The magnetic circular dichroism (MCD) spectrum of the metalloenzyme plastocyanin is calculated using time-dependent density functional theory. The calculation reproduces most of the main features of the experimental spectrum. The calculated spectrum is analyzed to elucidate the contributions to the spectrum and the source of the MCD intensity. As expected, the temperature-dependent MCD intensity is found to be dominant. More surprisingly, contributions from the ground-state mixing with excited states are found to be significant.