An Overview of Drug Binding to Human Serum Albumin: Protein Folding and Unfolding

Human Serum Albumin (HSA) is a principal extracellular protein with a high concentration in blood plasma and a carrier of many drugs to different molecular targets. Drug binding to HSA can alter the protein biophysical and biochemical properties of protein. The structural analysis of human serum albumin complexes, with naturally occurring flavonoids quercetin(antioxidant), kaempferol (antioxidant), delphinidin (antioxidant), AZT (3'-azido-3'-deoxythymidine) (anti-AIDS), aspirin (anti inflammatory), taxol (anticancer), cisplatin (anticancer), atrazine (herbicide), 2,4-D (herbicide), polyamines (biogenic), chlorophyll (antimutagenic), chlorophyllin (antitumor), poly(ethylene glycol) (polymer), vandyl cation and vanadate anion in aqueous solution are reported. Using capillary electrophoresis, FTIR (Fourier transform infrared), UV-Visible and CD (Circular dichroism) spectroscopic methods, the drug binding mode, the binding constant and the effects of drug complexation on protein secondary structure are determined. The concentrations of HSA used were 0.6 to 0.3 mM, while different drug concentrations were 1~$\mu$M to 1 mM. Structural analysis showed drugs are mostly located along the polypeptide chains, with both specific and non-specific interactions. The stability of drug-HSA complexes were in the order: K_VO^2+=1.2\times 10^8M^-1 > K_AZT=1.9 \times 10^6M^-1>K_del=4.7\times 10^5M^-1>K_PEG=4.1\times 10^5M^-1>K_kae=2.6\times 10^5M^-1>K_que=1.4\times 10^5M^-1>K_atrazine=3.5\times 10^4M^-1>K_chlorophyll=2.9\times 10^4M^-1>K_2,4-D=2.5\times 10^4M^-1>K_spermine=1.7\times 10^4M^-1>K_taxol=1.43\times 10^4M^-1>K_aspirin=1.04\times 10^4M^-1>K_chlorophyllin=7.0\times 10^3M^-1>K_VO3^-=6.0\times 10^3M^-1>K_spermidine=5.4\times 10^3M^{-1}>K_putrescine=3.9\times 10^3M^1>K_cisplatin=1.2\times 10^2M^-1. At low drug concentration (1 \muM), protein conformation was not altered (infrared and CD results), while, at high drug content(1 mM), a major reduction of \alpha-helix from 60-55% (free HSA) to 49-40% and an increase of \beta-structure from 22-15% (free HSA) to 33-23% in the drug-protein complexes occurred. These observations indicated that low drug content induced protein stabilization (folding), whereas, at high drug concentration, a partial protein destabilization (unfolding)occurred in these drug-HSA complexes.