MPSC 2023 Prelims Paper-I · Question 69 of 100

Correct — option (1). This is a fill-in-the-blank offered as a statement list: the sentence '_________ are precursors of enzymes' is to be completed from the four labelled items a to d, and the option chosen must name every item that completes it truly and no item that does not. A precursor, in this sense, is a substance that is itself converted into the enzyme — not a substance that makes the enzyme, not a substance the enzyme is composed of, and not a substance that carries instructions for building it. On that definition only statement a qualifies, so the answer is 'only a'. Zymogens, also called proenzymes, are exactly what the sentence describes. A zymogen is an inactive precursor form of an enzyme, secreted in a state in which it cannot act, and converted into the active enzyme by a chemical modification — characteristically the cleaving away of a stretch of the polypeptide chain, which allows the remainder to fold into a shape with a functional active site. The examples are standard and worth holding: pepsinogen is converted to pepsin in the stomach under the influence of hydrochloric acid, trypsinogen is converted to trypsin in the small intestine by the enzyme enterokinase, chymotrypsinogen is converted to chymotrypsin by trypsin, and prothrombin is converted to thrombin in the cascade of blood clotting. The reason the body works this way is protective, and it is the point most worth understanding rather than memorising: the digestive proteases would attack the tissue that produced them if they were synthesised in active form, so the pancreas manufactures them as inert zymogens and they are activated only after reaching the gut lumen where their substrate is. The same logic governs the clotting factors, which must be held ready but inactive until a vessel is injured. The other three items each name something with a genuine relation to enzymes — ribosomes build them, RNA encodes them, protein composes them — but none of the three is converted into an enzyme, and being related to a process is not the same as being a precursor in it. Option (1) is the answer.

• (2)only b and c — This option nominates statements b and c, and neither completes the sentence. Ribosomes are the cellular structures on which proteins, enzymes among them, are assembled from amino acids according to the sequence carried by messenger RNA; they are the machinery of synthesis and are not themselves converted into any enzyme, remaining intact and available to build the next polypeptide. Calling a ribosome a precursor of an enzyme is like calling a loom a precursor of cloth. RNA fails for a related reason set out under option (3). The distinction the question is enforcing is between the several different relationships a substance can bear to an enzyme — being its template, its site of manufacture, its chemical composition, or its immediate inactive form — and only the last of these is what the word precursor names.
• (3)only a and c — Statement a is correctly included here but statement c is not, and an option that adds a false item to a true one is as wrong as one that omits the true item altogether. RNA is not a precursor of enzymes. Messenger RNA carries the coded instructions transcribed from the gene and serves as the template from which the polypeptide is assembled, but it is not converted into the enzyme; it is read and then degraded, and the atoms of the finished enzyme come from amino acids and not from the RNA. There is a genuine fact in the neighbourhood that makes this option seductive — some RNA molecules, called ribozymes, are themselves catalytic and act as enzymes — but that establishes at most that RNA can be an enzyme, not that it is the precursor of one. This option is the reason the item cannot be answered by spotting the one obviously correct statement and choosing any option containing it.
• (4)only d — This option nominates statement d alone, and it is the subtlest of the three because the statement is nearly true in a different sense. Almost all enzymes are proteins, so protein is what an enzyme is made of — its chemical class — and not a distinct substance converted into it. A precursor is a specific inactive form that undergoes a change to become the active enzyme, whereas saying 'protein is the precursor of enzymes' is like saying that metal is the precursor of a key. The point becomes sharper when one notices that a zymogen is itself a protein: pepsinogen and trypsinogen are polypeptides, and what makes them precursors is not their chemical class but their inactive state and the specific cleavage that activates them. Composition and precursor status are different relations, and this option tests whether the candidate can keep them apart.

Enzymes are biological catalysts, overwhelmingly proteins, which accelerate reactions by lowering the activation energy without being consumed or permanently altered. Their action depends on a precisely shaped active site, so anything that alters the shape of the molecule alters its activity — which is the principle behind the whole system of zymogens. A zymogen, or proenzyme, is an enzyme synthesised in an inactive form carrying an extra stretch of polypeptide that blocks or distorts the active site. Removing that stretch by a specific cleavage allows the molecule to take up its active conformation, and the conversion is irreversible, so the cell controls the process by controlling where and when the activating cleavage occurs. The digestive proteases are the standard illustration: pepsinogen from the chief cells of the stomach becomes pepsin in the acid of the gastric lumen; trypsinogen from the pancreas becomes trypsin in the small intestine through the action of enterokinase secreted by the intestinal mucosa; and trypsin in turn activates chymotrypsinogen, procarboxypeptidase and further trypsinogen, so that one activation triggers a cascade. The blood clotting system works on the same principle, with prothrombin converted to thrombin and a chain of inactive factors activating one another in sequence, which allows a small initiating signal to produce a large and rapid response. The protective purpose is plain from what happens when it fails: premature activation of pancreatic zymogens within the pancreas leads it to digest its own tissue. Related vocabulary is often tested alongside — an apoenzyme is the protein part of an enzyme, a cofactor the non-protein part, a coenzyme an organic cofactor often derived from a vitamin, and the two together form the holoenzyme.

MPSC's biology questions frequently turn on the precision of a single technical word, and this item is a clean example: all four labelled statements name something the candidate has certainly met in connection with enzymes, and the question is decided entirely by what 'precursor' means. The construction is worth recognising because it recurs — the Commission builds a statement list in which every item is related to the topic and only one bears the specific relation the stem names, so a candidate who reads loosely and asks 'is this connected with enzymes?' finds four plausible answers, while one who asks 'is this converted into an enzyme?' finds one. Note the trap built into the option set: statement a appears in two options, alone in option (1) and paired with statement c in option (3), so recognising that zymogens are precursors is not by itself enough to reach the answer, and the candidate must also positively reject RNA. That is a general feature of statement-list items and the reason each statement must be judged individually before the options are looked at. This paper labels the statements here with lower-case a to d in the English column and with the Devanagari अ ब क ड in the Marathi column, while the four answer choices are printed (1) to (4); the same letter can therefore mean a statement in the stem and a storage key in the options, and keeping the two apart is a habit worth forming. The closing line of the English stem prints the singular 'Which of the option given above', one of several grammatical slips in this paper's English column.

• A zymogen, also called a proenzyme, is an inactive precursor of an enzyme that becomes active when a part of its polypeptide chain is cleaved away, allowing the remainder to fold into a functional active site.
• Standard examples are pepsinogen converted to pepsin by the hydrochloric acid of the stomach, trypsinogen converted to trypsin by enterokinase in the small intestine, chymotrypsinogen converted to chymotrypsin by trypsin, and prothrombin converted to thrombin during blood clotting.
• Digestive proteases are secreted as zymogens so that they cannot digest the tissue that produced them, becoming active only after reaching the lumen where their substrate lies; the same principle keeps the clotting factors inert until injury occurs.
• Ribosomes are the sites at which proteins including enzymes are assembled and messenger RNA is the template read during that assembly, but neither is converted into an enzyme and neither is therefore a precursor of one.
• Nearly all enzymes are proteins, so protein describes the chemical composition of an enzyme rather than a precursor of it; a zymogen is itself a protein, and what makes it a precursor is its inactive state and the specific activating cleavage.

• Treating anything associated with enzymes as a precursor of them, when the word names only a substance that is itself converted into the enzyme
• Confusing the composition of an enzyme with its precursor, so that 'protein' is offered as the answer when protein is the chemical class enzymes belong to
• Assuming that recognising one true statement settles the answer, when this option set places that statement both alone and paired with a false one
• Reading messenger RNA as a precursor because the enzyme is built from its instructions, when the template is read and degraded and contributes none of the enzyme's substance
• Overlooking that ribozymes are catalytic RNA molecules, which makes RNA capable of being an enzyme but still not a precursor of one

Enzyme questions in MPSC papers divide into definition items of the kind used here, function items pairing an enzyme with the substrate it acts on, and condition items about the effect of temperature and pH on activity. The Commission's favourite construction for the first sort is the near-miss statement list, in which every item is genuinely connected with the topic and only the precise definition separates them, so preparation should record definitions as sentences with their boundaries — what a zymogen is and, equally, what a coenzyme, an apoenzyme and a cofactor are, since those four terms are routinely swapped between options. The digestive enzymes are the most heavily worked examples: their site of secretion, the zymogen each is secreted as, the agent that activates it and the substrate it digests together answer most of what can be asked. A candidate who prepares the pancreatic and gastric enzymes in that four-column form covers both the definition and the function variants at once.

No directly related past PYQ was found.

• practice — not a real PYQ

  Trypsinogen secreted by the pancreas is converted into active trypsin in the small intestine by which of the following ?

  • (a)Hydrochloric acid
  • (b)Enterokinase
  • (c)Bile salts
  • (d)Pepsin

  Answer(b) Enterokinase — also called enteropeptidase, it is secreted by the mucosa of the small intestine and cleaves trypsinogen to yield trypsin, which then goes on to activate further trypsinogen as well as chymotrypsinogen and procarboxypeptidase in a cascade. Hydrochloric acid is what activates pepsinogen in the stomach, and bile salts emulsify fats rather than activating any zymogen, so both belong to different steps of digestion.

• practice — not a real PYQ

  The non-protein organic molecule that binds to an enzyme and is required for its activity, often derived from a vitamin, is known as which of the following ?

  • (a)Zymogen
  • (b)Apoenzyme
  • (c)Coenzyme
  • (d)Holoenzyme

  Answer(c) Coenzyme — it is the organic non-protein component whose presence the enzyme requires in order to act, and many coenzymes are derived from vitamins of the B group. The apoenzyme is the protein part on its own, the holoenzyme is the complete functional assembly of apoenzyme and cofactor together, and a zymogen is something different again — the inactive precursor form of an enzyme awaiting activation by cleavage.