(full paper is archived in the Miller Library)
Title: Effects of temperature, trimethylamine-N-oxide and urea on protein function and stability
Student Author(s): Ranchod, Tushar
Faculty Advisor(s): Somero, George
Location: Final papers Biology 175H
Date: June 1998
Abstract: The presence of trimethylamine N-oxide (TMAO) was recently discovered in the tissues of some Antarctic notothenioid fishes. This discovery raises the questions of whether the effects of TMAO on protein function vary with temperature and whether TMAO plays the same role in notothenioids as it does in elasmobranchs. Two temperature-sensitive kinetic properties - apparent Km of pyruvate and kcat - of muscle A4-LDH in Gillichthys mirabilis, a warm-temperature fish with negligible amounts of TMAO in its cells, and Parachaenichthys charcoti, an Antarctic fish, are not significantly affected by the presence of 150 mM TMAO at most temperatures from 0 degrees to 30 degrees C. The presence of TMAO did not appear to significantly affect the loss of activity in the LDH of any of the five teleost species studied over a period of 24 days. The Vmax of the LDH reaction in these species was slightly inhibited by 150 mM TMAO at both 0 degrees and 20 degrees C. TMAO may function by increasing the rigidity of the enzyme at hinge regions which play a role in establishing the rates of conformational change.
TMAO is known to counteract urea's perturbing effects on protein function in elasmobranchs at an approximately 2:1 [urea]:[TMAO] ratio. However, the effects of temperature on this counteraction had not been studied. Below 20 degrees C TMAO is able to fully counteract the denaturing effect of urea, while TMAO's ability to counteract urea at those same concentrations appears diminished at 30 degrees C or above. This varying counteracting effect, as observed on the Michaelis-Menten constant, appears to be consistent among species, including the subtropical cow-nosed ray (Rhinoptera bonasus), the temperate catshark (Parmaturus xaniurus) and the Antarctic skate (Bathyraja easoni), suggesting a lack of species-specific adaptation to solute effects.