Tryptophan contains a five membered ring and a six membered ring on its side group. Tyrosine and phenylalanine both contain one six membered ring. Histidine contains a five membered ring on its side chain. Threonine's R-group is , and does not have a ring structure.

Because proline has part of its R-group side chain attached to its amino group, it has a unique structure that differs from all other amino acids. The ring structure that it forms with its amino groups causes it to be unable to be found in alpha helix protein structures - if it were to be in one the structure would be disrupted. However, proline is important in kinks and turns because of its small, unique structure.

Amino acids can be identified by their full names, three-letter codes, and one-letter symbols. Glutamine has the three-letter code "gln" and the one-letter symbol "Q." Glutamic acid has the three-letter code "glu" and the one-letter symbol "E." Lysine has the three-letter code "lys" and the one-letter symbol "K." Aspartic acid has the three-letter code "asp" and the one-letter symbol "D."

The structure of this amino acid classifies it as histidine.

Considering that the protein was run through a cation exchange column, one would expect that the more positively charged proteins would flow very slowly through the column. A cation exchange column is coated with negatively charged beads that would become attracted to the positively charged elements of the protein, causing them to elute out of the column slowly. Lysine and Arginine are positively charged amino acids that would cause this to happen, and a protein rich in these amino acids would elute slowly when run through a cation exchange column.

Amino acids can be identified by their three-letter codes and one-letter symbols. Tryptophan has the one-letter symbol "W" and the three-letter code "Trp." Glutamine has the one-letter symbol "Q" and the three-letter code "Gln." Aspartic acid has the one-letter symbol "D" and the three-letter code "Asp." Lastly, lysine has the one-letter symbol "K" and the three-letter code "Lys."

Cysteine and methionine are the only two amino acids that are incorporated into proteins which contain sulfur. However, only cysteine can form disulfide bonds due to its free SH group. Methionine does not have a free SH group and thus cannot form these bonds.

Beta-pleated sheets and alpha helical conformation are types of protein secondary structures. Recall that secondary structures of proteins are formed through the formation of hydrogen bonds and disulfide bonds between adjacent amino acids in the polypeptide chain. Converting one type of secondary structure to another involves breaking these hydrogen and disulfide bonds. Disulfide bonds between amino acids occur between cysteine amino acids. Hydrogen bonds occur between a hydrogen atom on one amino acid and either a fluorine, oxygen or nitrogen atom on another amino acid. Hydrogen bonds can be formed between any amino acids because all amino acids have the essential atoms required for hydrogen bonds; therefore, both amino acids listed are involved in bonds that are broken during the conversion of one secondary structure to another.

There are three main branched amino acids that accumulate in a patient with MSUD. These are isoleucine, leucine and valine. Accumulation of these branched amino acids occurs because of depletion of enzymes essential for their breakdown. The breakdown products of branched amino acids are used for several processes, including energy production via glycolysis and oxidative phosphorylation. Accumulation of these amino acids is toxic and is often fatal to a child with MSUD.

A transmembrane protein consists of two hydrophilic regions (that are found on the ends facing the cytoplasmic and extracellular sides) and one hydrophobic region (inserted into the hydrophobic interior of the phospholipid bilayer). The question states that the region is saturated with valine amino acid residues. Recall that valine is a hydrophobic amino acid; therefore, the region containing lots of valine residues must be the hydrophobic region of the transmembrane protein.