Active site, binding site or receptor: the word AQA is waiting for

Four questions, one wrong word

Every summer, after the papers are marked, AQA publishes a Report on the Examination for each one: a few pages in which the senior examiners say, question by question, what students wrote and what it earned. The reports are public, they are short, and almost nobody reads them. They are also the only place the board says out loud which word cost the mark.

In June 2019 the A-level Biology Paper 2 report did that for the same phrase three times.¹ In a question about a mutation changing a polypeptide, the mark for a change in tertiary structure “was negated by stating that there would be a change in ‘the active site’”. In a question about muscle contraction, “a few students referred to ‘active site’ rather than binding site”. And in a question about insulin, students “disqualified the mark by suggesting attachment to an ‘active site’”.

Three years later it was still there. The June 2022 Paper 1 report, on a question about antibodies, says that references to the technique the scientists used “were simply ignored unless they suggested, incorrectly, that antibodies had an active site”.² The Paper 2 report from the same summer opens its general comments with the next rung of the same ladder: in three questions “reference to ‘receptor’ was required rather than ‘binding site’”, and it notes, as good news, that “fewer students used the term ‘active site’ to describe the receptor” than in earlier years.³

Read the verbs. Negated. Disqualified. Ignored unless. This is not a word that fails to score. It is a word that takes away a mark the rest of the sentence had already earned.

What an active site is, and is not

An active site is the region of an enzyme where the substrate binds and the reaction is catalysed. The phrase makes two claims at once. The first is that the protein has a pocket with a shape complementary to something else, which is true of a great many proteins. The second is that a reaction happens in that pocket, which is true of enzymes and of nothing else on the A-level specification.

So when a student writes “active site” about an antibody, they have not used a slightly loose synonym. They have asserted that an antibody catalyses a reaction. It does not: it binds an antigen and stays bound. When they write it about the site on an actin filament that a myosin head attaches to, they have said that actin is an enzyme. When they write it about the protein insulin attaches to on a liver cell, they have described a signal being received as though it were a substrate being broken down. In each case the marker is looking at a sentence that is false, and a mark scheme cannot credit a false sentence however close it sits to the true one.

The word is not decoration on top of the idea. The word is the idea, because it names what the pocket is for.

Three shapes, three names

An enzyme has an active site. The substrate is complementary to it, an enzyme-substrate complex forms, and the reaction is catalysed there. This is the first protein with a pocket that anyone meets, in the first term of Year 12, and the word attaches itself to the picture of a pocket so firmly that it comes out for every pocket afterwards. That is the whole problem, and it is why the same wrong word turns up in a question on muscles, a question on hormones and a question on immunity.

An antibody has a binding site, in the variable region at the tip of each arm, complementary to one antigen. The right phrase is “antigen-antibody complex”, and the June 2022 Paper 1 ruling is the one to remember: an answer that gave an antibody an active site was not merely unrewarded, it was the one form of the answer the examiners singled out as incorrect.²

A receptor is a protein in a cell-surface membrane with a site complementary to a hormone or a neurotransmitter. Here the ladder has a third rung, and it catches students who have already learned to avoid the first mistake. Insulin attaches to a receptor. Acetylcholine attaches to a receptor. The June 2022 Paper 2 report required “receptor” in three separate questions and refused “binding site”.³ The reason is the question’s own grammar: it asks what the molecule attached to, and a binding site is part of the answer to that, not the answer. The receptor is the protein; the binding site is the pocket in it.

A filament has a binding site too. In the June 2019 muscle question, the site on actin that the myosin head attaches to, and that tropomyosin covers at rest, is a binding site, and “active site” was the recorded near-miss.¹ Nothing is catalysed at that site. Something attaches there.

Why the board draws the line there

The June 2019 report puts the general principle in one sentence before it gets to any question: “the imprecise use of scientific terminology and limited powers of expression prevented some students from accessing specific marking points.”¹ A marking point is a statement the examiner may credit. It is written in particular words because the words carry the biology, and an examiner reading a script is not asked whether the student probably knew the right thing. They are asked whether the sentence on the page is one the scheme allows.

The rulings above show the two ways that goes wrong. Sometimes the wrong word simply does not reach the marking point: “binding site” for a receptor names the right protein imprecisely, and the mark that needed the word “receptor” is not given. Sometimes the wrong word contradicts a marking point the student had already reached: the mutation answer in June 2019 correctly said the tertiary structure would change, and then said the active site would change, and the second sentence cancelled the first, because the protein in the question was not an enzyme and the examiner cannot credit a change in a structure the protein does not have. Right plus wrong is wrong. That is not a quirk of one examiner. It is how a set number of marking points is counted.

It is worth noticing which direction the error always runs. Nobody writes “receptor” for an enzyme. The enzyme word leaks outward, onto every protein with a shape, because it was learned first and learned best. So the fix is not to learn three words. It is to stop the first one from answering every question.

The rule to carry into the exam

Before writing any of the three words, ask one question of the protein: what is it doing in this sentence?

• If it is catalysing a reaction, it is an enzyme and the pocket is its active site.
• If it is binding an antigen, or it is a site on a filament that something attaches to, the pocket is a binding site.
• If it is receiving a hormone or a neurotransmitter at a membrane, the protein is a receptor, and that is the word the question wants, not the pocket in it.

And one more, from the mutation question: if the question is about a protein in general, the thing that changes is its tertiary structure. A change in the active site is a consequence, and only for an enzyme. Say the structure, not the consequence, unless the protein is an enzyme and the question is about the reaction.

Three pockets, three words. Enzymes have active sites. Antibodies and filaments have binding sites. Membranes have receptors.

Credited and refused

Everything below is from the three reports cited, AQA only. Other boards draw the same distinction in their specifications, and their examiners may say the same things in their reports, but those reports have not been read for this post, so the claim here is about AQA and nothing wider.