title: Interdomain zinc site on human albumin.
abstract: Albumin is the major transport protein in blood for Zn(2+), a metal ion required for physiological processes and recruited by various drugs and toxins. However, the Zn(2+)-binding site(s) on albumin is ill-defined. We have analyzed the 18 x-ray crystal structures of human albumin in the PDB and identified a potential five-coordinate Zn site at the interface of domains I and II consisting of N ligands from His-67 and His-247 and O ligands from Asn-99, Asp-249, and H(2)O, which are the same amino acid ligands as those in the zinc enzymes calcineurin, endonucleotidase, and purple acid phosphatase. The site is preformed in unliganded apo-albumin and highly conserved in mammalian albumins. We have used (111)Cd NMR as a probe for Zn(2+) binding to recombinant human albumin. We show that His-67 --> Ala (His67Ala) mutation strongly perturbs Cd(2+) binding, whereas the mutations Cys34Ala, or His39Leu and Tyr84Phe (residues which may H-bond to Cys-34) have no effect. Weak Cl(-) binding to the fifth coordination site of Cd(2+) was demonstrated. Cd(2+) binding was dramatically affected by high fatty acid loading of albumin. Analysis of the x-ray structures suggests that fatty acid binding to site 2 triggers a spring-lock mechanism, which disengages the upper (His-67Asn-99) and lower (His-247Asp-249) halves of the metal site. These findings provide a possible mechanism whereby fatty acids (and perhaps other small molecules) could influence the transport and delivery of zinc in blood.
authors.affiliation: School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, United Kingdom.
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Stewart
authors.forename: Claudia A
authors.initials: CA
authors.lastname: Blindauer
authors.forename: Stephen
authors.initials: S
authors.lastname: Berezenko
authors.forename: Darrell
authors.initials: D
authors.lastname: Sleep
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Sadler
mesh_terms: D019943:Amino Acid Substitution; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D018360:Crystallography, X-Ray / Q000379:methods; D006639:Histidine; D006801:Humans; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012709:Serum Albumin / Q000737:chemistry* / Q000378:metabolism; D015032:Zinc / Q000737:chemistry / Q000378:metabolism*
chemical_list: D011994:Recombinant Proteins; D012709:Serum Albumin; D006639:Histidine; D015032:Zinc; D002118:Calcium