Part of the heterotrimer that catalyzes the conversion of D-gluconate to 2-dehydro-D-gluconate. A + D-gluconate = 2-dehydro-D-gluconate + AH2 Optimum pH is 5.0. Optimum temperature is 30 degrees Celsius. Heterotrimer. Binds 3 heme c groupd covalently per subunit.