Coordination of Zn2+ (and Cd2+) by prokaryotic metallothionein. Involvement of his-imidazole.
In mammalian metallothionein Zn2+ is exclusively coordinated to Cys-thiolate to form clusters in which the metal is thermodynamically stable but also kinetically labile. By contrast, little is known about coordination to prokaryotic metallothionein, SmtA. 3 nmol of Zn2+ nmol-1 SmtA were displaced by 8 nmol of p-(hydroxymercuri)phenylsulfonate implicating eight of the nine Cys in the coordination of three metal ions. None of the Zn2+ associated with SmtA was accessible to 4-(2-pyridylazo)resorcinol prior to the addition of p-(hydroxymercuri)phenylsulfonate. An unusual feature of SmtA is the presence of three His residues, and we have investigated whether these contribute to metal coordination. Less Zn2+ was associated with purified SmtA(H40R/H49R/H55R), in which all three His residues were substituted with Arg, and approximately one equivalent of Zn2+ was immediately accessible to 4-(2-pyridylazo)resorcinol. Following incubation of SmtA with 111Cd, three 111Cd resonances were detected, two in a range expected for CdS4 and the third indicative of either CdNS3 or CdN2S2 coordination. Two-dimensional TOCSY 1H NMR and 111Cd-edited 1H NMR showed two His residues bound to 111Cd, confirming CdN2S2 coordination. The pH of half-dissociation of Zn2+ increased from 4.05 for SmtA to 5.37 for SmtA(H40R/H49R/H55R). Equivalent values for single His mutants SmtA(H40R), SmtA(H49R), and SmtA(H55R) were 4.62, 4.48, and 3.81, respectively, revealing that conversion of His40 or His49 to Arg impairs Zn2+ binding at the CdN2S2 and CdS4 sites. Only approximately two equivalents of Zn2+ were associated with purified SmtA(H49R). The appearance of a fourth 111Cd resonance at lower pH suggests that an alternative CdN2S2 site also exists.
Department of Biochemistry and Genetics, The Medical School, University of Newcastle, NE2 4HH, United Kingdom.
M J
MJ
Daniels
J S
JS
Turner-Cavet
R
R
Selkirk
H
H
Sun
J A
JA
Parkinson
P J
PJ
Sadler
N J
NJ
Robinson
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002104:Cadmium / Q000737:chemistry / Q000378:metabolism*; D002413:Cations, Divalent / Q000737:chemistry / Q000378:metabolism; D019915:DNA Primase / Q000737:chemistry; D006639:Histidine; D008668:Metallothionein / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D019906:Nuclear Magnetic Resonance, Biomolecular; D011387:Prokaryotic Cells; D011522:Protons; D011993:Recombinant Fusion Proteins; D015032:Zinc / Q000737:chemistry / Q000378:metabolism*
D001426:Bacterial Proteins; D002413:Cations, Divalent; D011522:Protons; D011993:Recombinant Fusion Proteins; C080048:smtA protein, Synechococcus; D002104:Cadmium; D006639:Histidine; D008668:Metallothionein; D019915:DNA Primase; D015032:Zinc