Albumin in Patients with Liver Disease Shows an Altered Conformation

Bibliographic info

• Authors: Margret Paar, Vera H. Fengler, Daniel J. Rosenberg, Angelika Krebs, Rudolf E. Stauber, Karl Oettl, Michal Hammel (Oettl and Hammel equal senior contribution)
• Journal: Communications Biology (Nature portfolio), 2021, vol. 4, art. 507; DOI: 10.1038/s42003-021-02030-3
• Institutions: Medical University of Graz, Austria; Lawrence Berkeley National Laboratory, USA

Key question

Does HSA from patients with chronic liver disease (CLD) adopt a different 3D conformation than albumin from healthy donors, and does this conformational change correlate with clinical markers of disease severity?

Methods

• Technique: SAXS (small-angle X-ray scattering) + SEC (size-exclusion chromatography) + MALS (multi-angle light scattering) — SEC-SAXS-MALS; first in-solution structural study of patient vs healthy HSA
• In vitro fractionation: HMA, HNA1, HNA2 fractions separated from a healthy donor; conformational comparison between Cys34 redox states
• Patient samples: plasma HSA from CLD patients (various etiologies, severity) vs healthy controls
• Modeling: Normal mode analysis (NMA) on crystal structure (PDBID: 1AO6) to model conformational variability; one-state SAXS model fitting; goodness of fit (χ²)
• Correlations: MELD score, bilirubin, fatty acid (FA) content, redox state (HMA/HNA1/HNA2)

Main findings

1. Patient HSA shows significant “domain opening” compared to healthy donor HSA — extended, more open conformation visible in Kratky plots and P(r) pair distribution functions
2. Redox state alone (Cys34) does NOT alter overall HSA conformation in vitro: HMA, HNA1, HNA2 fractions from a healthy donor show only minor SAXS differences — domain I/III conformational variability exists but is minor
3. In vivo, the conformational change in CLD patients correlates with:
   • MELD score (disease severity)
   • Bilirubin concentration (reflects binding occupancy at site II)
   • Fatty acid (FA) content (binding occupancy at multiple fatty acid binding sites)
4. Structural interpretation: the conformational opening in patient HSA reflects combined effects of heavy ligand loading (bilirubin, FAs) rather than oxidation per se
5. Domain-specific opening: domain I and III show greater conformational variability in patient models; domain II (contains main ligand binding sites Sudlow I and II) mediates the opening
6. No single conformational state: large dynamic variability in patient HSA — not a single open form but a distribution of open conformers

Structural context

• HSA crystal structure (1AO6): 3 domains (I, II, III); 7 fatty acid binding sites (FABS); 2 Sudlow drug binding sites
• Domain opening = increased accessibility of binding sites but potentially altered binding affinity/selectivity
• The opening described here provides a structural explanation for why heavily loaded (bilirubin/FA) patient albumin has altered drug binding — relevant for pharmacokinetics

Clinical context

• Disease: CLD — Liver fibrosis / cirrhosis
• MELD correlation: structural opening severity tracks with disease stage
• Implication: Albumin binding function impairment in CLD results not only from PTM (oxidation, glycation) but from conformational change driven by accumulated cargo (bilirubin, FAs). The two mechanisms are likely additive.

Connections

• HSA — first in-solution structural study of CLD vs healthy albumin
• Oxidation — clarifies that Cys34 oxidation alone does not change structure; the conformational change is ligand-load driven
• Liver fibrosis — disease context; MELD correlation
• domenicali-2014 — provides the PTM landscape; this paper provides the 3D structural correlate
• baldassarre-2021-ealb — effective albumin; SAXS data here shows that even if eAlb is intact by LC-MS (native fraction), accumulated bilirubin/FA cargo changes conformation → functional impairment mechanism beyond PTMs
• spinella-2016-review — reviews binding function aspects; this SAXS paper provides the structural evidence
• oettl-2013 — Oettl is senior author here too; links to Graz group’s redox albumin work

Take home notes

• Key insight: the conformational change is not primarily PTM-driven but cargo-driven (bilirubin, fatty acids). This is distinct from what the PTM isoform profiling captures. Means that in severe CLD, both PTM damage AND cargo overloading contribute to albumin dysfunction independently.
• This SAXS study is unique — most clinical albumin studies are MS-based (structure by mass). SAXS reveals the 3D shape in solution, which is not detectable by MS.
• The finding that Cys34 redox state alone does not change conformation is somewhat counterintuitive given the large functional differences between HMA and HNA2. Suggests the functional loss in HNA2 is local (Cys34 site specificity) rather than global conformational.
• Relevance for ALBOM: our isoform panel captures PTM-based structural damage. Bilirubin and FA cargo cannot be measured by the same LC-MS method. Future studies could combine isoform profiling + SAXS or EPR binding function to capture both mechanisms.