Hemocyanin from E. californicum encapsulated in silica gels: Oxygen binding and conformational states

Immobilization of Allantoinase for the Development of an Optical Biosensor of Oxidative Stress States

Allantoin, the natural end product of purine catabolism in mammals, is non-enzymatically produced from the scavenging of reactive oxygen species through the degradation of uric acid. Levels of allantoin in biological fluids are sensitively influenced by the presence of free radicals, making this molecule a candidate marker of acute oxidative stress in clinical analyses. With this aim, we exploited allantoinase—the enzyme responsible for allantoin hydrolization in plants and lower organisms—for the development of a biosensor exploiting a fast enzymatic-chemical assay for allantoin quantification. Recombinant allantoinase was entrapped in a wet nanoporous silica gel matrix and its structural properties, function, and stability were characterized through fluorescence spectroscopy and circular dichroism measurements, and compared to the soluble enzyme. Physical immobilization in silica gel minimally influences the structure and the catalytic efficiency of entrapped allantoinase, which can be reused several times and stored for several months with good activity retention. These results, together with the relative ease of the sol-gel preparation and handling, make the encapsulated allantoinase a good candidate for the development of an allantoin biosensor.

Entrapment and characterization of functional allosteric conformers of hemocyanin in sol–gel matrices

Entrapment of hemocyanin in sol–gel stabilizes conformations scarcely populated in solution, allowing for their structural and functional analysis.

Tertiary and quaternary effects in the allosteric regulation of animal hemoglobins

In the last decade, protein allostery has experienced a major resurgence, boosted by the extension of the concept to systems of increasing complexity and by its exploitation for the development of drugs. Expansion of the field into new directions has not diminished the key role of hemoglobin as a test molecule for theory and experimental validation of allosteric models. Indeed, the diffusion of hemoglobins in all kingdoms of life and the variety of functions and of quaternary assemblies based on a common tertiary fold indicate that this superfamily of proteins is ideally suited for investigating the physical and molecular basis of allostery and firmly maintains its role as a main player in the field. This review is an attempt to briefly recollect common and different strategies adopted by metazoan hemoglobins, from monomeric molecules to giant complexes, exploiting homotropic and heterotropic allostery to increase their functional dynamic range. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Tertiary and quaternary allostery in tetrameric hemoglobin from Scapharca inaequivalvis

The clam Scapharca inaequivalvis possesses two cooperative oxygen binding hemoglobins in its red cells: a homodimeric HbI and a heterotetrameric A2B2 HbII. Each AB dimeric half of HbII is assembled in a manner very similar to that of the well-studied HbI. This study presents crystal structures of HbII along with oxygen binding data both in the crystalline state and in wet nanoporous silica gels. Despite very similar ligand-linked structural transitions observed in HbI and HbII crystals, HbII in the crystal or encapsulated in silica gels apparently exhibits minimal cooperativity in oxygen binding, in contrast with the full cooperativity exhibited by HbI crystals. However, oxygen binding curves in the crystal indicate the presence of a significant functional inequivalence of A and B chains. When this inequivalence is taken into account, both crystal and R state gel functional data are consistent with the conservation of a tertiary contribution to cooperative oxygen binding, quantitatively similar to that measured for HbI, and are in keeping with the structural information. Furthermore, our results indicate that to fully express cooperative ligand binding, HbII requires quaternary transitions hampered by crystal lattice and gel encapsulation, revealing greater complexity in cooperative function than the direct communication across a dimeric interface observed in HbI.

Polymerized and polyethylene glycol-conjugated hemoglobins: a globin-based calibration curve for dynamic light scattering analysis

Dynamic light scattering (DLS) is a technique capable of determining the hydrodynamic radius of proteins. From this parameter, a molecular weight can be assessed provided that an appropriate calibration curve is available. To this goal, a globin-based calibration curve was used to determine the polymerization state of a recombinant hemoglobin-based oxygen carrier and to assess the equivalent molecular weight of hemoglobins conjugated with polyethylene glycol molecules. The good agreement between DLS values and those obtained from gel filtration chromatography is a consequence of the high similarity in structure, shape, and density within the globin superfamily. Moreover, globins and heme proteins in general share similar spectroscopic properties, thereby reducing possible systematic errors associated with the absorption of the probe radiation by the chromophore.