FIELDS, P.A.*; STROTHERS, C.; Franklin and Marshall College, Lancaster, PA; Franklin and Marshall College, Lancaster, PA: Muscle-type lactate dehydrogenase of the Galapagos marine iguana, Amblyrhynchus cristatus, is adapted to high temperature.

The Galapagos marine iguana, Amblyrhynchus cristatus, is unique among lizards in diving to forage. Air temperatures often exceed 40�C in the Galapagos; however, because of upwelling, sea surface temperatures can fall below 15�C. This temperature variability presents a challenge to the ectothermic A. cristatus, because it must be active in both environments. To determine whether this thermal variability has led to biochemical adaptation in A. cristatus, we compared kinetics and structure of the glycolytic enzyme muscle-type lactate dehydrogenase (A₄-LDH) in A. cristatus and the confamilial green iguana, Iguana iguana. In addition, we examined kinetics and structure of the TCA enzyme citrate synthase (CS) in both species, to determine whether there were differences in temperature adaptation between aerobic and anaerobic pathways. The Michaelis-Menten constant (K_m) for pyruvate of A. cristatus A₄-LDH was lower at every measurement temperature compared to the I. iguana ortholog, indicating adaptation to higher temperature in A. cristatus. In contrast, there was no difference in Arrhenius activation energy (Ea) between the two orthologs, and the A. cristatus A₄-LDH was less thermally stable than the I. iguana enzyme. CS showed no difference in K_m of oxaloacetic acid or Ea between the two species, suggesting this enzyme has not undergone temperature adaptation in A. cristatus. An alignment of the 333 a.a. LDH-A monomers reveals four substitutions, of which two are non-conservative; there are two non-conservative substitutions between the 469 a.a. CS orthologs. Our results suggest that selection has favored optimization of anaerobic function to the marine iguana�s warm terrestrial habitat, rather than to the cooler temperatures experienced during foraging dives.