Fig. 7. (A) Gillichthys muscle-type lactate dehydrogenase (A₄-LDH) monomer with Tyr residues (gray wire-frame) and `major mover' (dark gray ribbon) secondary structures labeled. Substrate and cofactor enter the active site through the opening to the left among the major movers, which close down to form the catalytic vacuole. (B) LDH-A dimer, showing major movers, as in A, and the positions of Tyr246 residues hydrogen-bonded to a trapped water molecule (gray sphere) in the intersubunit contact area. Helix 3G is shown as a white ribbon below helix 1G—2G and Tyr246. Structures were based on the homology model of dogfish A₄-LDH (Abad-Zapatero et al., 1987) and pig A₄-LDH (Dunn et al., 1991) produced using the SWISS-PROT program (Guex and Peitsch, 1997) and visualized using Rasmol.