Table 1.

Glossary and parameter estimates

ParameterDefinitionValueUnits
Model 1 (MM2_irrevers, M-M equations)
VmaxHMaximum rate that the enzyme catalyzes Hx → Xa1.836 ± 0.015μM/s
KmHDissociation constant for Hx-XO complexes3.68 ± 0.18μM
VmaxXMaximum rate that the enzyme catalyzes Xa → Ua1.96 ± 0.11μM/s
KmXDissociation constant for Xa-XO complexes5.95 ± 0.27μM
Model 2 (MMinhib, Escribano et al.'s M-M equations with Ua product inhibition)
VmaxHMaximum rate that the enzyme catalyzes Hx → Xa1.755 ± 0.01μM/s
KmHDissociation constant for Hx-Xo complexes2.46 ± 0.12μM
VmaxXMaximum rate that the enzyme catalyzes Xa → Ua1.95 ± 0.01μM/s
KmXDissociation constant for Xa-XO complexes4.1 ± 0.18μM
KiUDissociation constant for Ua-XO complexes178 ± 88μM
Model 3 (VfNet, reversible M-M equations)
VfmaxHMaximum forward rate catalyzing Hx → Xa1.79 ± 0.026μM/s
VbmaxHMaximum backward rate catalyzing Xa → Hx0.06 ± 36μM/s
KmHDissociation constant for Hx binding E (forward)2.47 ± 0.39μM
KpHDissociation constant for E for binding Xa → Hx19.1 ± 1e4μM
VfmaxXMaximum forward rate catalyzing Xa → Ua1.95 ± 0.015μM/s
VbmaxXMaximum backward rate catalyzing Ua → Xa0.86 ± 0.66μM/s
KmXDissociation constant for Xa binding E (forward)4.06 ± 0.60μM
KpXDissociation constant for E for binding Ua → Xa (reverse)244 ± 80μM
Model 4 (FullXO, reversible reactions with substrate and product binding to enzyme)
EtotTotal concentration of XO1.0μM
kfHForward reaction rate binding Hx to E100 ± 3.9e41·μM−1·s−1
KdHDissociation constant for H binding to E = kbH/kfH (forward)2.77 ± 2.4μM
kpfHRate of EHx → Xa, forward reaction forming Xa1.74 ± 0.0321/s
KpHDissociation constant for E for binding Xa → Hx (reverse)2,000 ± 65,000μM
kfXForward reaction rate binding Xa to E100 ± 5051·μM−1·s−1
KdXDissociation constant for Xa binding to E = kbX/kfX (forward)4.96 ± 0.82μM
kpfXRate of Exa → Ua, forward reaction forming Ua1.93 ± 0.0231/s
KpXDissociation constant for E for binding Ua → Xa (reverse)600 ± 370μM
Derived parameters
VfmaxHMaximum forward rate catalyzing Hx → Xa1.74 ± 0.04μM/s
VbmaxHMaximum backward rate catalyzing Xa → Hx277 ± 739μM/s
VfmaxXMaximum forward rate catalzying Xa → Ua1.93 ± 0.02μM/s
VbmaxXMaximum backward rate catalyzing Ua → Xa496 ± 146μM/s
KmHEffective Michaelis constant for Hx → Xa2.78 ± 1,200μM
KmXEffective Michaelis constant for Xa → Ua4.91 ± 150μM
kpbHReverse reaction rate for Xa + E → EHx8.7e−4 ± 1.6e−71·μM−1·s−1
kpbXReverse reaction rate for Ua + E → EXa0.0032 ± 2.5e−51·μM−1·s−1
  • Values are means ± SD. M-M, Michealis-Menten; Hx, hypoxanthine; Xa, xanthine; XO, xanthine oxidase; Ua, uric acid; E, free unbound XO.