| Third , peptide analogs of cdc 2 ( 6 20 ) with single substitutions of amino acid residues Lys 9 , Glu 12 , Thr 14 , Gly 16 , Val 18 , and Tyr 19 by amino acid residues at corresponding positions of rsk were synthesized and tested as spleen tyrosine kinase substrates . ^^^ It was determined either by the kinetic parameters ( Km and Vmax ) of phosphorylation of the peptide or by the initial phosphorylation rate of the peptide by the spleen tyrosine kinase . ) An analog with double substitution at Glu 12 An analog with double substitution at Glu 12 and Thr 14 was found to be almost as poor a substrate as the rsk peptide . ^^^ |