Exciton chirality of bilirubin homologs

Boiadjiev, Stefan E.; Lightner, David A.

doi:10.1002/(sici)1520-636x(1997)9:5/6<604::aid-chir31>3.0.co;2-q

Cited by 14 publications

(4 citation statements)

References 18 publications

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“…4). In aqueous solutions, bilirubin exists as an equimolar mixture of two intramolecularly hydrogen-bonded conformational isomers of P-and M-helicity referred to as "ridge tile" structures (36,37) (Fig. 4).…”

Section: Resultsmentioning

confidence: 99%

A Dynamic Model for Bilirubin Binding to Human Serum Albumin

Petersen¹,

Ha²,

Harohalli³

et al. 2000

Journal of Biological Chemistry

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Site-directed mutagenesis of human serum albumin was used to study the role of various amino acid residues in bilirubin binding. A comparison of thermodynamic, proteolytic, and x-ray crystallographic data from previous studies allowed a small number of amino acid residues in subdomain 2A to be selected as targets for substitution. The following recombinant human serum albumin species were synthesized in the yeast species Pichia pastoris: K195M, K199M, F211V, W214L, R218M, R222M, H242V, R257M, and wild type human serum albumin. The affinity of bilirubin was measured by two independent methods and found to be similar for all human serum albumin species. Examination of the absorption and circular dichroism spectra of bilirubin bound to its high affinity site revealed dramatic differences between the conformations of bilirubin bound to the above human serum albumin species. The absorption and circular dichroism spectra of bilirubin bound to the above human serum albumin species in aqueous solutions saturated with chloroform were also examined. The effect of certain amino acid substitutions on the conformation of bound bilirubin was altered by the addition of chloroform. In total, the present study suggests a dynamic, unusually flexible high affinity binding site for bilirubin on human serum albumin.The binding of bilirubin, a toxic metabolite of heme, to human serum albumin (HSA) 1 has been studied extensively for many years. Early medical interest in the bilirubin-HSA interaction arose when it became clear to physicians that prolonged high blood concentrations of bilirubin, which often occur in premature infants, could result in bilirubin encephalopathy (1-5). In this condition, significant amounts of bilirubin, which is toxic to all tissues, partitions from the blood to neuronal tissue causing irreversible brain damage. The prolonged high blood concentrations of bilirubin in premature infants results from underdevelopment of the liver, the organ responsible for conversion of bilirubin to a soluble form and its excretion into the bile. HSA binds bilirubin (K d ϭ 10 Ϫ7 -10 Ϫ8 M) at a high affinity site and acts as a buffer preventing the transfer of bilirubin from blood to the tissues, thus playing a critical role in impairing the development of bilirubin encephalopathy. In total, other studies on the pathology of bilirubin encephalopathy in premature infants have highlighted the importance of HSA as a bilirubin transport molecule in normal neonates (who experience a transient hyperbilirubinemia after birth) and in normal adults. These findings provided the motivation for many years of study on the bilirubin-HSA binding mechanism, making it one of the most studied of the HSA-ligand interactions (6 -11).Although some studies have suggested that lower affinity binding components (K d ϭ 10 Ϫ6 -10 Ϫ3 M) contribute to HSAbilirubin binding, most studies have primarily attempted to locate the high affinity binding site and to identify amino acid residues involved in the high affinity binding process. A number of studies th...

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Section: Resultsmentioning

confidence: 99%

A Dynamic Model for Bilirubin Binding to Human Serum Albumin

Petersen¹,

Ha²,

Harohalli³

et al. 2000

Journal of Biological Chemistry

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“…In solution, these chiral forms are clearly in equilibrium so that no CD spectrum is seen. Bilirubin adopts two enantiomeric 'ridge tile' configurations [19,27], whereas Effect of pH on the dimeric Synechocystis BVR-A biliverdin is suggested to oscillate between two helical 'lock washer' configurations, and one of these, the Pconfiguration, is clearly stabilized in a biliverdin-myoglobin complex [28]. Although it is tempting to suggest that trapping oscillations between two helical forms is the phenomenon responsible for the P(lus) and M(inus) spectra of biliverdin when bound to human serum albumin (HAS) and BSA respectively [26], this remains to be confirmed.…”

Section: Discussionmentioning

confidence: 99%

The effect of pH on the initial rate kinetics of the dimeric biliverdin‐IXα reductase from the cyanobacterium Synechocystis PCC6803

Hayes

Mantle

2009

The FEBS Journal

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Biliverdin‐IXα reductase from Synechocystis PCC6803 (sBVR‐A) is a stable dimer and this behaviour is observed under a range of conditions. This is in contrast to all other forms of BVR‐A, which have been reported to behave as monomers, and places sBVR‐A in the dihydrodiol dehydrogenase/N‐terminally truncated glucose–fructose oxidoreductase structural family of dimers. The cyanobacterial enzyme obeys an ordered steady‐state kinetic mechanism at pH 5, with NADPH being the first to bind and NADP+ the last to dissociate. An analysis of the effect of pH on kcat with NADPH as cofactor reveals a pK of 5.4 that must be protonated for effective catalysis. Analysis of the effect of pH on kcat/KmNADPH identifies pK values of 5.1 and 6.1 in the free enzyme. Similar pK values are identified for biliverdin binding to the enzyme–NADPH complex. The lower pK values in the free enzyme (pK 5.1) and enzyme–NADPH complex (pK 4.9) are not evident when NADH is the cofactor, suggesting that this ionizable group may interact with the 2′‐phosphate of NADPH. His84 is implicated as a crucial residue for sBVR‐A activity because the H84A mutant has less than 1% of the activity of the wild‐type and exhibits small but significant changes in the protein CD spectrum. Binding of biliverdin to sBVR‐A is conveniently monitored by following the induced CD spectrum for biliverdin. Binding of biliverdin to wild‐type sBVR‐A induces a P‐type spectrum. The H84A mutant shows evidence for weak binding of biliverdin and appears to bind a variant of the P‐configuration. Intriguingly, the Y102A mutant, which is catalytically active, binds biliverdin in the M‐configuration.

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“…Small, low-energy internal rotations about the C(9)-C(10) and C(10)-C (11) bonds cause to open or close somewhat, while maintaining hydrogen bonding. 6,14 Large rotations, however, break hydrogen bonds and lead to unstable conformations. Such large bond rotations are associated with the interconversion of mirror-image ridge-tile conformations.…”

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confidence: 99%

Relative steric size of SCH3, OCH3, and CH3 groups from circular dichroism measurements

Boiadjiev

Lightner

2000

Chirality

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The relative steric size of methyl, methoxy, and methylthio groups was determined from circular dichroism (CD) spectroscopy using a sensitive system based on the bilirubin model. In the cyclohexane model, equatorial vs. axial orientation and conformational analysis led to quantitative measurements of orientation preference or steric demand: conformational A-values CH 3 > SCH 3 > OCH 3 . A more sterically demanding model for assessing group size has been found in bilirubin analogs, which are yellow pigments that adopt a ridge-tile shape stabilized by a matrix of intramolecular hydrogen bonds. Optically active bilirubins have been shown to exhibit intense bisignate CD Cotton effects from exciton coupling of their two dipyrrinone chromophores held in either of two enantiomeric ridge-tile conformations. Interconversion of these M and P conformational enantiomers of helical chirality is rapid at room temperature but may be displaced toward either enantiomer by intramolecular nonbonded steric interactions that arise when substituents are introduced at equivalent sterically demanding sites, viz., the ␣ or ␤ carbons of the pigment's propionic acid chains. Such substituents shift the conformational equilibrium toward the M or the P-chirality conformer, depending only on the S or R stereochemistry at the ␣ and ␤ sites, and the resulting exciton CD for the ∼430 nm transition(s) was used to evaluate the relative steric size, SCH 3 > CH 3 > OCH 3 .

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Exciton chirality of bilirubin homologs

Cited by 14 publications

References 18 publications

A Dynamic Model for Bilirubin Binding to Human Serum Albumin

A Dynamic Model for Bilirubin Binding to Human Serum Albumin

The effect of pH on the initial rate kinetics of the dimeric biliverdin‐IXα reductase from the cyanobacterium Synechocystis PCC6803

Relative steric size of SCH3, OCH3, and CH3 groups from circular dichroism measurements

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