Albumin-Hyaluronan Interactions: Influence of Ionic Composition Probed by Molecular Dynamics

Abstract

The lubrication mechanism in synovial fluid and joints is not yet fully understood. Nevertheless, intermolecular interactions between various neutral and ionic species including large macromolecular systems and simple inorganic ions are the key to understanding the excellent lubrication performance. An important tool for characterizing the intermolecular forces and their structural consequences is molecular dynamics. Albumin is one of the major components in synovial fluid. Its electrostatic properties, including the ability to form molecular complexes, are closely related to pH, solvation, and the presence of ions. In the context of synovial fluid, it is relevant to describe the possible interactions between albumin and hyaluronate, taking into account solution composition effects. In this study, the influence of Na⁺, Mg²⁺, and Ca²⁺ ions on human serum albumin-hyaluronan interactions were examined using molecular dynamics tools. It was established that the presence of divalent cations, and especially Ca²⁺, contributes mostly to the increase of the affinity between hyaluronan and albumin, which is associated with charge compensation in negatively charged hyaluronan and albumin. Furthermore, the most probable binding sites were structurally and energetically characterized. The indicated moieties exhibit a locally positive charge which enables hyaluronate binding (direct and water mediated).

Keywords: human serum albumin; hyaluronan; hyaluronic acid; hydrogen bonds; ionic interactions; molecular dynamics simulations; water mediated interactions.

Figure 1

Structure of the repeating disaccharide unit of hyaluronate, the deprotonated form of hyaluronic acid. GCU stands for D-glucuronic acid with pKa of about 3, and NAG means N-acetyl-D-glucosamine. The different oxygen atoms are numbered, and this numbering will be utilized when discussing the interaction with human serum albumin.

Figure 2

Structure of human serum albumin with different coloring for the different HSA subdomains: IA—red; IB—cyan; IIA—yellow; IIB—green; IIIA—grey; IIIB—blue. Hyaluronate is colored pink. The figure represents one of many structures of the HAS–hyaluronate complex. This particular complex is referred to as complex number 1 in Table 1.

Figure 3

Electrostatic potential map of HSA where blue and red represent positively and negatively charged regions, respectively. Effects of different ions are presented: (a) no ions, (b) Na⁺, (c) Ca²⁺, (d) Mg²⁺.

Figure 4

(a) HSA–HA binding energy vs. time average for complex 1 (constant line represents average over last 60 ns). (b) Binding energies for different complexes in presence of different cations for the simulation time of 40–100 ns. Complexes are sorted according to the average for all three ions.

Figure 5

(a) Number of direct intermolecular H-bonds. (b) Water bridges between HSA and HA for different complexes evaluated by MD.

Figure 5

(a) Number of direct intermolecular H-bonds. (b) Water bridges between HSA and HA for different complexes evaluated by MD.

Figure 6

Number of ionic interactions between HSA and HA: (a) direct, (b) cation mediated (cation bridges).

Figure 6

Number of ionic interactions between HSA and HA: (a) direct, (b) cation mediated (cation bridges).

Figure 7

Hydrogen bond distribution between different oxygen classes in HA and different amino acids in HSA. Data were obtained in solutions containing (a) Na⁺, (b) Ca²⁺, (c) Mg²⁺. In all cases, Cl⁻ was the anion.

Figure 8

Water bridge distribution between different oxygen classes in HA and different amino acids in HSA. Data were obtained in solutions containing (a) Na⁺, (b) Ca²⁺, (c) Mg²⁺. In all cases, Cl⁻ was the anion.

Figure 9

Number of hydrophobic interactions between HSA and HA.

Figure 10

Water bridge visualization.

Figure 11

Cation bridge visualization.