Several ways that peptides within proteins can react and be modified include the formation of covalent disulfide bonds and other weaker interactions between the R groups of amino acids in the peptide backbone.
Each amino acid is attached to its neighboring amino acid by a covalent bond, known as a peptide bond. During the formation of this peptide bond, the carboxyl (-COOH) group of one amino acid and the amino (-NH2) group of the other amino acid combine and release a molecule of water. Therefore, to break a peptide bond, it must be cleaved with the addition of a water, in a process called hydrolysis.
A disulfide bond between two cysteine amino acids (with thiol groups) can be formed in the presence of oyxgen. This oxidized, covalent, disulfide bond is known as a cystine.
Other interactions between the R groups of amino acids in a protein structure include ionic bonds, hydrogen bonds, and hydrophobic interactions. A section of folded protein showing several interactions which can occur within a protein, such as a disulfide bond, is shown here:
Practice Questions
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Key Points
• Amino acids are covalently bound by a peptide bond – carboxyl group of one to amino group of the other – which will release water.
• Hydrolysis (addition of water) can break a peptide bond.
• Two cysteines in a polypeptide can interact with each other to form a disulfide bond, known as a cystine.
Key Terms
Cysteine: amino acid with a thiol group (-SH)
Cystine: disulfide bond between two cysteine amino acids
Peptide bond: bond between two amino acids (-COOH group of one to -NH2 group of the other)
Hydrolysis: addition of water (for peptides, will break peptide bond)