the temperature increases, within limits. High temperatures rapidly denature most enzymes. The molecular conformation (3-D shape) of the protein becomes altered as the hydrogen bonds responsible for its secondary, tertiary, and quaternary structures are broken. Because this inactivation is usually not reversible, activity is not regained when the enzyme is cooled.

The multiple molecular forms of an enzyme occuring in the same organism and having a similar substrate activity are called isoenymes or isozymes. They have similar properties but different molecular weights and locations. Over 100 enzymes are known to have isoenzymes. α-amylase of wheat endosperm has 16 isozymes, lactate dehydrogenase has 5 isozymes. Glucokinase is an isozyme of hexokinase.