Energy coupling to K+ transport in Paracoccus denitrificans.

Paracoccus denitrificans requires potassium for normal growth and transports this cation by at least two systems, one with low (Km approximately 1 to 2 mM) and another with high (Km approximately 0.1 microM) affinity. Neither of the two systems seems to be dependent on periplasmic components since each retains full activity in cells subjected to an osmotic shock. P. denitrificans accumulates potassium at high velocity (270 nmol of K+/min/mg dry weight of cells) and against a large concentration gradient. The intracellular concentration of K+ in media of high osmolarity (about 320 mosmol) is 0.4 M; this gives a concentration gradient [K+]i/[K+]e of greater than or equal to 2 X 10(4). The uptake of potassium against its concentration gradient requires a source of energy and is eliminated by the addition of uncouplers. The increased rate of energy usage for potassium transport results in an increased rate of ATP synthesis by the respiratory chain and is expressed in enhanced rates of respiration and substrate utilization. The stimulation of respiration is accompanied by increased steady state reduction level of the components of the respiratory chain. The calculations show that two K+ are most likely to be transported per one ATP hydrolyzed.