Video Transcript
Which of the following best explains why proteins differ in their structure and function? (A) Different proteins contain the same composition of amino acids that form the same polypeptide chain, but this chain folds in different ways. (B) Different proteins have different numbers and arrangements of amino acids in their polypeptide chains, which interact in different ways. (C) Each protein consists of only one type of amino acid joined into polypeptide chains, but this amino acid differs between proteins. Or (D) this statement is incorrect; all proteins have very similar structures and functions.
This question asks us to explain why different proteins exhibit different structures and functions. In order to answer this correctly, let’s think about the composition of proteins themselves. But first, let’s clear the answer choices so we have more room to work with.
You might recall that proteins are formed from amino acids that are linked via peptide bonds to form a polypeptide. Each protein contains a unique number and sequence of amino acids, which contributes to its distinct structure and function. With 20 different amino acids contributing to protein synthesis in humans, there are a lot of different combinations.
Consider this polypeptide, which is six amino acids long. If there are 20 different amino acids for each of these six positions, then we can multiply this together to see the total number of possible combinations. With 64 million different sequence combinations, we can appreciate that there’s a lot of possibilities. This polypeptide can then fold into a protein. Amino acids contribute to protein structure because they interact with one another to create 3D conformations. Recall the basic structure of an amino acid, and remember that each one has a different R group or side chain. These R groups can have different chemical structures and chemical properties that make these amino acids polar, nonpolar, or charged. Depending on the sequence of amino acids in a particular protein, these R groups will interact in different ways and contribute to different 3D conformations.
Now that we’ve discussed how amino acids can contribute to differences in protein structure and function, let’s bring back the answer choices to answer our question. The answer choice that best explains why proteins differ in their structure and function is “Different proteins have different numbers and arrangements of amino acids in their polypeptide chains, which interact in different ways.”
