Expansion
Nonpolar, polar, acidic and basic, determined by the R group
Expansion
- Nonpolar (hydrophobic): glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline. Cluster in the core of soluble proteins and in membrane-spanning regions
- Polar uncharged: serine, threonine, cysteine, tyrosine, asparagine, glutamine. Serine, threonine and tyrosine are the targets of phosphorylation
- Acidic (negatively charged): aspartate and glutamate
- Basic (positively charged): lysine, arginine and histidine
Special residues worth knowing individually:
- Glycine: smallest, so it provides flexibility and is obligatory at every third position in collagen
- Proline: its ring introduces a kink and breaks alpha helices
- Cysteine: forms disulphide bridges, stabilising tertiary structure
- Histidine: its pKa near physiological pH makes it the key buffering residue in haemoglobin and a common catalytic residue
The distribution of charge determines the isoelectric point, which is exploited in electrophoresis, and explains why a single charge-changing substitution such as glutamate to valine in sickle cell disease alters solubility so dramatically.