_________ are precursors of enzymes.
a. Zymogens
b. Ribosomes
c. RNA
d. Protein
Which of the option given above is/are correct ?

Question

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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.

Why the other options are wrong — (b) 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.  (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.  (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.

Concept: 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.

State PCS 2023 Q69 — Science & Technology PYQ with Answer

State PCS 2023 Q69 — Science & Technology PYQ with Answer