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2013 ; 52
(9
): 1594-602
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Inverse solvent isotope effects arising from substrate triggering in the factor
inhibiting hypoxia inducible factor
#MMPMID23351038
Hangasky JA
; Saban E
; Knapp MJ
Biochemistry
2013[Mar]; 52
(9
): 1594-602
PMID23351038
show ga
Oxygen homeostasis plays a critical role in angiogenesis, erythropoiesis, and
cell metabolism. Oxygen homeostasis is set by the hypoxia inducible factor-1?
(HIF-1?) pathway, which is controlled by factor inhibiting HIF-1? (FIH). FIH is a
non-heme Fe(II), ?-ketoglutarate (?KG)-dependent dioxygenase that inhibits HIF-1?
by hydroxylating the C-terminal transactivation domain (CTAD) of HIF-1? at
HIF-Asn(803). A tight coupling between CTAD binding and O2 activation is
essential for hypoxia sensing, making changes in the coordination geometry of
Fe(II) upon CTAD encounter a crucial feature of this enzyme. Although the
consensus chemical mechanism for FIH proposes that CTAD binding triggers O2
activation by causing the Fe(II) cofactor to release an aquo ligand, experimental
evidence of this has been absent. More broadly, this proposed coordination change
at Fe(II) has not been observed during steady-state turnover in any ?KG oxygenase
to date. In this work, solvent isotope effects (SIEs) were used as a direct
mechanistic probe of substrate-triggered aquo release in FIH, as inverse SIEs
(SIE < 1) are signatures for pre-equilibrium aquo release from metal ions. Our
mechanistic studies of FIH have revealed inverse solvent isotope effects in the
steady-state rate constants at limiting concentrations of CTAD or ?KG
[(D2O)kcat/KM(CTAD) = 0.40 ± 0.07, and (D2O)kcat/KM(?KG) = 0.32 ± 0.08],
providing direct evidence of aquo release during steady-state turnover.
Furthermore, the SIE at saturating concentrations of CTAD and ?KG was inverse
((D2O)kcat = 0.51 ± 0.07), indicating that aquo release occurs after CTAD binds.
The inverse kinetic SIEs observed in the steady state for FIH can be explained by
a strong Fe-OH2 bond. The stable Fe-OH2 bond plays an important part in FIH's
regulatory role over O2 homeostasis in humans and points toward a strategy for
tightly coupling O2 activation with CTAD hydroxylation that relies on substrate
triggering.