An inquiry into the source of stereospecificity of lactate dehydrogenase using substrate analogues and molecular modeling.

Lactate dehydrogenase catalyzes the stereospecific hydride transfer to and from the re face of the nicotinamide coenzyme. The demonstrated probability of transfer to the si face of less than 2 x 10(-8) indicates that the free energy of any diastereotopic transition state leading to a si transfer must be over 10 kcal/mol greater than the free energy for transfer to or from the re face. The general notion of closed, desolvated active sites suggests the a priori hypothesis that steric hindrance prevents the nicotinamide ring from assuming a conformation that would lead to transfer of the pro-S hydrogen. In this paper we report that the probability of transfer of the pro-S proton is less than 9 x 10(-7) with 3-pyridinealdehyde adenine dinucleotide as coenzyme and less than 4 x 10(-7) during the lactate dehydrogenase catalyzed disproportionation of glyoxylate. Examination of the crystal structure of lactate dehydrogenase further suggests that steric exclusion does not enforce the extreme stereospecificity of the reaction. An electrostatic interaction with the macrodipole associated with the alpha 2F helix is suggested as a potential molecular source of the stereospecificity.     

The energy cost of echolocation in pipistrelle bats (Pipistrettus pipistrellus)

Summary A positive relationship was established between energy expenditure and pulse rate of echolocation for 8 pipistrelle bats (Pipistrellus pipistrellus) when hanging at rest in a respirometry chamber at 28 °C. The least squares fit equation: Energy expenditure (J·^–1·h^–1)=110.09+ 40.3 pulse rate (n/s) explained 14% of the minute by minute variation in energy expenditure. For a 6 g bat therefore each pulse costs approximately 0.067 Joules to produce. The net cost of echolocation at 10 pulses per second for a 6 g pipistrelle bat was predicted to be 9.5 × BMR with a range of 7.0–12.2 × BMR. We suggest that since a major portion of the cost of echolocation may result from contraction of the pectoralis and scapularis groups of muscles, the cost of echolocation is reduced for flying animals which contract these muscles anyway during flight. This may account for the high incidence of echolocation systems amongst flying vertebrates, when compared with terrestrial species.     

Trophic interactions in soils as they affect energy and nutrient dynamics. IV. Flows of metabolic and biomass carbon

Flows of biomass and respiratory carbon were studied in a series of propylene-oxide sterilized soil microcosms. One-half of the microcosms received three pulsed additions of 200 ppm glucose-carbon to mimic rhizosphere carbon inputs. Biotic variables were: bacteria (Pseudomonas) alone, or amoebae (Acanthamoeba) and nematodes (Mesodiplogaster) singly, or both combined in the presence of bacteria.Over the 24-day experiment, respiration was significantly higher in the microcosms containing the bacterial grazers. Biomass accumulation by amoebae was significantly higher than that by nematodes. The nematodes respired up to 30-fold more CO₂ per unit biomass than did amoebae. Similar amounts of carbon flowed into both respiratory and biomass carbon in microcosms with fauna, compared with the bacteria-alone microcosms. However, partitioning of available carbon by the microfauna varied considerably, with little biomass production and relatively more CO₂-C produced in the nematode-containing microcosms. The amoebae, in contrast, allocated more carbon to tissue production (about 40% assimilation efficiency) and correspondingly less to CO₂.