| Comments |
"Another problem in reconciling H322 with its assigned
role in the cyclic process leading to H+-lactose symport is
that its proton turnover rate should match the turnover rate of
the symporter, i.e., 20–50 Hz (primary sources). Nachliel and Gutman
(39,40) have shown that protons diffuse rapidly within the
large, water-filled central cavity of LacY, and protonation
rates of H322, over a wide pH range, are two orders of
magnitude faster than the turnover rate of the symporter in
the presence or absence of lactose. This is discordant with a
symport model, which requires synchronized mechanical
linkages between the various stages of proton-lactose transit
and the conformational changes they evoke. It would seem
that the rapid rate of Brownian motion within the central
cavity militates against such a precise clockwork model." "The relatively slow rate of proton pumping of LacY
permease (turnover rate ~20–50 s^-1) (primary sources) is consistent
with the low probability of proton escape from the central
cavity. However, as seen above, this does not correlate with
the microscopic rates of deprotonation of H322 within the
central cavity." See BNID 104098,104176,105064 |