TABLE 1.

Rate constants, isotope effects, and temperature dependences (ΔΔH^‡) of the hydride and proton transfer steps for the various POR enzymes

All rates were measured at 25 °C as described under “Experimental Procedures.”

	T. elongatus	Synechocystis sp. PCC6803	G. violaceus	Arabidopsis POR B
Hydride transfer
KH− (s−1 × 106)	1.87 ± 0.17	1.86 ± 0.18	1.86 ± 0.12	1.81 ± 0.24
KIE	1.91 ± 0.17	1.90 ± 0.16	1.94 ± 0.13	1.91 ± 0.23
ΔΔH‡ (kJ mol−1)a	8.2 ± 0.6	7.0 ± 0.3	6.9 ± 0.7	7.6 ± 0.9
ΔΔH‡ (kJ mol−1)b	(0.4 ± 1.0)	(0.4 ± 1.5)	(0.5 ± 1.2)	(0.3 ± 1.2)
Proton transfer
kH+ (s−1 × 103)	27.4 ± 1.0	30.4 ± 0.6	152.0 ± 5.7	123.3 ± 3.7
SIE	2.01 ± 0.14	1.80 ± 0.08	1.54 ± 0.07	1.50 ± 0.07
ΔΔH‡ (kJ mol−1)	15.6 ± 3.1	17.6 ± 3.7	2.6 ± 2.4	1.0 ± 1.9

^a These values of ΔΔH^‡ were calculated from the temperature dependence of the KIE measured above the observed breakpoint (−27 °C) (Fig. 3).

^b These values of ΔΔH^‡ were calculated from the temperature dependence of the KIE measured below the observed breakpoint (−27 °C) (Fig. 3).