title: Binding of lipoic acid induces conformational change and appearance of a new binding site in methylglyoxal modified serum albumin.
abstract: The binding of lipoic acid (LA), to methylglyoxal (MG) modified BSA was studied using isothermal titration calorimetry in combination with enzyme kinetics and molecular modelling. The binding of LA to BSA was sequential with two sites, one with higher binding constant and another comparatively lower. In contrast the modified protein showed three sequential binding sites with a reduction in affinity at the high affinity binding site by a factor of 10. CD results show appreciable changes in conformation of the modified protein as a result of binding to LA. The inhibition of esterase like activity of BSA by LA revealed that it binds to site II in domain III of BSA. The pH dependence of esterase activity of native BSA indicated a catalytic group with a pK(a) = 7.9 +/- 0.1, assigned to Tyr411 with the conjugate base stabilised by interaction with Arg410. Upon modification by MG, this pK(a) increased to 8.13. A complex obtained by docking of LA to BSA and BSA in which Arg410 is modified to hydroimidazolone showed that the long hydrocarbon chain of lipoic acid sits in a cavity different from the one observed for unmodified BSA. The molecular electrostatic potential showed that the modification of Arg410 reduced the positive electrostatic potential around the protein-binding site. Thus it can be concluded that the modification of BSA by MG resulted in altered ligand binding characteristics due to changes in the internal geometry and electrostatic potential at the binding site.
authors.affiliation: Department of Life Sciences, University of Mumbai, Santacruz (E), Mumbai 400098, India.
authors.forename: George
authors.initials: G
authors.lastname: Suji
authors.forename: Santosh A
authors.initials: SA
authors.lastname: Khedkar
authors.forename: Sreelekha K
authors.initials: SK
authors.lastname: Singh
authors.forename: Nand
authors.initials: N
authors.lastname: Kishore
authors.forename: Evans C
authors.initials: EC
authors.lastname: Coutinho
authors.forename: Vikrant M
authors.initials: VM
authors.lastname: Bhor
authors.forename: S
authors.initials: S
authors.lastname: Sivakami
mesh_terms: D001665:Binding Sites / Q000187:drug effects; D002151:Calorimetry; D002942:Circular Dichroism; D004950:Esterases / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D009596:Nitrophenols / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation / Q000187:drug effects; D011765:Pyruvaldehyde / Q000737:chemistry / Q000494:pharmacology*; D012710:Serum Albumin, Bovine / Q000187:drug effects / Q000378:metabolism*; D008063:Thioctic Acid / Q000378:metabolism*
chemical_list: D008024:Ligands; D009596:Nitrophenols; D012710:Serum Albumin, Bovine; D011765:Pyruvaldehyde; D008063:Thioctic Acid; C008642:4-nitrophenyl acetate; D004950:Esterases