Some examples of endogenous ligands
Aliphatic fatty acids. Crystallographic analysis has revealed the existence of 7 common sites for medium and long-chain fatty acids (C10:0 – C18:0); C10:0 and C12:0 – C14:0 has 3 and 1 additional binding sites, respectively (Bhattacharya, A.A., Grüne, T. & Curry, S. (2000). Crystallographic analysis reveals common modes of binding of medium and long-chain fatty acids to human serum albumin. J. Mol. Biol. 303, 721-732). Further studies showed that also the unsaturated fatty acids C18:1 and C20:4 bind to the 7 common sites (Petitpas, I., Grüne, T., Bhattacharya, A.A. & Curry, S. (2001). Crystal structures of human serum albumin complexed with monounsaturated and polyunsaturated fatty acids. J. Mol. Biol. 314, 955-960). Binding of C8:0 – C14:0 has been studied using recombinant and genetic variants of the protein (Kragh-Hansen, U., Watanabe, H., Nakajou, K., Iwao, Y. & Otagiri, M. (2006). Chain length-dependent binding of fatty acid anions to human serum albumin studied by site-directed mutagenesis. J. Mol. Biol. 363, 702-712). Binding of fatty acids, enriched with 13C, has been studied with NMR, and more recent with 2D NMR, spectroscopy, see the review of Hamilton, J.A. [(2013). NMR reveals molecular interactions and dynamics of fatty acid binding to albumin. Biochim. Biophys. Acta 1830, 5418-5426]. Finally, fatty acid binding, and its effects on the albumin molecule, have been examined using molecular simulation approaches such as docking, molecular dynamics and binding free energy calculations as reviewed by Fujiwara, S.-i. & Amisaki, T. [(2013). Fatty acid binding to serum albumin: molecular simulation approaches. Biochim. Biophys. Acta 1830, 5427-5434].
Bilirubin and hemin. Crystallographic analysis of ligand-human serum albumin crystals have revealed high-affinity binding of bilirubin (and fusidic acid) to a preformed pocket in subdomain IB of the protein (Zunszain, P.A., Ghuman, J., McDonagh, A.F. & Curry, S. (2008). Crystallographic analysis of human serum albumin complexed with 4Z,15E-Bilirubin-IXα. J. Mol. Biol. 381, 394-406). Hemin binds to principally the same pocket in albumin (Wardell, M., Wang, Z., Ho, J.X., Robert, J., Ruker, F., Ruble, J. & Carter, D.C. (2002). The atomic structure of human methemalbumin at 1.9 Å. Biochem. Biophys. Res. Commun. 291, 813-819 and Zunszain, P.A., Ghuman, J., Komatsu, T., Tsuchida, E. & Curry, S. (2003). Crystal structural analysis of human serum albumin complexed with hemin and fatty acid. BMC Struct. Biol. 3, 6). Modified complexes of heme and albumin can function as artificial O2-carriers (Tsuchida, E., Sou, K., Nakagawa, A., Sakai, H., Komatsu, T. & Kobayashi, K. (2009). Artificial oxygen carriers, hemoglobin vesicles and albumin-hemes, based on bioconjugate chemistry. Bioconjug. Chem. 20, 1419-1440).
Thyroid hormones. Abnormalities of thyroid hormone transport proteins may produce alterations in the concentration of iodothyronines in serum. Familial dysalbuminemic hyperthyroxinemia (FDH-T4) is a relatively common cause of euthyroid hyperthyroxinemia, and it is characterized by greater elevation in serum total thyroxine (T4) than triiodothyronine (T3) concentration. Consistent with its dominant inheritance, the disorder is mostly associated with heterozygous albumin gene defects, generating mutant proteins with enhanced T4 binding affinity. The mutation 218Arg→His is the most common causal variant, but the condition can also be caused by the presence of the variants 218Arg→Pro or 218Arg→Ser. In addition to the mutations in position 218, a mutation in position 222 (Arg→Ile) has been found to cause FDH-T4. Very recently, the first example of a homozygous person with FDH-T4, caused by the mutation 218Arg→His, has been published (Mimoto, M.S., Karaca, A., Scherberg, N., Dumitrescu, A.M. & Refetoff, S. (2018) Homozygous mutation in human serum albumin and its implication on thyroid tests. Thyroid, in press). A fifth mutation is caused by the substitution of the normal leucine-66 with a proline (66Leu→Pro), and it results in a variant albumin with 40-fold higher affinity for T3 but only 1.5-fold increase in the affinity for T4. This condition is also dominantly inherited and presents clinically as familial dysalbuminemic hypertriiodothyroninemia (FDH-T3). So far, this condition has only been found in its heterozygous form. For more information, see the recent review by Kragh-Hansen, U., Galliano, M. & Minchiotti L. (BOOKS, SPECIAL ISSUE AND REVIEWS) and GENETIC VARIANTS OF HUMAN SERUM ALBUMIN and the corresponding REFERENCE LIST.
Retinol and retinoic acid. These vitamin A components bind to albumin with association constants in the order of 1-3 × 105 M-1 (N’soukpoé-Kossi, C.N., Sedaghat-Herati, R., Ragi, C., Hotchandani, S. & Tajmir-Riahi, H.A.(2007). Retinol and retinoic acid bind human serum albumin: stability and structural features. Int. J. Biol. Macromol. 40, 484-490).
Uremic toxins. Hippurate (104 M-1), indole acetate (105 M-1), indoxyl sulfate (106 M-1) and 3-carboxy-4-methyl-5-propyl-2-furanpropionate (107 M-1) bind to human serum albumin with the association constants given in the parenthesis (Sakai, T., Takadate, A. & Otagiri, M. (1995). Characterization of binding site of uremic toxins on human serum albumin. Biol. Pharm. Bull. 18, 1755-1761). Crystallographic analysis has shown that the latter compound binds in subdomain IIA (Sudlows site I) (Ghuman, J., Zunszain, P.A., Petitpas, I., Bhattacharya, A.A., Otagiri, M. & Curry, S. (2005). Structural basis of the drug-binding specificity of human serum albumin. J. Mol. Biol. 353, 38-52).
Binding to 34Cys. Thiol-containing endogenous compounds such as L-cysteine and homocysteine, metal ions like Ag+, Hg2+ and Au+ as well as several drugs can interact covalently with the sulfhydryl group of 34Cys. For a review, see Kragh-Hansen, U., Chuang, V.T.G. & Otagiri, M. (2002). Practical aspects of the ligand-binding and enzymatic properties of human serum albumin. Biol. Pharm. Bull. 24, 695-704.
Homocysteinylation of albumin. Homocysteine thiolactone, formed from methionyl-tRNA, and S-nitrosohomocysteine, formed in endothelial cells, link covalently to blood proteins, particularly hemoglobin and albumin. The e-nitrogen of 525Lys, and not 34Cys, is the predominant site of attachment to albumin. The presence of a mixed disulfide at 34Cys, alb-S-S-Cys, accelerates the attachment (Glowacki, R. & Jakubowski, H. (2004). Cross-talk between Cys34 and lysine residues in human serum albumin revealed by N-homocysteinylation. J. Biol. Chem. 279, 10864-10871).
Nitric oxide (NO). This ubiquitous molecule can bind to 34Cys, and other sulfhydryl groups if present, and thereby increase its biological half-life very much. S-nitrosylation of albumin is enhanced by simultaneous binding of oleate, bilirubin or Cu2+ but not by L-tryptophan, progesterone, ascorbate, Zn2+ or Fe2+. For a recent review, see Ishima, Y., Kragh-Hansen, U., Maruyama, T. & Otagiri, M. (2013). Poly-S-nitrosated albumin as a safe and effective multifunctional antitumor agent: characterization, biochemistry and possible future therapeutic applications. BioMed Res. Int. Article ID 353892.
Cu2+ and Ni2+. Most albumins bind Cu2+ and Ni2+ at their N-terminal Asp-X-His site with high affinity. According to competitive UV-vis spectroscopy, the binding constant for Cu2+ is as high as 1012 M-1 (Rózga, M., Sokolowska, M., Protas, A.M. & Bal, W. (2007). Human serum albumin coordinates Cu(II) at its N-terminal binding site with 1 pM affinity. J. Biol. Inorg. Chem. 12, 913-918).
Transition metal ions. In addition to Cu2+ and Ni2+, Zn2+, Co2+ and Cd2+ can bind to albumin. So far, a total of 4 metal ion-binding sites on human serum albumin have been described, and their localization, metal ion preference, stability constants and other aspects have been reviewed by Bal, W., Sokolowska, M., Kurowska, E. & Faller, P. (2013). Binding of transition metal ions to albumin: sites, affinities and rates. Biochim. Biophys. Acta 1830, 5444-5455. Zn2+ binds at a five-coordinate site between domains I and II, and its interplay with fatty acid binding has been reviewed by Barnett, J.P., Blindauer, C.A., Kassaar, O., Khazaipoul, S., Martin, E.M., Sadler, P.J. & Stewart, A.J. (2013). Allosteric modulation of zinc speciation by fatty acids. Biochim. Biophys. Acta 1830, 5456-5464.
Some examples of exogenous ligands
Binding of bacteria. Bacteria, including human pathogens, containing a GA module (protein G-related albumin-binding module) on its surface can bind to helices 2,3,7 and 8 of domain II of the albumin molecule. In addition to conferring increased virulence and growth of the bacteria, binding is thought to aid in the capture of albumin-bound nutrients such as fatty acids (Lejon, S., Frick, I.-M., Björck, L., Wikström, M. & Svensson, S. (2004). Crystal structure and biological implications of a bacterial albumin binding module in complex with human serum albumin. J. Biol. Chem. 279, 42924-42928).
Resveratrol. According to a fluorescence spectroscopic study, trans-resveratrol, a popular antioxidant found in red wines, binds to bovine serum albumin with an association constant in the order of 105 M-1 (Xiao, J.B., Chen, X.Q., Jiang, X.Y., Hilczer, M. & Tachiya, M. (2008). Probing the interaction of trans-resveratrol with bovine serum albumin: a fluorescence quenching study with Tachiya model. J. Fluoresc. 18, 671-678). Binding of resveratrol and two other polyphenolic compounds, genistein and curcumin, to bovine serum albumin caused a partial unfolding of its structure as shown in a reduction in α-helix and increase in β-sheet and turn structure (Bourassa, P., Kanakis, C.D., Tarantilis, P., Pollissiou, M.G. & Tajmir-Riahi, H.A. (2010). Resveratrol, genistein, and curcumin bind bovine serum albumin. J. Phys. Chem. B 114, 3348-3354).
Noble gases. According to computational docking simulation, the anesthetics xenon, krypton and argon bind to the enflurane binding site of human serum albumin, whereas the non-anesthetics neon and helium bind to sites different from the xenon binding site. Binding of the former was dominated by van der Waals energy, while binding of the latter was dominated by solvent-effect energy. Binding site preference was determined by the ratios of local binding energy (van der Waals energy) and nonspecific binding energy (solvent-effect energy) to the total binding energy. Van der Waals energy dominance was found to be necessary for anesthetic binding (Seto, T., Isogai, H., Ozaki, M. & Nosaka, S. (2008). Noble gas binding to human serum albumin using docking simulation: nonimmobilizers and anesthetics bind to different sites. Anesth. Analg. 107, 1223-1228).
Human serum albumin is able to stereoselectively bind a great number of various endogenous and exogenous compounds. For a review, see Chuang, V.T.G. & Otagiri, M. (2006). Stereoselective binding to human serum albumin. Chirality 18, 159-166.
For original articles, see for example:
Ghuman, J., Zunszain, P.A., Petitpas, I., Bhattacharya, A.A., Otagiri, M. & Curry, S. (2005). Structural basis of the drug-binding specificity of human serum albumin. J. Mol. Biol. 353, 38-52.
Wang, Z.-m., Ho, J.X., Ruble, J.R., Rose, J., Rüker, F., Ellenburg, M., Murphy, R., Click, J., Soistman, E., Wilkerson, L. & Carter, D.C. (2013). Structural studies of several clinically important oncology drugs in complex with human serum albumin. Biochim. Biophys. Acta 1830, 5356-5374.
For reviews, see for example:
Kragh-Hansen, U., Chuang, V.T.G. & Otagiri, M. (2002). Practical aspects of the ligand-binding and enzymatic properties of human serum albumin. Biol. Pharm. Bull. 25, 695-704.
Otagiri, M. (2005). A molecular functional study on the interactions of drugs with plasma proteins. Drug Metab. Pharmacokinet. 20, 309-323.
Fanali, G., di Masi, A., Trezza, V., Marino, M., Fasano, M. & Ascenzi, P. (2012). Human serum albumin: from bench to bedside. Mol. Aspects Med. 33, 209-290.
The Worldwide Protein Data Bank (www.wwpdb.org) has the structure of 106 different ligand-human serum albumin complexes (accessed 21 august, 2014).