Steric hindrance glutamate

However, trypsin can not hydrolyze the lysine-proline bond (probably a result of steric hindrance), and if a protein sequence contains adjacent basic residues (KK, KR, RK or RR) the resulting digestion products include single amino acids (K or R) that are difficult to detect. As a check on this and other possibilities, other proteases are used in addition to trypsin, e.g., Lys-C, Arg-C and Glu-C are proteases that hydrolyze peptide bonds on the C-terminal side of only lysine, arginine and glutamic acid, respectively. Cyanogen bromide chemically hydrolyses peptide bonds on the C-terminal side of methionine to form peptides with a homoserine residue (X = -CH2-CH2-OH) at the C-terminus. 

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