def v_2(self, x):
        # Glucose-6-phosphate isomerase
        # G6P <=> F6P
        return (self.VmPGI*(-(x[3]/self.KeqPGI) + x[4]))/(self.KmPGIG6P*(1 + x[3]/self.KmPGIF6P + x[4]/self.KmPGIG6P))