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Steroid Ring Hydroxylation Patterns Govern Cooperativity in Human Bile Acid Binding Protein†
journal contribution
posted on 2003-09-20, 00:00 authored by Gregory P. Tochtrop, Jamie L. Bruns, Changguo Tang, Douglas F. Covey, David P. CistolaHuman ileal bile acid binding protein (I-BABP) is a member of the intracellular lipid binding
protein family. This protein is thought to function in the transcellular transport and enterohepatic circulation
of bile salts. Human I-BABP binds two molecules of glycocholate, the physiologically most abundant
bile salt, with modest intrinsic affinity but a remarkably high degree of positive cooperativity. Here we
report a calorimetric analysis for the binding of a broad panel of bile salts to human I-BABP. The interaction
of I-BABP with nine physiologically relevant derivatives of cholic acid, chenodeoxycholic acid, and
deoxycholic acid in their conjugated (glycine and taurine) and unconjugated forms was monitored by
isothermal titration calorimetry. All bile salts bound to I-BABP with a 2:1 stoichiometry and similar
overall affinity, but the derivatives of cholic acid displayed much higher Hill coefficients, a measure of
macroscopic positive cooperativity. To test whether the cooperativity was dependent on individual structural
features of the bile salt side chain, a series of side-chain-extended bile salts that lacked a hydrogen bond
donor or acceptor at C-24 were chemically synthesized. These synthetic variants exhibited the same
energetic and cooperativity profile as the naturally occurring bile salts. Our findings indicate that
cooperativity in bile salt−I-BABP recognition is governed by the pattern of steroid B- and C-ring
hydroxylation and not the presence or type of side-chain conjugation.