collected from the bacterial fleece of one A. pompejana specimen from the East
Pacific Rise (9°N). Using standard gene insertion techniques, they created a
laboratory bacterium that could generate those enzymes in sufficient quantity to
extract for in vitro characterization of the effect of temperature on the rates of the
reactions that they catalyze. The most dramatic result (Fig. 15.6) was for glutamate
dehydrogenase, a metabolic enzyme common and essential to all cells. Lee et al.
measured the substrate conversion rate at 40°C, then pulsed the temperature to a much
higher level and measured the rate over time. Initially, the rate was progressively
higher for all temperatures up to 90°C, then fell off over 10 min for temperatures
above 60°C. The longer-term increase in rates was less than a doubling for an increase
from 40° to 75°C, when most enzyme reaction rates would increase ∼10-fold. Lee et
al. propose that at least some enzymes of such thermophilic organisms have two
forms in a shifting equilibrium: active and inactive-undamaged. At sufficiently high
temperature, the inactive form will progress at some rate to a denatured
(unrecoverable) form. As the temperature rises, more of the enzyme takes the inactive
form, stabilizing the overall reaction rate as the substrate turnover by the active form
accelerates. A full description of the chemical kinetics of this scheme predicts the
form of the rate response to temperature. Since the bacterial surface symbionts have
evolved this mode of temperature regulation, perhaps something similar has evolved
in the worms. Certainly some quite extraordinary enzymology exists to allow A.
pompejana to survive temperatures and rapid temperature variation well outside the
tolerances of most animals.
Fig. 15.6 Substrate conversion rate of glutamate dehydrogenase enzyme constructed
using its gene taken from an epsilonproteobacterial strain episymbiotic with Alvinella
pompejana. The black line shows rates immediately after raising temperature from
40°C to that on the abscissa. The dashed line shows rates after 10 min at that higher
temperature.
(^) (After Lee et al. 2008.)
(^) Sexes are separate in pompeii worms, and, unlike related annelids, the females have
sperm storage chambers (spermathecae). Both sexes have a single pair of gonoducts