A comprehensive model for the steady state behavior of xanthine oxidase is proposed. The model includes 36 species which account for all the possible levels of reduction and binding states for the enzyme. A set of seven intrinsic reactions are modulated by the fraction of available reaction center to provide 160 macroscopic rates between these species. Unlike previous steady-state models, our model includes features known from rapid kinetic experiments such as the formation of a bound product species and the ability of the enzyme to react three times sequentially with substrate under anaerobic conditions. The complex reoxidation reactions producing either hydrogen peroxide or superoxide radical depending on the overall state of reduction of the enzyme are also accounted for in our model. Measured room temperature mid-point potentials are used to predict the fraction of available reaction center according to the accepted rapid-equilibrium model and literature values for the intrinsic reactions are used when available. We are able to predict product turnover and superoxide turnover for both xanthine and lumazine using our model.
The reductive rapid kinetic reaction of xanthine oxidase with lumazine has been proposed to follow a simple three step reaction sequence of substrate binding, bound-product formation and product release. Absorbance changes during the first turnover with lumazine can be simulated using the three step mechanism. Molar absorbances for the intermediates are obtained from the transient kinetics of substoichiometric lumazine with enzyme. The observed molar absorbance at 650 nm for two electron enzyme with violapterin bound is much larger than expected if only Mo(IV) forms the charge transfer band. To verify that Mo(V) contributes to the charge transfer band at 650 nm, optical and epr potentiometric titrations can be performed with enzyme in excess violapterin. Midpoint potentials for molybdenum with violapterin bound are determined from the epr potentiometric titration. The Mo(VI)/Mo(V) and Mo(V)/Mo(IV) half-reactions are $-$352 and $-$347 mV, respectively. Fitting the optical potentiometric titration data, the molar absorbance of the Mo(V) and Mo(IV) species with violapterin bound are 9.23 and 8.53 mM$\sp{-1}$cm$\sp{-1}$ at 650 nm, respectively.
| Identifer | oai:union.ndltd.org:RICE/oai:scholarship.rice.edu:1911/16510 |
| Date | January 1992 |
| Creators | Howarth, Jack Wayne |
| Source Sets | Rice University |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | application/pdf |
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