Cox 11 Inhibits HKI Activity and the LD of RanBP2 Reverses the Inhibition of Cox11 over HKI
To probe whether the interaction of Cox11 and HKI with the LD of RanBP2 modulates the enzymatic activity of HKI, we first examined the effect of increasing concentrations of Cox11 on the initial rates of HKI enzymatic activity (Figure 2A). Cox11 strongly inhibits HKI activity in a concentration-dependent fashion, and at ~15 nM of Cox11, HKI activity could not be recorded (Figure 2A). Cox11 behaves as a partial noncompetitive inhibitor of HKI by affecting the V max of HKI for glucose (Figure 2B). Then, we evaluated the effect of the LD of RanBP2 on the HK activity in the presence of a fixed inhibitory concentration of Cox11, saturating concentration of glucose substrate, and increasing concentrations of LD. As shown in Figure 2C, the LD domain sharply reversed the inhibitory effect of Cox11 on HKI activity in a concentration-dependent manner, but under saturating (and stochiometric) amounts of LD, the velocity of the reaction did not reach that observed for HKI activity in the absence of Cox11 (Figure 2A), suggesting the LD by itself may also have an effect on HKI activity. Indeed, a saturating concentration of LD reduced the V max but not the K m of HKI (Figure 2D) by ~20% under similar conditions.
Figure 2  Effect of Cox11 and RanBP2 on HKI Activity
(A) Saturation kinetics, rate versus glucose of HKI (0.24 μg) in the absence (solid circles) and presence of Cox11 (open circles, 0.25 nM; solid triangles, 7.5 nM). The activity of HKI decreases with increasing concentrations of Cox11. No measurable HKI activity was recorded in the presence of 15 nM of Cox11 (unpublished data).
(B) Hanes-Wolf plot of (A) (1/rate versus glucose) in the absence and presence of fixed concentrations of Cox11. Linearity of reciprocal plots also supported the hyperbolic behavior of the reactions (unpublished data). Cox11 behaves as a noncompetitive inhibitor of HKI by reducing the V max of HKI but not its K m toward glucose.
(C) HKI rate is plotted as a function of LD concentration at saturating glucose and fixed Cox11 (7.5 nM) concentrations. Note that increasing concentrations of the LD of RanBP2 reverse the inhibition of HKI activity by Cox11. A half-maximal effect of the LD of RanBP2 on HKI activity in the presence of 7.5 nM of Cox11 was observed at a concentration of ~0.05 nM of LD.
(D) Rate versus glucose plot in the absence and presence of the LD of RanBP2. At a saturating concentration of the LD of RanBP2 (3.75 nM), the HKI activity was reduced by about 20%. v, rate; S, glucose.