Catalyzes the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate and CO(2), with concomitant reduction of NAD to NADH. 6-phospho-D-gluconate + NAD(+) = CO2 + D-ribulose 5-phosphate + NADH The catalytic efficiency with NAD(+) is about 2000-fold higher than with NADP(+), indicating that NAD(+) is the physiological electron acceptor. Optimum pH is 10.5. Carbohydrate degradation; pentose phosphate pathway. Homotetramer. Deletion mutants do not grow on glucose. Belongs to the 6-phosphogluconate dehydrogenase family.