Oligomer structure and functional regulation in cation pumps

自从Na/K-ATPase被发现以来，越来越多的证据表明，1摩尔的ATP水解酶是通过Na~(++)封闭的ADP敏感的磷酸酶、K~(2+)敏感的磷酸酶和K~(2+)封闭的酶发生的，同时伴随着3Na~+和2K~+的主动转运。然而，对于该酶的功能单位是αβ原构体还是更高的寡聚体，存在一些争议，这可能与连续或同时的运输机制有关。对寡聚体相互作用和酶的反应活性的详细研究，以及在有或没有三磷酸腺苷水解和其他情况下磷酸化程度与配体结合能力的比较等有力地表明，膜中酶的功能单元是四原构体，(αβ)_4。他们还提出，后阿尔伯斯方案的每个反应中间体，反映了中间体一半的位置性质，另一半与ATP结合。这些数据不仅有助于回答长期存在的机制是否在Na~(++)和K~(++)存在下起作用的问题，而且有助于更好地理解P型泵ATPase的机制。

Since the discovery of Na/K-ATPase, evidence has accumulated to suggest that 1 mol of ATP hydrolysis occurs via the Na^+-occluded ADP-sensitive phosphoenzyme, the K^+- sensitive phoshphoenzyme and the K^+-occluded enzyme accompanying active transport of 3Na^+ and 2K^+ according the Post-Albers sheme. However, some controversial issues have arisen concerning whether the functional unit of the enzyme is an αβ-protomer or a much higher oligomer, which would be related to the mechanism of transport, either sequential or simultaneous. Detailed studies of oligomer interaction and the reactivity of the enzyme and a comparison of the extent of phosphorylation with ligand-binding capacities in the presence or absence of ATP hydrolysis and others strongly suggest that the functional unit of the enzyme in the membrane is a tetraprotomer, (αβ)_4. They also suggest that each reaction intermediate of the Post-Albers scheme, respectively, reflects half of the site property of the intermediate and that another half binds ATP.These data may be useful not only to answer the long-standing question of whether the mechanism functions in the presence of both Na^+ and K^+ but also contribute to a better understanding of the mechanism of P-type pump ATPase in general.