Fig. 5. Catalysis-dependent mobilization of belt sulphurs.

(a) Relative electron densities of individual sulphurs or sulphur groups in various Av1 proteins or protein chains. The relative electron density (solid circle, triangle or rectangle) is expressed as the difference between the sigma level of an individual sulphur or sulphur group (Sσ) and the average sigma level of all sulphurs in the protein (average Sσ). The average densities of individual sulphurs or sulphur groups in all structures depicted in this graph (open blue circles) are expressed as mean ± s.e.m. (n=4 independent biological samples for Fe1 S and Cys/Met; and n=3 for Belt S). Designations: Belt S, sulphurs at the belt region of the M-cluster; Fe1 S, sulphurs ligated to Fe1 of the M-cluster; P, sulphurs in the P-clusters; Cys/Met, sulphurs in the Cys and Met residues. PDB entries: Av1*, 6UG0;¹⁸ Av1*(TOD), 6VXT;¹⁸ Av1*(TOS), 7MCI (this work) (b, c) GC-MS analysis of the release of the acid labile, cluster-bound ³⁴S²⁻ ions in Av1*(TOS) (b) and its isolated M-clusters (c) as H₂³⁴S. Shown are (1) H₂S standard, (2) Av1*(TOS), (3) Av1*(TOS) upon turnover with excess ³⁴SO₃²⁻ for 10 min [designated Av1*(TO³⁴S)], and (4) Av1*(TO³⁴S) upon turnover with excess unlabelled ³²SO₃²⁻ for 60 min [designated Av1*(TO³⁴S)^{60 min}]. H₂³⁴S was traced at m/z = 36. (d) ICP-OES determination of Se/Mo ratios of the M-clusters extracted from Av1*, Av1*(TOS), Av1*(TOSe) and ${\text{Av1*}\left(\text{TOSe}\right)}^{{\text{N}}_2}$. Data in d are expressed as mean ± s.d. (n=3 independent experiments).