A protein motif (aka supersecondary structure) is a defined arrangement of secondary structures within a protein. It is commonly occurring enough to have an identified structure. An example would be the beta-alpha-beta loop. While the arrangement is made up of secondary structure, the overall motif itself can be considered supersecondary or possibly even tertiary, though its components are secondary structures. Motif's do not necessarily have a defined function across different proteins. Protein domains on the other hand, do.

Hydrogen bonds stabilize interactions among the amide and carboxyl groups in the main chain of the polypeptide. These interactions may induce the formation of alpha-helices and/or beta-pleated sheets.

脯氨酸is bound to two alkyl groups thus giving it a planar configuration, giving the nitrogen only the ability to accept hydrogen bonds not donate them. While this is not a problem at the beginning of an alpha helix this can disturb the bonds if place further down the chain. Thus proline is often referred to as the "alpha helix buster."

Alpha helices and beta sheet, the dominant secondary structural motifs in polypeptides are formed by hydrogen bonds between the carbonyl and amino groups of the amino acid backbone.

In an antiparallel beta sheet, the hydrogen bonding angle is 180 degrees and optimal; this is the most stable angle. In parallel sheets, it is a less stable 150 degrees. Whether a sheet is parallel or antiparallel does not tell us anything about what amino acids it is composed of, so each of the other answers is incorrect.

In an alpha helix, hydrogen bonding occurs every four amino acids, starting from the 1st binding to the 4th in the sequence; the 2nd amino acid binds to the 6th, the 3rd to the 7th, and so on.

Alpha helices and beta pleated sheets are two forms of secondary structure. Alpha helices can be either right handed (counterclockwise) or left handed (clockwise). Beta pleated sheets can be either parallel (amino and carbonyl groups do not line up) or anti parallel (amino and carbonyl groups line up).

Glycine and proline are the two amino acids that are found in beta turns. These 180 degree turns are composed of four total amino acids.

All the answer choices are different examples of protein supersecondary structures. Beta barrels are commonly found in transmembrane porin proteins.

Covalent bonding is when two nonmetals share electrons in order to form a bond. This type of bonding can be observed in the primary (peptide bonds), tertiary (disulfide bonds), and quaternary (disulfide bonds) levels of protein structure. The secondary structure of proteins only uses hydrogen bonding as the folding force.