Table 1.

Mean single molecule parameters

[ATP]	d, nm		ton, ms
	pH 7.4	pH 6.4	pH 7.4	pH 6.4
0.5 μM	N/A	2±2	N/A	200±114
1 μM	10±2	2±1*	178±67	119±22
10 μM	11±2	6±5	21±31	70±31*
1 mM	10†	9±3	10†	31±9

The values displayed, obtained by mean variance analysis, are the means ± SD from 4 to 10 single molecule data records, each 1–3 min in length, containing hundreds of events. d, unitary step size of myosin; t_on, strong binding variable; N/A, not applicable.

^†Extrapolated or estimated value from Baker et al. (3).

^*Significantly different (P < 0.05) from the value at the corresponding [ATP] at pH 7.4. The data were analyzed using a two-way ANOVA (pH × ATP) with a Tukey-HSD post hoc test to locate specific differences. A subset of data using smooth muscle myosin at 1 mM ATP showed that d was unaffected by low pH (9 nm at pH 7.4 vs. 9 nm at pH 6.4), whereas t_on was threefold longer (27 ms at pH 7.4 vs. 100 ms at pH 6.4), consistent with the observations in skeletal myosin.