Interdomain Zinc Site on Human Albumin

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 100; no. 7; pp. 3701 - 3706
• Main Authors: Stewart, Alan J., Blindauer, Claudia A., Berezenko, Stephen, Sleep, Darrell, Sadler, Peter J.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 01.04.2003; National Acad Sciences; The National Academy of Sciences
• Series: Bioinorganic Chemistry Special Feature
• Subjects: Albumins; Amino Acid Substitution; Amino acids; Atoms; Binding Sites; Biochemistry; Bioinorganic Chemistry Special Feature; Calcium - metabolism; Caprylates; Chlorides; Crystallography, X-Ray - methods; Fatty acids; Histidine; Humans; Kinetics; Ligands; Magnetic Resonance Spectroscopy; Models, Molecular; Mutagenesis, Site-Directed; Physical Sciences; Protein Structure, Secondary; Proteins; Recombinant Proteins - chemistry; Recombinant Proteins - metabolism; Serum Albumin - chemistry; Serum Albumin - metabolism; Serum albumins; Zinc; Zinc - chemistry; Zinc - metabolism
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0436576100

Albumin is the major transport protein in blood for Zn2+, a metal ion required for physiological processes and recruited by various drugs and toxins. However, the Zn2+-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 H2O, 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 111Cd NMR as a probe for Zn2+ binding to recombinant human albumin. We show that His-67 → Ala (His67Ala) mutation strongly perturbs Cd2+ 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 Cd2+ was demonstrated. Cd2+ 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-67/Asn-99) and lower (His-247/Asp-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.