Dr. Appleby is distinguished for his contributions to the understanding of hemoprotein structure and function. He purified and crystallized cytochrome b2, showed it to be a flavohemoprotein, and determined its reaction mechanism. His major study on soybean leghemoglobin and Rhizobium cytochromes provided the foundations for elucidating the energetic processes of nitrogen fixation. He showed that nitrogen-fixing rhizobia respire efficiently at low levels of oxygen, that leghemoglobin has unique kinetic properties which permit rapid transport of oxygen to the respiratory oxidases at these low levels of oxygen, and that the mobility of the distal histidine within an unusually flexible oxygen-binding pocket of leghemoglobin accounts for these properties.