The rate of an enzyme catalyzed reaction depends:

Correct — A, upon substrate concentration, temperature and pH. All three act, and they act by different routes. Raising substrate concentration raises the rate steeply at first, then less and less, and finally not at all: once every active site is occupied the enzyme is saturated and the rate has reached its maximum. Raising temperature speeds the reaction in the ordinary way, by giving molecules more energy and more frequent collisions, but only up to an optimum — beyond it the protein's folded shape breaks up, the active site loses its geometry, and the rate falls away far faster than it climbed. Changing pH shifts the ionisation of the acidic and basic side chains that form the active site and hold the substrate, so each enzyme works fastest in a narrow band and poorly outside it. Pepsin in the stomach is at its best near pH 2, trypsin in the small intestine near pH 8, and neither would work in the other's compartment. Since the other three options each deny at least one of these, only (a) can stand.

• (b)only on substrate concentration and pH, but not on temperature. — Temperature dependence is the easiest of the three to demonstrate — the same reaction runs faster as the mixture is warmed towards the optimum and stops altogether once the enzyme is denatured. A boiled extract has no activity left.
• (c)only on pH and temperature, but not on substrate concentration. — This describes only the saturated state. Below saturation the rate climbs with substrate concentration, which is precisely the relationship the Michaelis-Menten equation was written to express.
• (d)only on temperature, but not on pH and substrate concentration. — It denies two real dependencies at once. Salivary amylase stops working as soon as it meets stomach acid, which settles the pH half on its own.

An enzyme is a biological catalyst, almost always a protein, that lowers the activation energy of a reaction without being consumed. Its power comes from a small pocket, the active site, whose shape and charge distribution fit the substrate. Anything that changes the concentration of substrate arriving at that pocket, or changes the pocket's shape and charge, changes the rate. That gives the standard list of factors: substrate concentration, enzyme concentration, temperature, pH, and the presence of activators, inhibitors or cofactors.

The trap in this item is that the temperature and pH curves both have a peak while the substrate curve does not, so students who have memorised bell-shaped graphs sometimes conclude that substrate concentration behaves differently in kind and drop it from the list. It behaves differently in shape, not in kind — the curve rises and then levels off instead of falling, because excess substrate does no harm while excess heat or acid destroys the protein. That asymmetry is itself examinable: past a certain point more substrate simply does nothing, whereas past the optimum more heat actively reduces the rate below what it was before.

• Rate rises with substrate concentration until the active sites are saturated, after which it holds at the maximum velocity.
• Human enzymes generally work best near 37 degrees Celsius; above the optimum the protein denatures and activity falls sharply.
• Denaturation changes the shape of the enzyme, not its amino acid sequence, and for most enzymes it cannot be reversed by cooling.
• Pepsin has an optimum near pH 2 and trypsin near pH 8, which is why one works in the stomach and the other in the small intestine.
• Enzyme concentration, inhibitors, activators and cofactors also change the rate, so the three factors named here are not an exhaustive list.

• Concluding that substrate concentration does not matter because the curve eventually flattens.
• Assuming higher temperature always means a faster reaction; past the optimum the enzyme is destroyed.
• Treating one optimum pH as universal — the value is a property of the individual enzyme.

Asked as an all-or-nothing list item, where three options each drop one genuine factor and the answer is the option that keeps them all.

• practice — not a real PYQ

  Beyond its optimum temperature, the rate of an enzyme-catalysed reaction falls because

  • (a)the substrate is used up faster
  • (b)the enzyme is denatured and its active site loses shape
  • (c)the pH of the medium rises
  • (d)the enzyme is consumed in the reaction

  Answer(b) the enzyme is denatured and its active site loses shape — the protein unfolds, so the substrate no longer fits.

• practice — not a real PYQ

  Salivary amylase stops acting on starch once food reaches the stomach mainly because

  • (a)the temperature there is too high
  • (b)there is no starch left
  • (c)the acidic pH is far from its optimum
  • (d)it is diluted by gastric juice

  Answer(c) the acidic pH is far from its optimum — amylase works around neutral pH, and gastric acid takes the medium well outside its working band.