These three tyrosines (Y363, Y118, and Y536) are directly involved in catalysis and are connected via water (HOH701) with a proton relay system composed of Y115, Y467, and Y466. Our bioinformatic analysis showed that these six tyrosines are highly conserved in the majority of the analysed KstD sequences (65%), while the second most common motif consists of only catalytic tyrosines (Y118, Y367, and Y536) encountered in 24% of the 82 analysed sequences (Figure and Table ). This type of active site organization without an extended proton relay system is present in the other known structure of KstD1 (Figure ).