However, tyrosine and tryptophan also can form hydrogen bonds. Cysteine can form disulfide bonds with another cysteine. The peptide bond is a specialized form of amide bond. The reaction is thermodynamically unfavorable. Both in nature and in the laboratory, the reaction is driven by using activated derivatives of the amino acids whose reactivity is much stronger.
Some modifications are reversible and others are permanent.
Many modifications are listed here, but they shall not be covered in detail until later in the course. The structures of a few key modifications are shown in VVP, p.90. Proteolytic cleavage associated with protein sorting. The targeting sequences are usually removed by proteolytic cleavage within the organelle. Name the four amino acids.