Proteins and Enzymes

ID #2381

So the side-chain R groups are attached to the C-terminus (which is always negatively charged) and never to the always positively charged N-terminus? Also, the N-terminus isn't always on the left and the C-terminus isn't always on the right?


The sidechains are always connected to the alpha carbon, labelled Ca in your lecture slides. The order of a single amino acid is: Amino group - alpha carbon - carboxylic acid group. The only exception (of sorts) is proline, which forms a ring that is connected to both the alpha carbon and the amino group.
The C-terminus and the N-terminus are not always charged - though they will be charged at a normal physiological pH.  If the pH of the surrounding solution is suitably acidic or basic, these groups may lose their charge. Margaret explained this a few posts above, and Professor Mehrtens also mentioned how it is best to be careful of "always" statements in biology.
In most of the figures you will see in the lecture notes, the N-terminus is on the left and the C-terminus is on the right. However, in the cell, proteins are folded into more complex levels of organization and may be rotated, so you should not always assume that the left-most portion of a drawn protein is the amino terminus unless it is labelled.

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