AP Biology FRQ -- Polypeptides & Proteins

Question

1. Describe how a change in the primary structure of a polypeptide could affect the quaternary structure of an enzyme.

Answer 1

The primary structure of a protein is the amino acid sequence and the quaternary sturcture is the association of multiple protein subunits. The quaternary structure is dependent on the tertiary structure (3-D structure), which is dependent on the secondary structure (smaller structural motifs), which in turn is dependent on primary structure. A change in the primary structure will affect each level, including quaternary structure.

The charge and size characteristics of each amino acid side chain has an effect on how the polypeptide or protein folds to form secondary structures. A change in amino acids means the protein may not be able to form the same secondary structure. Differences in secondary structure impact the overall 3-D, or tertiary, structure of the protein. If the overall shape of the protein (tertiary structure) has changed, then how the proteins come together would be affected.

Answer 2

The four levels of protein organization are primary, secondary, tertiary, and quaternary structure. We know that a polypeptide is a chain of amino acids that form the primary structure of a protein and a protein's quaternary structure refers to the interactions between multiple polypeptide chains.

Enzymes are proteins that catalyze chemical reactions, and they are particularly sensitive to changes in protein structure.

 As stated, the primary structure of a polypeptide is the sequence of amino acids that are linked together by peptide bonds and this order of amino acids is essential because it determines how the polypeptide will fold into a functional protein. Even a small alteration in the primary structure of a protein can have a significant impact on its function. This is the age-old function/structure argument that is seen throughout biology. Proteins must maintain their shape to function properly, and they do so by folding into specific shapes that are determined by their primary structure. The quaternary structure of a protein is the arrangement of multiple polypeptide chains and enzymes are proteins that are made up of multiple polypeptide chains. Just like all proteins the arrangement of these chains is critical for enzyme function. Enzymes have an active site where substrates bind and catalytic activity occurs, and changes to the quaternary structure of an enzyme can alter the shape of the active site, rendering it less effective or entirely inactive. Changes in the primary structure of a protein can affect its quaternary structure because when a protein's primary structure changes, it can alter how the protein folds into its tertiary and quaternary structures. This can cause significant changes in the protein's function, especially if it affects the active site. This could affect the stability of the enzyme, its activity, and ultimately, its function.