AP Biology · Free-response skill

Conceptual analysis: structure to function

The move. Describe a biological structure, explain how one of its properties causes its function, and predict the effect of a change to it.

How to do it

  1. Start with the structure. Say what it is made of and how its parts are arranged, using exact terms such as amino acid sequence, R group or hydrogen bond.
  2. Then name the property that matters, such as a charge, polarity or shape, and say how that property makes the function happen. Use a because sentence: the site holds the substrate because opposite charges attract.
  3. For a prediction, state the direction first: more, less or no change. Then give the reason by naming what the change does to the property you described.
  4. Keep each part to two or three sentences, and use the details given in the prompt, such as the position or charge of an amino acid.

Practice prompt Try it

Enzyme K (fictional) comes from a bacterium that lives in mud. Its single polypeptide chain is 260 amino acids long and folds into a shape with a pocket, the active site. The substrate of enzyme K has a negatively charged carboxyl group. Inside the pocket, the amino acid at position 88 has a positively charged R group that sits next to that carboxyl group when the substrate binds. In a mutant strain, the amino acid at position 88 is replaced by one with a nonpolar R group. The rest of the sequence is unchanged.

Using the information above, respond to each part.

  1. Describe how the order of amino acids in enzyme K leads to the shape of its active site.
  2. Explain how the charge on the R group at position 88 helps enzyme K hold its substrate.
  3. Predict how the change at position 88 in the mutant affects how well enzyme K holds its substrate, and explain your prediction using the property of the new R group.

Write each part, then check yourself against the checklist. Nothing you type here is saved or graded.

Check yourself and see a model answer

Checklist

Our own checklist of what a full answer does, one line per point. Tick each one your response earns.

0 of 4 points

Model answer, annotated

(a) The order of amino acids is the primary structure of enzyme K. That order decides where each R group sits along the chain, and the R groups form hydrogen bonds, ionic bonds and hydrophobic clusters with each other. These interactions fold the chain into its 3D shape, and the fold creates the pocket that is the active site.

  • Point 1 Ties the sequence to folding through R group interactions, and the folding to the active site.

(b) The R group at position 88 is positively charged, and the substrate's carboxyl group is negatively charged. Opposite charges attract, so this ionic attraction holds the substrate in place inside the active site.

  • Point 2 Names both charges, says they attract and links the attraction to holding the substrate.

(c) The mutant enzyme will hold its substrate less well. The new R group at position 88 is nonpolar, so it has no charge. Without a positive charge there is nothing to attract the substrate's negative carboxyl group, so the substrate binds more weakly or not at all.

  • Point 3 States the direction of the change first: weaker binding.
  • Point 4 Gives the reason from the new R group: no charge, so the attraction is lost.