Proteins and Enzymes

ID #1608

When discussing quaternary structure is found in proteins, what would be a better description: 1. With multiple polypeptide chains. (or) 2. Structure that changes over time. Why is chice 1 better than 2, as in the textbook on page 46, there is an example that uses hemoglobin to illustrate quaternary structure. "As hemoglobin binds one O2 molecule, its four subunits shift their relative positions slightly, changing the quaternary structure... The quaternary structure changes again when hemoglobin releases its O2 molecules to the cells of the boy." On the other hand, although quaternary structure contains multiple polypeptide chains, beta pleated sheets that are a part of secondary structure ALSO are formed from multiple polypeptide chains.

While it is true that a multi-subunit protein can change over time, this is not restricted to ONLY multi-subunit proteins. Plenty of single polypeptide chain proteins change when bound or modified, so the statement that quaternary structure is found in proteins which change over time is not the most accurate solution to the problem. By contrast, quaternary structure can ONLY be found in proteins with multiple polypeptide chains -- if it has only one chain, it does NOT have quaternary structure, period. So that's the only option that is specific, exclusive, and complete.

As for the beta pleated sheets, the regions of beta sheet that are part of a single polypeptide interacting with each other are part of just one chain, and if one beta sheet region interacts with the beta sheet region of another polypeptide chain, then the interactions between the two regions of secondary structure are actually what we'd call quaternary structure!

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