There are several ways in which amino acids can be modified, combined, or separated, to augment their structure or function.

Amino acids are the building blocks for the proteins responsible for the biological functions within our body. Amino acids are chemical compounds consisting of a carbon atom bonded to an amine group, a hydrogen atom, a carboxylic group, and a varying side-chain (R group); it is this side chain that distinguishes each amino acid from another. Higher-ordered structures such as peptide chains and proteins are formed when amino acids bond to each other.

 

Peptide Linkage: Polypeptides and Proteins
A peptide is a molecule composed of two or more amino acids. The bond connecting together the two amino acids is a peptide bond. It occurs when the amino group of one amino acid nucleophilically attacks the carboxyl group carbon of another amino acid, linking the two molecules together and releasing a water molecule. In the final product, the amino and carboxyl group have been transformed into an amide functional group (carbon double bonded to an oxygen and single bonded to a nitrogen). Because this reaction releases a water molecule, it is a specific example of a condensation reaction. Peptide bond formation is endergonic (a process which requires energy), the energy for which typically comes from ATP. 

 

 

Hydrolysis
Long chain polypeptides can be formed by linking many amino acids to each other via peptide bonds. The peptide bond can only be broken by hydrolysis, where the bonds are cleaved with the addition of a water molecule.

Because this reaction is the reverse of peptide bond formation, it is exergonic (releases energy) and occurs spontaneously. Despite the fact that peptide bonds will spontaneously want to break down, the activation energy for this reaction is high enough that peptide bonds are metastable and will break down very slowly. Living organisms have enzymes that are capable of both forming and breaking peptide bonds.

 

Sulfur linkage for cysteine
A covalent disulfide bond (sulfur linkage) can form between the sulfur-containing R groups of two cysteine molecules (called sulfhydryl groups). Disulfide bonds between cysteine residues can affect protein folding and stability. Disulfide bonds form between cysteine residues under oxidizing (high pH) conditions. Disulfide bonds can be broken under reducing (low pH) conditions.

 

Practice Questions

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Key Points

• Amino acids are the basic building block of proteins; they are composed of a carbon atom attached to a hydrogen, a carbonyl group, an amine group, and an R group.
• Cysteines can interact with each other under oxidizing conditions to form a disulfide bond.
• Large proteins are formed by linking amino acids with peptide bonds. This occurs when the carboxylic group of one molecule reacts with the amino group of the other molecule, linking the two molecules and releasing a water molecule.
• Hydrolysis (the addition of a water molecule) can break a peptide bond.

Key Terms

• Cysteine: a sulfur-containing amino acid
• Condensation: any reaction where two molecules combine to form a single molecule, often releasing a water molecule as a byproduct
  
• Disulfide bond (also referred to as an S-S bond, disulfide bridge, or sulfur linkage): a covalent bond derived from two cysteine thiol groups
• Peptide bond (also called peptide linkage): a chemical bond formed between two amino acids when the amino group of one amino acid attacks the carboxyl group of another, releasing a molecule of water. This is an endergonic reaction.
  
• Hydrolysis: any reaction where a bond is broken due to the addition of water