Substrate-inhibition

In the oxidation of Dsorbitol and Dmannitol by potassium periodate in alkaline mediam, sub
strate inhibition was observed with both the substrates, i.e., a decrease in the rate of the reaction was observed
with an increase in the concentration of substrate. The substrate inhibition was attributed to the formation of
stable complex between the substrate and periodate. The reactions were found to be first order in case of peri
odate and a positive fractional order with hydroxide ions. Arrhenius parameters were calculated for the oxi
dation of sorbitol and mannitol by potassium periodate in alkali media.

This paper reintroduces the Wayman and Tseng model for representing substrate inhibition effects on speci®c growth rate by further documenting its potential predictive capabilities. It also introduces a modi-®cation to this model in which an Andrews inhibition function is used in place of the Monod noninhibitory substrate function. This modi®cation better represents the relationship between speci®c growth rate and substrate concentration for those substrates that show Andrews type inhibition at lower substrate concentrations, rather than the Monod type noninhibitory behavior described in the model of Wayman and Tseng. Results from nonlinear, least squares regression analysis are used to evaluate the ability of these models to empirically represent experimental data (both new and from the literature). The statistical goodness of ®t is evaluated by comparing the regression results against those obtained using other empirical models. Finally, possible mechanisms of toxicity responsible for the observed inhibition trends are used to further justify use of these empirical models. The dominant mechanism considered to be relevant for conceptually explaining complete inhibition at high concentrations of solvents is the deterioration of cell membrane integrity. Literature citations are used to support this argument. This work should lead to improvements in the mathematical modeling of contaminant fate and transport in the environment and in the simulation of microbial growth and organic compound biodegradation in engineered systems. ã 2001 John Wiley & Sons, Inc. Biotechnol Bioeng 75: 393±405, 2001.

X-ray crystal structures of dehaloperoxidasehemoglobin A (DHP A) from Amphitrite ornata soaked with substrate, 2,4,6-tribromophenol (2,4,6-TBP), in buffer solvent with added methanol (MeOH), 2-propanol (2-PrOH), and dimethyl sulfoxide (DMSO) reveal an internal substrate binding site deep in the distal pocket above the α-edge of the heme that is distinct from the previously determined internal inhibitor binding site. The peroxidase function of DHP A has most often been studied using 2,4,6-trichlorophenol (2,4,6-TCP) as a substrate analogue because of the low solubility of 2,4,6-TBP in an aqueous buffer solution. Previous studies at low substrate concentrations pointed to the binding of substrate 2,4,6-TCP at an external site near the exterior heme βor δ-edge as observed in the class of heme peroxidases. Here we report that the turnover frequencies of both substrates 2,4,6-TCP and 2,4,6-TBP deviate from Michaelis−Menten kinetics at high concentrations. The turnover frequency reaches a maximum in the range of 1400−1700 μM, with a decrease in rate at higher concentrations that is both substrate-and solvent-dependent. The X-ray crystal structure is consistent with the presence of an internal active site above the heme α-edge, in which the substrate would be oxidized in two consecutive steps inside the enzyme, followed by attack by H 2 O via a water channel in the protein. The physiological role of the internal site may involve interactions with any of a number of aromatic toxins found in benthic ecosystems where A. ornata resides.