residue
Summary
A residue is the unit of an amino acid that remains in a peptide or protein after the elements of water are lost in forming peptide bonds. The most testable point is that a residue's side chain (R group) usually determines its role in folding, charge, and function.
Detail
Meaning: When two amino acids join, a condensation (dehydration) reaction links the carboxyl group of one to the amino group of the other, forming an amide (peptide) bond and releasing one H₂O. Because each bond releases one H₂O, the units collectively lose the elements of water: an internal unit lacks an H from its amino group and an OH from its carboxyl group, while an end unit lacks just one of these. Each unit is called a residue, named for its parent amino acid.
On the MCAT: Passages ask about one residue, such as Asp52. Typical questions: a mutation swaps one residue for another (Glu → Val changes charge and polarity, so folding or binding may be disrupted); which residues sit in the core or on the surface; which can be phosphorylated (Ser, Thr, Tyr) or form disulfides (Cys). A peptide of n residues has n − 1 peptide bonds and releases n − 1 H₂O.
Common confusion: A residue is not a free amino acid. Internal α-amino and α-carboxyl groups are tied up in peptide bonds and no longer ionize, so charge at a given pH comes mainly from ionizable side chains plus the two termini.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Lehninger Principles of Biochemistry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.