The first enzyme in the metabolic pathway that produces isoleucine is threonine deaminase. Which graph illustrates the relationship between threonine deaminase activity and threonine concentration?

[Source: © International Baccalaureate Organization 2019]

B

The programme specifies the threonine to isoleucine pathway as the example of end-product inhibition that should be studied (8.1 of AHL). Candidates were therefore expected to know that threonine is the substrate for the enzyme threonine deaminase, but they could also have also deduced this from the name of the enzyme. It was expected that candidates would recognise the curve for the effect of increasing substrate concentration on enzyme activity and choose answer B. Two thirds of candidates successfully did this, but the discrimination index was rather low, showing that some candidates chose another answer. Perhaps they remembered that this enzyme can be affected by an end product inhibitor and therefore chose a curve where enzyme activity decreased with increased threonine concentration.