We study a general asymmetric spin-boson model for electron transfer processes in the nonadiabatic regime. Our path-integral approach reduces to the conventional Marcus result in the high-temperature limit, whereas quantum effects lead to distinct deviations from classical behaviors at low temperatures. Adopting a simple but realistic model spectral density for the environmental modes, we show that for low temperatures, the rate becomes sensitive to the details of the spectral density. In particular, due to nuclear tunneling, the rate maximum does not correspond to the usual classical estimate for the activationless case and the rate is asymmetric around the maximum. A centroid factorization using a properly defined nuclear reaction coordinate provides new insight into the mechanism of nonadiabatic electron transfer.