Strong hydrogen bonding between Glu78 and C4-OH has been proposed to cause an elongation of the C5-O7 bond, stabilizing the transition state in B. subtilis. In P. Aeruginosa, the backbone carbonyl group of Pro48 serves a similar function, as it acts as a hydrogen bond acceptor in forming a hydrogen bond with C4-OH. However, the hydrogen bond formed in PchB is at a somewhat unfavorable angle for this interaction, thus suggesting why the enthalpic effect is greater in BsCM where the anionic oxygen is also stabilized by the amide group of Cys75. This is supported by mutagenesis studies showing that mutations of this glutamate to residues reducing this interaction result in a reduction of the catalytic rate. Furthermore, the elongation of this C5-O7 ether bond by Glu78/Cys75 (in BsCM) or Pro48 (in PchB) results in a large negative charge on the ether oxygen. In BsCM, Arg90 provides a positive charge to counterbalance the large negative charge on this ether oxygen. Lys41 in PchB also provides a positive to stabilize the negative charge on this oxygen. To ensure proper formation of the prephenate product, Arg7 and Tyr108 in BsCM form hydrogen bonds with the carboxylate group in the pyruvate moiety of the transition state.