Biomolecules

Enzymes catalyzing a transfer of a group, G (other than hydrogen) between a pair of substrate S and S' belong to Class 2: Transferases.

In peroxidase and catalase enzymes (which break down toxic hydrogen peroxide into water and oxygen), haem acts as the tightly bound non-protein prosthetic group.

A holoenzyme consists of a protein part called the apoenzyme combined with a non-protein cofactor (which may be a prosthetic group, coenzyme, or metal ion).

Adenosine is a nucleoside (adenine + ribose) (III). Adenylic acid is a nucleotide (adenosine monophosphate) (II). Adenine is a purine nitrogen base (I). Alanine is an amino acid (IV).

Both starch and cellulose are made of glucose molecules, but the difference between them is that starch is a branched polymer while cellulose is a linear polymer. Starch also forms helical secondary structures, so it can hold $I_2$ molecules in the helical portion. The starch-$I_2$ complex is blue in colour. Cellulose does not contain complex helices and hence cannot hold $I_2$.

Lipids are mostly made of long non-polar hydrocarbon chains (fatty acids) that cannot form hydrogen bonds with water.
Such water-repelling molecules are called hydrophobic; they dissolve in non-polar (organic) solvents such as chloroform, ether and benzene, but not in water.
So lipids are insoluble in water because they are hydrophobic.

Reference: NCERT Class XI, pages 108, 109, 110.

Inulin is a fructan (polysaccharide of fructose) in which adjacent fructose units are linked by glycosidic bonds. Insulin is a protein of 51 amino acids, in which adjacent amino acids are linked by peptide bonds. So inulin and insulin, respectively.

Secondary metabolites such as nicotine, caffeine, quinine, strychnine and opium are produced by plants as defences against grazers and browsers.

Lysine is a basic amino acid (it has an extra amino group). Valine is neutral, glutamic acid is acidic, and tyrosine is an aromatic amino acid.

Glycine is a simple amino acid in which the R group is hydrogen. It is not a sulphur-containing amino acid; cysteine and methionine are sulphur-containing amino acids.

Amino acids have the general formula $R-CH(NH_2)-COOH$; they differ in the side chain R.
In glycine the R group is just a hydrogen atom: $H_2N-CH_2-COOH$.
It is the smallest and simplest amino acid, and it is the only common amino acid that is not optically active, because its $\alpha$-carbon carries two hydrogen atoms.
Alanine has a methyl group ($R = CH_3$), asparagine has $-CH_2CONH_2$, and tyrosine has a p-hydroxybenzyl group.
So the simplest amino acid is glycine.

Essential amino acids cannot be made in the human body and must be obtained from the diet.
They are valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan and lysine (with histidine and arginine for children).
Glycine, aspartic acid and serine are non-essential; the body can synthesise them.
So phenylalanine is an essential amino acid.

Collagen is the most abundant protein in the animal world. (RuBisCO is the most abundant protein in the whole biosphere.)

Collagen is the most abundant protein in the animal world; it is the main structural protein of connective tissue.
RuBisCO is the most abundant protein in the whole of the biosphere.

Adenine is a purine (double-ringed base in DNA and RNA); anthocyanin is a pigment (secondary metabolite); chitin is a homopolysaccharide of N-acetylglucosamine found in fungal cell walls and arthropod exoskeletons; codeine is an alkaloid (secondary metabolite).

Proteins are linear chains of amino acids in which successive amino acids are joined by peptide bonds, hence they are called polypeptides.

Peroxidase and catalase catalyse the breakdown of hydrogen peroxide to water and oxygen; haem is their prosthetic group.

Many enzymes need a non-protein cofactor for their catalytic activity; when the cofactor is a metal ion it forms coordination bonds with the active site and the substrate.
Carboxypeptidase is a proteolytic enzyme that removes amino acids from the carboxyl end of a peptide.
NCERT gives zinc as the cofactor of carboxypeptidase.
Copper is associated with enzymes such as cytochrome oxidase, iron with haem of catalase and peroxidase, and niacin is a vitamin forming the coenzymes NAD and NADP.
Hence carboxypeptidase requires zinc.

Many enzymes consist of a protein part and a non-protein part.
The protein part is called the apoenzyme, and the non-protein part that is essential for the activity of the enzyme is called the cofactor.
The complete, catalytically active enzyme (apoenzyme + cofactor) is called the holoenzyme.
Cofactors are of three kinds: prosthetic groups (organic, tightly bound), coenzymes (organic, bound only transiently during catalysis) and metal ions.
Coenzymes are organic compounds, many of which contain vitamins; for example NAD and NADP contain the vitamin niacin.
Isoenzymes are different molecular forms of the same enzyme.
So an organic substance bound to an enzyme and essential for its activity is called a coenzyme.

Sugar is a common term used to denote carbohydrate. Carbohydrates are polyhydroxy aldehydes, ketones or their derivatives, which means they have carbonyl and hydroxyl groups.

A holoenzyme is a conjugated enzyme in which the protein part is the apoenzyme while the non-protein part is the cofactor. Coenzymes are organic compounds whose association with the apoenzyme is only transient; they serve as cofactors.

Nucleic acids are polymers of nucleotides. Proteins are polymers of amino acids. Polysaccharides are polymers of monosaccharides. Lipids are esters of fatty acids and alcohol; they are not polymers.

A fat molecule consists of a glycerol moiety linked with three fatty acid molecules by three ester bonds (triglyceride).

Succinic dehydrogenase is the Krebs cycle enzyme that converts succinate into fumarate.
A competitive inhibitor is a substance that closely resembles the substrate in its molecular structure and competes with it for the active site of the enzyme.
Malonate ($^-OOC-CH_2-COO^-$) is very similar in structure to succinate ($^-OOC-CH_2-CH_2-COO^-$); both are dicarboxylic acids.
Malonate binds to the active site of succinic dehydrogenase but cannot be dehydrogenated, so the substrate cannot bind and the enzyme action declines.
This inhibition can be overcome by increasing the concentration of the substrate (succinate), which is the characteristic feature of competitive inhibition.
$\alpha$-ketoglutarate, malate and oxaloacetate are intermediates of the Krebs cycle, not the classical competitive inhibitor of this enzyme.
So the competitive inhibitor of succinic dehydrogenase is malonate.

A competitive inhibitor closely resembles the substrate in its molecular structure.
It competes with the substrate for the active site of the enzyme; when the inhibitor occupies the active site, the substrate cannot bind and the enzyme action declines.
For example, malonate competes with succinate for the active site of succinic dehydrogenase.
Competitive inhibition can be overcome by increasing the concentration of the substrate.
A non-competitive inhibitor binds at a site other than the active site and changes the shape of the enzyme; adding more substrate does not overcome it.
In the first statement the roles are confused (the substrate does not compete with the enzyme), and the second statement is not a general feature.
So the correct statement is that competitive inhibition is seen when the substrate and the inhibitor compete for the active site on the enzyme.

Chitin is a complex polysaccharide made of N-acetylglucosamine, so it contains nitrogen.

Inulin is a polysaccharide of fructose stored as reserve food in the roots and tubers of plants like dahlia and Jerusalem artichoke.
Pectin (pectic compounds) is a polysaccharide found in the middle lamella and cell walls, and can also act as a stored carbohydrate in fruits.
So, in plants, inulin and pectin are reserve materials.

In sucrose the reducing groups of glucose and fructose are both used in the glycosidic bond, so it has no free aldehyde or ketone group and is non-reducing.

When the substrate binds to the enzyme a new, high-energy transition state structure is formed. It is transient and unstable.

Almost all enzymes are proteins. The exceptions are some nucleic acids that behave like enzymes, called ribozymes (RNA enzymes). No enzyme is a lipid.
So the statement 'most enzymes are proteins but some are lipids' is wrong.
Each enzyme shows its highest activity at a particular optimum pH (true).
Enzymes are generally denatured at high temperatures, but enzymes isolated from thermophilic organisms living in hot vents and sulphur springs are stable and retain their catalytic power even at 80–90 °C (true).
Enzymes are highly specific for their substrates (true).
Hence the wrong statement is that some enzymes are lipids.

The essential chemical components of many coenzymes are vitamins; for example NAD and NADP contain niacin.

The curve is bell-shaped: the value on the y-axis rises to a peak and then falls.
In such graphs the dependent variable, enzyme activity, is plotted on the y-axis and the condition being varied on the x-axis.
Each enzyme shows its highest activity at a particular optimum temperature (and optimum pH); the activity declines both below and above the optimum, giving a bell-shaped curve.
At low temperature the enzyme is temporarily inactive, and at high temperature it is denatured.
With substrate concentration the curve is not bell-shaped: the velocity rises and then levels off at $V_{max}$.
Among the choices, the one consistent with the curve is x-axis: temperature and y-axis: enzyme activity.

The fat-soluble vitamins are A, D, E and K.
Retinol is vitamin A, a fat-soluble vitamin. Its deficiency causes night blindness and xerophthalmia (dryness of the conjunctiva and cornea). So this pair is correct on both counts.
Cobalamine is vitamin $B_{12}$, which is water soluble; its deficiency causes pernicious anaemia. Beri-beri is caused by the deficiency of thiamine (vitamin $B_1$).
Calciferol is vitamin D, which is fat soluble, but its deficiency causes rickets in children and osteomalacia in adults. Pellagra is caused by the deficiency of niacin (vitamin $B_3$).
Ascorbic acid is vitamin C; it is correctly matched with scurvy, but it is a water-soluble vitamin.
So the fat-soluble vitamin correctly matched with its deficiency disease is Retinol - Xerophthalmia.

Fat soluble vitamins are A, D, E and K; water soluble vitamins are the B group and C.
Vitamin A (retinol) is fat soluble. It forms the visual pigment rhodopsin in the rod cells of the retina. Its deficiency causes night blindness (and xerophthalmia).
Vitamin K is fat soluble and is needed for blood clotting; its deficiency delays clotting (haemorrhage). It is not water soluble.
Beri-beri is caused by deficiency of vitamin $B_1$ (thiamine), and pellagra by deficiency of niacin (vitamin $B_3$).
So the correct matching is Vitamin A – fat soluble – night blindness.

Enzymes are biocatalysts that speed up and control the rates of metabolic reactions.
Vitamins act mostly as coenzymes or their precursors, without which many enzymes cannot work.
Hormones are chemical messengers that regulate the activities of target cells and organs.
So all three help in regulating metabolism, which is why they can be grouped together.
They are not all synthesised in the body (most vitamins must be obtained from the diet), they are not all proteins (vitamins and steroid hormones are not), and they do not all enhance oxidative metabolism.
So the common feature is that they help in regulating metabolism.

Collagen is the most abundant protein in the animal body. It is found in connective tissues such as tendons, ligaments, cartilage, bone and skin.
It is a fibrous protein: three polypeptide chains are wound together into a triple helix, forming long, strong, insoluble fibres.
Globular proteins, such as enzymes and haemoglobin, are folded into compact, soluble shapes.
So collagen is a fibrous protein.

Vitamin E is tocopherol; correctly matched.
Vitamin D is calciferol (not riboflavin); vitamin $B_2$ is riboflavin.
Vitamin $B_1$ is thiamine; vitamin A is retinol.
So the correct match is vitamin E – tocopherol.

Cellulose is the main structural component of plant cell walls; wood is about 50 percent cellulose and cotton about 90 percent.
Because plants make up most of the earth's biomass, cellulose is the most abundant organic compound on earth.
So the most abundant organic compound is cellulose.

Rhodopsin (visual purple) in the rods of the retina is made of opsin and retinal, which is derived from vitamin A.
Deficiency of vitamin A reduces rhodopsin and causes night blindness.
Carrot and ripe papaya are rich in $\beta$-carotene, which is converted into vitamin A in the body.
So carrot and ripe papaya should be eaten.

In feedback inhibition, the end product of a metabolic pathway inhibits an enzyme acting early in the pathway (usually an allosteric enzyme).
When the end product accumulates, the pathway slows down; when it is used up, the pathway restarts. For example, isoleucine inhibits threonine deaminase.
So the end product is responsible for inhibition during feedback.

A nucleoside is a nitrogenous base + a pentose sugar; a nucleotide is a nucleoside + phosphate group(s).
ATP (adenosine triphosphate) is adenine + ribose + three phosphate groups. So it is a nucleotide (a ribonucleoside triphosphate).
So ATP is a nucleotide.

Conjugated proteins have a non-protein (prosthetic) group attached.
In glycoproteins, the prosthetic group is a carbohydrate (e.g. mucin, some hormones, blood group antigens).
Chromoproteins have a pigment (e.g. haemoglobin), lipoproteins a lipid, and nucleoproteins a nucleic acid.
So glycoproteins have carbohydrate as their prosthetic group.

Enzymes are biological catalysts (mostly proteins, not inorganic). They form an enzyme–substrate complex and provide an alternative pathway with a lower activation energy.
This greatly increases the rate of the reaction without changing its equilibrium or $\Delta G$.
So the role of an enzyme is to decrease the activation energy.