Table 3.

pK_a values as output by PROPKA for the catalytic Cys and the nearby His predicted from the structure of the human Srx and the models of AtSrx based on the PDB entries 3cyi (C99S human Srx) and 1xw3 (human Srx)

Human Srx (1xw3)
Residue	pKa	Locate	Desolvation effects				Sidechain hydrogen bond			Backbone hydrogen bond
			Massive		Local
Cys 99	4.98	Surface	0.00	363	0.00	0	–1.46	Ser	55	–0.73	Val	56
							–1.50	Arg	51	–0.33	Cys	99
His 100	6.29	Surface	0.00	278	–0.21	3	0.00	–	0	0.00	–	0

Model of the tertiary structure of AtSrx using 1xw3 as template
Residue	pKa	Locate	Desolvation effects				Sidechain hydrogen bond			Backbone hydrogen bond
			Massive		Local
Cys 72	6.46	Surface	0.00	358	0.07	1	–1.60	Arg	28	–0.36	Thr	33
							0.00	–	0	–0.62	Cys	72
							0.00	–	0	–0.03	His	73
His 73	6.36	Surface	0.00	295	–0.14	2	0.00	–	0	0.00	–	0

Model of the tertiary structure of AtSrx using 3cyi as template
Residue	pKa	Locate	Desolvation effects				Sidechain hydrogen bond			Backbone hydrogen bond
			Massive		Local
Cys 72	4.29	Surface	0.00	337	0.14	2	–1.60	Thr	33	–0.81	Thr	33
							0.00	–	0	–0.78	Cys	72
							0.00	–	0	–1.67	His	73
His 73	7.22	Surface	0.00	293	0.07	1	0.79	Glu	76	0.00	–	0

The contribution of the pK_a shift is quantified in terms of three perturbations: desolvation, hydrogen bonding and charge-charge interactions.