title: Biosynthetic incorporation of tryptophan analogues into staphylococcal nuclease: effect of 5-hydroxytryptophan and 7-azatryptophan on structure and stability.
abstract: 5-Hydroxytryptophan (5HW) and 7-azatryptophan (7AW) are analogue of tryptophan that potentially can be incorporated biosynthetically into proteins and used as spectroscopic probes for studying protein-DNA and protein-protein complexes. The utility of these probes will depend on the extent to which they can be incorporated and the demonstration that they cause minimal perturbation of a protein's structure and stability. To investigate these factors in a model protein, we have incorporated 5HW and 7AW biosynthetically into staphylococcal nuclease A, using a trp auxotroph Escherichia coli expression system containing the temperature-sensitive lambda cI repressor, Both tryptophan analogues are incorporated into the protein with good efficiency. From analysis of absorption spectra, we estimate approximately 95% incorporation of 5HW into position 140 of nuclease, and we estimate approximately 98% incorporation of 7AW, CD spectra of the nuclease variants are similar to that of the tryptophan-containing protein, indicating that the degree of secondary structure is not changed by the tryptophan analogues. Steady-state fluorescence data show emission maxima of 338 nm for 5HW-containing nuclease and 355 nm for 7AW-containing nuclease. Time-resolved fluorescence intensity and anisotropy measurements indicate that the incorporated 5HW residue, like tryptophan at position 140, has a dominant rotational correlation time that is approximately the value expected for global rotation of the protein. Guanidine-hydrochloride-induced unfolding studies show the unfolding transition to be two-state for 5HW-containing protein, with a free energy change for unfolding that is equal to that of the tryptophan-containing protein. In contrast, the guanidine-hydrochloride-induced unfolding of 7AW-containing nuclease appears to show a non-two-state transition, with the apparent stability of the protein being less than that of the tryptophan form.
authors.affiliation: Department of Chemistry, University of Mississippi, University 38677, USA.
authors.forename: C Y
authors.initials: CY
authors.lastname: Wong
authors.forename: M R
authors.initials: MR
authors.lastname: Eftink
mesh_terms: D006916:5-Hydroxytryptophan / Q000737:chemistry*; D004795:Enzyme Stability; D008836:Micrococcal Nuclease / Q000737:chemistry*; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D014364:Tryptophan / Q000031:analogs & derivatives* / Q000737:chemistry*
chemical_list: C006657:7-azatryptophan; D014364:Tryptophan; D006916:5-Hydroxytryptophan; D008836:Micrococcal Nuclease