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Crystallographic analysis reveals the structural basis of the high-affinity binding of iophenoxic acid to human serum albumin.

Ryan AJ, Chung CW, Curry S - BMC Struct. Biol. (2011)

Bottom Line: It was replaced by Iopanoic acid, which has an amino rather than a hydroxyl group at position 3 on the iodinated benzyl ring and, as a result, binds to albumin with lower affinity and is excreted more rapidly from the body.Consistent with this interpretation, the structure also suggests that the lower-affinity binding of iopanoic acid arises because replacement of the 3-hydroxyl by an amino group eliminates hydrogen bonding to Arg 257.This finding underscores the importance of polar interactions in high-affinity binding to albumin.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biophysics Section, Blackett Laboratory, Imperial College, Exhibition Road, London, SW7 2AZ, UK.

ABSTRACT

Background: Iophenoxic acid is an iodinated radiocontrast agent that was withdrawn from clinical use because of its exceptionally long half-life in the body, which was due in part to its high-affinity binding to human serum albumin (HSA). It was replaced by Iopanoic acid, which has an amino rather than a hydroxyl group at position 3 on the iodinated benzyl ring and, as a result, binds to albumin with lower affinity and is excreted more rapidly from the body. To understand how iophenoxic acid binds so tightly to albumin, we wanted to examine the structural basis of its interaction with HSA.

Results: We have determined the co-crystal structure of HSA in complex with iophenoxic acid at 2.75 Å resolution, revealing a total of four binding sites, two of which--in drugs sites 1 and 2 on the protein--are likely to be occupied at clinical doses. High-affinity binding of iophenoxic acid occurs at drug site 1. The structure reveals that polar and apolar groups on the compound are involved in its interactions with drug site 1. In particular, the 3-hydroxyl group makes three hydrogen bonds with the side-chains of Tyr 150 and Arg 257. The mode of binding to drug site 2 is similar except for the absence of a binding partner for the hydroxyl group on the benzyl ring of the compound.

Conclusions: The HSA-iophenoxic acid structure indicates that high-affinity binding to drug site 1 is likely to be due to extensive desolvation of the compound, coupled with the ability of the binding pocket to provide a full set of salt-bridging or hydrogen bonding partners for its polar groups. Consistent with this interpretation, the structure also suggests that the lower-affinity binding of iopanoic acid arises because replacement of the 3-hydroxyl by an amino group eliminates hydrogen bonding to Arg 257. This finding underscores the importance of polar interactions in high-affinity binding to albumin.

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Co-crystal structure of HSA in complex with iophenoxic acid. (a) Simulated annealing omit map contoured at 3σ for iophenoxic acid bound in drug site 1 in sub-domain IIA. (b) Overview of HSA, coloured by sub-domain, showing the locations of the two primary drug sites and the four molecules of iophenoxic acid. (c) Close-up of iophenoxic acid bound in drug site 1. The ligand and selected side-chains are shown as sticks, coloured by atom type (carbon - grey (yellow in the iophenoxic acid); oxygen - red; nitrogen - blue; iodine - magenta). The surface of iophenoxic acid is shown as a semi-transparent grey surface. Hydrogen bonds and salt bridges are indicated by dotted orange lines.
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Figure 2: Co-crystal structure of HSA in complex with iophenoxic acid. (a) Simulated annealing omit map contoured at 3σ for iophenoxic acid bound in drug site 1 in sub-domain IIA. (b) Overview of HSA, coloured by sub-domain, showing the locations of the two primary drug sites and the four molecules of iophenoxic acid. (c) Close-up of iophenoxic acid bound in drug site 1. The ligand and selected side-chains are shown as sticks, coloured by atom type (carbon - grey (yellow in the iophenoxic acid); oxygen - red; nitrogen - blue; iodine - magenta). The surface of iophenoxic acid is shown as a semi-transparent grey surface. Hydrogen bonds and salt bridges are indicated by dotted orange lines.

Mentions: HSA is found at very high concentrations in blood plasma and tissue fluids, typically around 0.6 mM and can have a profound effect on the distribution of many natural and artificial small-molecules [2]. The protein circulates as a monomeric polypeptide that folds into three similar helical domains (I-III), each of which is split into two sub-domains (A and B) [6] (Figure 2b). Between them these six sub-domains provide binding sites for a wide variety of endogenous ligands that are predominantly apolar molecules with anionic or electronegative features and include fatty acids, bilirubin, hemin and thyroxine [2,7]. Two of the binding pockets on the protein have been identified as the primary sites for drugs and drug-like compounds [8,9]. Crystallographic analysis has shown that these pockets -- drug sites 1 and 2 -- are located in sub-domains IIA and IIIA respectively [10,11], although additional drug binding sites have also been found [10,12-14].


Crystallographic analysis reveals the structural basis of the high-affinity binding of iophenoxic acid to human serum albumin.

Ryan AJ, Chung CW, Curry S - BMC Struct. Biol. (2011)

Co-crystal structure of HSA in complex with iophenoxic acid. (a) Simulated annealing omit map contoured at 3σ for iophenoxic acid bound in drug site 1 in sub-domain IIA. (b) Overview of HSA, coloured by sub-domain, showing the locations of the two primary drug sites and the four molecules of iophenoxic acid. (c) Close-up of iophenoxic acid bound in drug site 1. The ligand and selected side-chains are shown as sticks, coloured by atom type (carbon - grey (yellow in the iophenoxic acid); oxygen - red; nitrogen - blue; iodine - magenta). The surface of iophenoxic acid is shown as a semi-transparent grey surface. Hydrogen bonds and salt bridges are indicated by dotted orange lines.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3104944&req=5

Figure 2: Co-crystal structure of HSA in complex with iophenoxic acid. (a) Simulated annealing omit map contoured at 3σ for iophenoxic acid bound in drug site 1 in sub-domain IIA. (b) Overview of HSA, coloured by sub-domain, showing the locations of the two primary drug sites and the four molecules of iophenoxic acid. (c) Close-up of iophenoxic acid bound in drug site 1. The ligand and selected side-chains are shown as sticks, coloured by atom type (carbon - grey (yellow in the iophenoxic acid); oxygen - red; nitrogen - blue; iodine - magenta). The surface of iophenoxic acid is shown as a semi-transparent grey surface. Hydrogen bonds and salt bridges are indicated by dotted orange lines.
Mentions: HSA is found at very high concentrations in blood plasma and tissue fluids, typically around 0.6 mM and can have a profound effect on the distribution of many natural and artificial small-molecules [2]. The protein circulates as a monomeric polypeptide that folds into three similar helical domains (I-III), each of which is split into two sub-domains (A and B) [6] (Figure 2b). Between them these six sub-domains provide binding sites for a wide variety of endogenous ligands that are predominantly apolar molecules with anionic or electronegative features and include fatty acids, bilirubin, hemin and thyroxine [2,7]. Two of the binding pockets on the protein have been identified as the primary sites for drugs and drug-like compounds [8,9]. Crystallographic analysis has shown that these pockets -- drug sites 1 and 2 -- are located in sub-domains IIA and IIIA respectively [10,11], although additional drug binding sites have also been found [10,12-14].

Bottom Line: It was replaced by Iopanoic acid, which has an amino rather than a hydroxyl group at position 3 on the iodinated benzyl ring and, as a result, binds to albumin with lower affinity and is excreted more rapidly from the body.Consistent with this interpretation, the structure also suggests that the lower-affinity binding of iopanoic acid arises because replacement of the 3-hydroxyl by an amino group eliminates hydrogen bonding to Arg 257.This finding underscores the importance of polar interactions in high-affinity binding to albumin.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biophysics Section, Blackett Laboratory, Imperial College, Exhibition Road, London, SW7 2AZ, UK.

ABSTRACT

Background: Iophenoxic acid is an iodinated radiocontrast agent that was withdrawn from clinical use because of its exceptionally long half-life in the body, which was due in part to its high-affinity binding to human serum albumin (HSA). It was replaced by Iopanoic acid, which has an amino rather than a hydroxyl group at position 3 on the iodinated benzyl ring and, as a result, binds to albumin with lower affinity and is excreted more rapidly from the body. To understand how iophenoxic acid binds so tightly to albumin, we wanted to examine the structural basis of its interaction with HSA.

Results: We have determined the co-crystal structure of HSA in complex with iophenoxic acid at 2.75 Å resolution, revealing a total of four binding sites, two of which--in drugs sites 1 and 2 on the protein--are likely to be occupied at clinical doses. High-affinity binding of iophenoxic acid occurs at drug site 1. The structure reveals that polar and apolar groups on the compound are involved in its interactions with drug site 1. In particular, the 3-hydroxyl group makes three hydrogen bonds with the side-chains of Tyr 150 and Arg 257. The mode of binding to drug site 2 is similar except for the absence of a binding partner for the hydroxyl group on the benzyl ring of the compound.

Conclusions: The HSA-iophenoxic acid structure indicates that high-affinity binding to drug site 1 is likely to be due to extensive desolvation of the compound, coupled with the ability of the binding pocket to provide a full set of salt-bridging or hydrogen bonding partners for its polar groups. Consistent with this interpretation, the structure also suggests that the lower-affinity binding of iopanoic acid arises because replacement of the 3-hydroxyl by an amino group eliminates hydrogen bonding to Arg 257. This finding underscores the importance of polar interactions in high-affinity binding to albumin.

Show MeSH
Related in: MedlinePlus