Test: Enzymes (NCERT) - NEET MCQ

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

• Enzymes are regarded as the biocatalysts.
• Enzymes are proteins made from amino acids. 
• It is made up of hundreds and thousands of amino acids stringed together in a very specific and unique order. 
• Any chemical reaction inside a cell or any work that goes on inside a cell is the handiwork of enzymes inside the cell. 

Inhibition of an enzyme controlling an early stage of a series of biochemical reactions by the end product when it reaches a critical concentration.

Ans: 2
Sol: During the state where substrate is bound to the enzyme active site, a new structure of the substrate called transition state structure is formed. Very soon, after the expected bond breaking/making is completed, the product is released from the active site

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 variable with which we have to find the relation should always have to be on the y axis and the variable which does not show variation and will always increase will be on the x-axis.

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.

• Isoenzymes (also called isozymes) are alternative forms of the same enzyme activity that exist in different proportions in different tissues.
• Isoenzymes differ in amino acid composition and sequence and multimeric quaternary structure; mostly, but not always, they have similar (conserved) structures.

(i) Correct: Enzymes only speed up reactions and do not affect the overall change in free energy.
(ii) Correct: The three-dimensional shape of an enzyme determines its ability to bind to specific substrates.
(iii) Correct: Enzymes lower the activation energy required to start a reaction.
(iv) Correct: Enzymes are highly specific and typically act on only one or a few substrates.
(v) Correct: Activation energy is the minimum energy required to initiate a chemical reaction.

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.