The enzymes which catalyze the same reaction but have different amino acid compositions are commonly referred as:

Correct — A, Isoenzymes. The stem is the standard definition, almost word for word: isoenzymes, also written isozymes, are different molecular forms of an enzyme found in the same species that carry out the same reaction but differ in their amino acid sequence. Because the sequence differs, so do the properties that follow from it — net charge and therefore mobility in electrophoresis, affinity for the substrate, sensitivity to inhibitors, and which tissue the form is found in. The textbook example is lactate dehydrogenase. It is a tetramer assembled from two kinds of subunit, H and M, and the five possible combinations give five isoenzymes, LDH-1 through LDH-5. LDH-1, made of four H subunits, predominates in heart muscle; LDH-5, four M subunits, in skeletal muscle and liver. All five convert pyruvate to lactate and back, which is why the phrase in the stem, 'the same reaction', is doing the real work. Medicine exploits the difference: creatine kinase exists as CK-MM, CK-MB and CK-BB, and it is the MB form appearing in blood that points to heart muscle rather than to skeletal muscle.

• (b)Isotopes — A term from nuclear chemistry, not biology. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, such as carbon-12, carbon-13 and carbon-14. Nothing to do with enzymes.
• (c)Isomers — Compounds sharing a molecular formula but differing in how the atoms are arranged — glucose and fructose are both C6H12O6. The word describes chemical structure in general and does not carry the meaning of catalysing the same reaction.
• (d)Coenzymes — Not forms of an enzyme at all. Coenzymes are small non-protein organic molecules that an enzyme needs in order to work — NAD+, FAD, coenzyme A — and many are derived from vitamins.

Enzymes are proteins, so each one has a definite amino acid sequence, and a cell may make more than one protein capable of the same catalysis. Those variants are isoenzymes. The term was introduced by Clement Markert and Freddy Moller in 1959 after electrophoresis revealed several bands of lactate dehydrogenase activity in a single tissue extract. Isoenzymes typically arise from different genes, and they let one organism run the same chemical step under different conditions — a liver that must handle a flood of glucose after a meal and a brain that must take up glucose steadily need different kinetics from the same reaction.

Three of these four options begin with 'iso-', which is exactly the confusion being tested, so separate them by the discipline each belongs to. Isotopes are about the nucleus of an atom. Isomers are about how atoms are arranged in a molecule. Isoenzymes are about proteins, and only the third has anything to say about catalysis. Coenzymes are the odd one out in a different way: they are helpers to an enzyme rather than a version of one. Two printed slips are worth noting and neither affects the answer — the paper writes 'catalyze' where Indian usage would normally spell it 'catalyse', and 'referred as' where the idiom is 'referred to as'.

• Isoenzymes, or isozymes, are multiple forms of an enzyme in the same species that catalyse the same reaction but differ in amino acid sequence.
• Lactate dehydrogenase has five isoenzymes, LDH-1 to LDH-5, built from different combinations of the H and M subunits in a tetramer.
• Creatine kinase exists as CK-MM, CK-MB and CK-BB; the MB form is used clinically to point at heart muscle.
• Isotopes differ in neutron number, isomers in molecular arrangement — neither term concerns catalysis.
• A coenzyme is a non-protein organic cofactor such as NAD+, FAD or coenzyme A, and is not a form of the enzyme itself.

• Reading the 'iso-' prefix and reaching for isotope or isomer without checking which discipline the stem is in.
• Treating a coenzyme as a kind of enzyme; it is a helper molecule, and it is not a protein.
• Assuming isoenzymes must have identical kinetics because they catalyse the same reaction — differing affinity is the point of having them.

As a definition-recall item like this one, or by naming lactate dehydrogenase or creatine kinase and asking what its several forms are called. The coenzyme-versus-cofactor distinction is asked separately.

• practice — not a real PYQ

  Lactate dehydrogenase occurs in five forms in human tissues, all catalysing the same reaction. These forms are best described as

  • (a)Coenzymes
  • (b)Isoenzymes
  • (c)Zymogens
  • (d)Apoenzymes

  Answer(b) Isoenzymes — LDH-1 to LDH-5 arise from different combinations of the H and M subunits and differ in amino acid composition while carrying out the same conversion.

• practice — not a real PYQ

  Which one of the following is a coenzyme rather than an enzyme?

  • (a)Pepsin
  • (b)Amylase
  • (c)NAD+
  • (d)Lactase

  Answer(c) NAD+ — nicotinamide adenine dinucleotide is a small non-protein molecule that carries hydrogen for dehydrogenase enzymes; the other three are proteins that catalyse reactions themselves.