Glutamate is an acidic amino acid. At a pH below 2.2 (acidic conditions), the side chain, the amino group, and the carboxyl backbone would all be protonated (+1 net charge). At a pH above 9.7 (basic conditions), all of those groups would be deprotonated (negative 2 net charge).  

In the situation we are given, the pH is 3, which is above the , but below the . The carboxyl group on the backbone is therefore deprotonated, but not the carboxyl side chain. The amino backbone also remains protonated in these still acidic conditions. Overall, the net charge must then be zero.

To simplify this problem, remember that at any pH above the  of a given group (basic conditions), that group will be deprotonated. On the other hand, at any pH below the  of a given group (acid conditions), that group will be protonated.

Methionine and cysteine are the only amino acids that contain sulfur. 

Only two cysteine amino acids can come together to form a disulfide bridge. This disulfide bridge is a covalent bond. Disulfide bridges are a component of tertiary structure in proteins. As a side note, the name of a cysteine amino acid changes to cystine after a disulfide bridge is formed. 

Only glycine is nonpolar. The rest of the answer choices are polar. Remember that being nonpolar implies that the substance is hydrophobic and, hence, insoluble in water. Besides methionine, you will notice that the rest of the nonpolar and nonaromatic amino acids have R-groups composed solely of carbon and hydrogen. Polarity of amino acids is quite important when looking at the tertiary, or three dimensional, structure of a protein. 

There are only three aromatic amino acids. They are phenylalanine, tyrosine, and tryptophan. Histidine has a positively charged R-group. Proline is a nonpolar amino acid. Serine is a polar amino acid. Threonine is a polar amino acid. 

The  value of the amino groups in amino acids are around 9. On the other hand, the  value of the carboxyl groups of amino acids are around 2. All biologically relevant amino acids are indeed found only in the L stereoisomer formation. There are three aromatic amino acids: phenylalanine, tyrosine, and tryptophan. All amino acids are joined together with a planar, rigid peptide bond. Glycine is the only achiral amino acid. This is due to the hydrogen found in the R-group. 

Quarternary structure is the association of multiple polypeptide chains into a functional protein. Not all proteins have this level of protein structure. Subunits within the quarternary structure are held together by noncovalent bonds and disulfide bonds.

A peptide bond is the chemical bond formed between two amino acids to create a polypeptide. The carboxyl group of one amino acid reacts with the amino group of the other amino acid, releasing a molecule of water. This is known as a dehydration reaction.

Glucose can be stored as glycogen, mostly in the liver, and fatty acids are stored in adipose cells. Amino acids, though, can not be stored for future anabolic use in the human body. Therefore, any time there is an excess of amino acids, they are broken down into their constituent parts so that they may be recycled into other molecules.

An essential amino acid is one that can not be synthesized by the human body, and so it must be consumed in the diet. Tyrosine can be synthesized, and is therefore not considered to be an essential amino acid.