Human Serum Albumin in the HSA-GA Complex

Drew Albrecht '23 and Joe Boesel '23

Contents:

I. Introduction

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Albumin proteins are classified as globular transport proteins that are only found in vertebrates. While there are a few types of albumin proteins, serum albumin is the most dominant, as it is the most abundant plasma protein in humans. The human homolog is simply named human serum albumin (HSA). This protein is responsible for transporting a plethora of molecules throughout the body, including fatty acids, amino acids, bile, and steroids. Additionally, HSA is tasked with the important function of regulating oncotic pressure of the blood vessels. HSA is encoded by the ALB gene (4q13.3) and is produced in the liver, where it is then dissolved into the bloodstream. There are many well studied protein:ligand interactions with HSA, but something that is less studied are the protein:protein interactions.

Many Gram-positive bacteria have surface proteins that interact with host proteins, like HSA. Finegoldia magna is an anaerobic bacterium that is present in the flora of the skin, the urogenital and gastrointestinal tracts, and the oral cavity. It has been found that F. magna bind HSA to their surface with high affinity. The bacterial protein responsible for this binding is called the PAB (peptostreptococcal albumin-binding) protein. More specifically, a stretch of 53 residues interact with HSA called the proteinG-related Albumin-binding (GA) module. The GA module consists of a left-handed, three helical bundle that interacts with HSA through multiple binding mechanisms. It has been hypothesized that the GA module is linked to bacterial masking and increased virulence. By understanding this protein:protein interaction, we can begin to think about how this knowledge can be used to stop pathogens before they severely harm the body.

II. HSA Structure

In the body, HSA is actually naturally found as a dimer, where it can bind to two or more ligands at a time. The HSA monomer is a single polypeptide chain that is 585 residues long and holds its shape with the help of 17 pairs of disulfide bridges (Dugiaczyk et al. 1982) . However, there is one cysteine, , that does not participate in a disulfide bridge. Because of this, it is heavily guarded to prevent the sulfhydryl group from coupling with external counterparts. Further observation shows that HSA is composed of three homologous domains, domain I, domain II , and domain III, each of which are divided into smaller subdomains, A and B, which are divided into even smaller subdomains (Fig. 1). These domains each contain pocketed areas that specific ligands can bind to and be carried throughout the body.
There are many ligand-binding domains (LBDs) where ligands like fatty acids usually bind. These LBDs are normally found within the protein, rather than on the surface (Shin-ichi Fujiwara and Takashi Amisaki 2008). HSA has the ability to carry multiple ligands at once, like 5 . Because normal ligands bind within HSA, this often creates a high conformational change for the protein. However, the GA module is unique because it interacts with HSA through their molecular surfaces, held together by many proteins:protein interactions.


Fig. 1. The schematic structure of the HSA domains and the secondary structure elements that hold it together. Helices are represented by rectangles, and loops and turns by thin lines. Disulfide bridges are represented by thick lines (Suggio et al. 1999).

III. The HSA-GA Complex

The HSA-GA complex forms with high affinity when protein PAB comes in contact with HSA. The GA module binds to HSA in domain II through hydrophobic interactions and two distinct hydrogen bond networks. The hydrophobic core contains a bundle of nonpolar amino acids from both HSA and GA . These amino acids include Phe-228, Ala-229, Ala-322, Val-325, Phe-326, and Met-329 from HSA and residues Phe-27, Ala-31, Leu-44, and Ile-48 from GA. The high electron density in this area creates a strong interaction between the two proteins. However, to secure the protein:protein interaction even more, the hydrophobic core is flanked by two hydrogen bond networks.

The first network contains 4 hydrogen bonds. The first 2 bonds form from Glu-321 to Thr-24 and Ser-25, the other 2 form from Asn-318 to Ser-25 and Tyr-28. The second network contains 3 hydrogen bonds. The first 2 bonds form from Glu-230 to Ala-35 and Thr-37, and the final from Asn-267 to Thr-37. A combination of the hydrophobic core and the hydrogen bond networks create a strong connection between the two proteins. However, it should be noted that no significant conformational changes to either HSA or GA are observed when the two form the complex. The process for how these two proteins interact is now understood, but why they interact brings to light an important pathogenic mechanism.

IV. Implications

A problem that pathogenic bacteria face as they try to infect their host comes from the retaliation of the host’s immune system. Bacteria and viruses have evolved to have strategies to combat this, which reveals the importance of the HSA-GA complex. It has been studied that bacteria like F. magna coat themselves in HSA to evade detection by the immune system and increase virulence (Fig. 2). Additionally, it is thought that by binding to HSA, the bacteria is able to capture the albumin bound nutrients (Cramer et al. 2007). This ability to mask itself and steal from their host allows pathogenic bacteria to survive and infect. With the knowledge of this binding activity, antibacterial medications could stem from this interaction. If it were possible to select against the binding of HSA to the GA module, then the pathogenic bacteria would not be able to evade the immune system, effectively killing it.

Fig. 2. Pathogenic bacteria coated with HSA in vivo. Arrows point to the HSA molecules, represented by the black dots (Egesten et al. 2011).

V. References

Cramer, Jacob Flyvholm, Peter A. Nordberg, Janos Hajdu, and Sara Lejon. “Crystal Structure of a Bacterial Albumin-Binding Domain at 1.4Å Resolution.” FEBS Letters 581, no. 17 (July 10, 2007): 3178–82. https://doi.org/10.1016/j.febslet.2007.06.003.

Dugaiczyk, A, S W Law, and O E Dennison. “Nucleotide Sequence and the Encoded Amino Acids of Human Serum Albumin MRNA.” Proceedings of the National Academy of Sciences of the United States of America 79, no. 1 (January 1982): 71–75.

Egesten, Arne, Inga-Maria Frick, Matthias Mörgelin, Anders I. Olin, and Lars Björck. “Binding of Albumin Promotes Bacterial Survival at the Epithelial Surface.” The Journal of Biological Chemistry 286, no. 4 (January 28, 2011): 2469–76. https://doi.org/10.1074/jbc.M110.148171.

Lejon, Sara, Inga-Maria Frick, Lars Björck, Mats Wikström, and Stefan Svensson. “Crystal Structure and Biological Implications of a Bacterial Albumin Binding Module in Complex with Human Serum Albumin.” The Journal of Biological Chemistry 279, no. 41 (October 8, 2004): 42924–28. https://doi.org/10.1074/jbc.M406957200.

Sugio, S., A. Kashima, S. Mochizuki, M. Noda, and K. Kobayashi. “Crystal Structure of Human Serum Albumin at 2.5 Å Resolution.” Protein Engineering, Design and Selection 12, no. 6 (June 1, 1999): 439–46. https://doi.org/10.1093/protein/12.6.439.

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