This was marked wrong please help One of the amino adds form

This was marked wrong please help.

One of the amino adds forms covalent bonds with another of its kind elsewhere in protein chains. This property is used to stiffen protein structures, especially those that will be sent into harsh environments outside the cell. Which amino acid is it? Select one: cysteine tryptophan threonine lysine asparagine

Solution

The answer is

a. cysteine

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms.

Cysteine is a semi-essential proteinogenic amino acid. The thiol side chain in cysteine is susceptible to oxidization to give the disulfide derivative cystine, which serves an important structural role in many proteins.

Most cysteine residues are covalently bonded to other cysteine residues to form disulfide bonds.

Cysteine residues play a valuable role by crosslinking proteins, which increases the rigidity of proteins and also functions to confer proteolytic resistance (since protein export is a costly process, minimizing its necessity is advantageous). Inside the cell, disulfide bridges between cysteine residues within a polypeptide support the protein\'s tertiary structure.