Question

two polypeptide chains have the same amino acid composition and molecular weight, yet only one binds to a cell membrane receptor. several possible scenarios have been listed to explain these observations. which is the only plausible one? one chain has bond linkages connecting amino acids that differ from the bond linkages in the other chain. one chain is longer than the other chain, causing the polypeptides to fold into different shapes. the two chains fold into different three - dimensional shapes because the sequences of amino acids differ in the chains. one chain was synthesized in a laboratory, while the other was synthesized in a bacterium, causing differences in the final structures of the proteins. the two chains fold into different three - dimensional shapes because the polypeptides were made by two different organisms.

Explanation:

The function of a polypeptide is determined by its three - dimensional shape which is in turn determined by the amino - acid sequence. Even with the same amino - acid composition and molecular weight, different amino - acid sequences can lead to different three - dimensional folds and thus different functions (such as binding to a receptor). Bond linkages between amino acids are usually peptide bonds in polypeptides. If they have the same composition and weight, one can't be longer than the other. The place of synthesis (lab or bacterium) or the organism making them doesn't directly explain the difference in receptor - binding if composition and weight are the same.

Answer:

The two chains fold into different three - dimensional shapes because the sequences of amino acids differ in the chains.