title: Molecular mechanism of the drop in the pKa of a substrate analog bound to medium-chain acyl-CoA dehydrogenase: implications for substrate activation.
abstract: The pKa value of a substrate analogue 3-thiaoctanoyl-CoA at alphaC-H is known to drop from ca. 16 in the free state to 5-6 upon binding to medium-chain acyl-CoA dehydrogenase (MCAD). The molecular mechanism underlying this phenomenon was investigated by taking advantage of artificial FADs, i.e., 8-CN-, 7,8-Cl2-, 8-Cl-, 8-OCH3-, 8-NH2-, ribityl-2'-deoxy-8-CN-, and ribityl-2'-deoxy-8-Cl-FADs, reconstituted into MCAD. The stronger the electron-withdrawing ability of the substituent, the smaller the pKa value became [e.g., 7.4 (8-NH2-FAD) and 4.0 (8-CN-FAD)], suggesting that the flavin ring itself affects the pKa value of the ligand via a charge-transfer interaction with the ligand. The destruction of the hydrogen bond between the thioester C(1)=O and the ribityl-2'-OH of FAD raised the pKa by ca. 2.5 units. These results indicate that the interaction between the ligand and the flavin ring also serves to lower the pKa of the ligand, in addition to the hydrogen bonds at C(1)=O of the ligand.
authors.affiliation: Department of Molecular Physiology, Graduate School of Medical Sciences, Kumamoto University, Honjo, Kumamoto 860-8556. nishina@medic.kumamoto-u.ac.jp
authors.forename: Yasuzo
authors.initials: Y
authors.lastname: Nishina
authors.forename: Kyosuke
authors.initials: K
authors.lastname: Sato
authors.forename: Haruhiko
authors.initials: H
authors.lastname: Tamaoki
authors.forename: Takeyuki
authors.initials: T
authors.lastname: Tanaka
authors.forename: Chiaki
authors.initials: C
authors.lastname: Setoyama
authors.forename: Retsu
authors.initials: R
authors.lastname: Miura
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Shiga
mesh_terms: D000214:Acyl Coenzyme A / Q000737:chemistry / Q000378:metabolism*; D042964:Acyl-CoA Dehydrogenase / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D002384:Catalysis; D004789:Enzyme Activation; D005182:Flavin-Adenine Dinucleotide / Q000737:chemistry; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D011485:Protein Binding; D013053:Spectrophotometry; D013379:Substrate Specificity; D013552:Swine
chemical_list: C097160:3-thia-octanoyl-coenzyme A; D000214:Acyl Coenzyme A; D005182:Flavin-Adenine Dinucleotide; D042964:Acyl-CoA Dehydrogenase