IGCSE Biology 0610: Enzyme Specificity and the Active Site (Rennin and Casein)

Rennin, also called chymosin, is an enzyme used in cheese-making. When rennin is added to milk, it acts on casein, a protein dissolved in the milk, causing the casein to clump together and form solid curds. Rennin has no effect on lactose, a sugar that is also dissolved in milk.

(a) State the type of biological molecule that acts as the substrate for rennin. [1]

(b) Using the term active site, describe why rennin causes casein to clump together but has no effect on lactose. [2]

(c) A cheese-maker accidentally uses a batch of rennin that was left inside a hot delivery van for several hours and has since lost its normal shape. Suggest what will happen when this rennin is added to milk, and explain your answer. [2]

Part (a): Identifying the substrate

Casein is described as a protein dissolved in milk, and rennin acts on casein. So the substrate for rennin is a protein.

Part (b): Explaining rennin’s specificity using the active site

Rennin, like every enzyme, has an active site whose shape is complementary to the shape of one particular substrate.

Casein’s molecular shape fits rennin’s active site, so casein molecules can bind there and be converted into clumped curds.
Lactose is a completely different type of molecule, a sugar, with a different shape from casein. Because it does not fit rennin’s active site, lactose cannot bind to the enzyme, so rennin has no effect on it.

Part (c): Predicting the effect of the damaged rennin

Being left inside a hot delivery van for several hours has exposed the rennin to a temperature far above its optimum working temperature. This is enough to denature the enzyme.

Once denatured, rennin cannot go back to working normally just by being added to milk at a suitable temperature. The change caused by the excess heat is permanent. So very little or no clumping (curd formation) will occur when this batch of rennin is added to the milk.

Final answers

(a) A protein.
(b) Rennin’s active site shape is complementary only to casein, so only casein binds and clumps; lactose has a different shape and cannot bind.
(c) Little or no curd will form, because the heat in the van has denatured the rennin, and this cannot be reversed just by returning it to a suitable temperature.