Conjugating Hemoglobin and Albumin by Strain-Promoted Azide- Alkyne Cycloaddition

A one-to-one conjugate of cross-linked human hemoglobin and human serum albumin results from a strain-promoted alkyne-azide cycloaddition (SPAAC) of the modified proteins. Additions of a strained alkyne-substituted maleimide to the Cys-34 thiol of human serum albumin and an azide-containing cross-link between the amino groups of each β-unit at Lys-82 of human hemoglobin provide sites for coupling by the SPAAC process. The coupled hemoglobin-albumin conjugate can be readily purified from unreacted hemoglobin. The oxygen binding properties of the two-protein bioconjugate demonstrate oxygen affinity and cooperativity that are suitable for use in an acellular oxygen carrier.

"Inside-Out" PEGylation of Bovine β-Cross-Linked Hemoglobin

The development of a blood substitute is urgent due to blood shortages and potential communicable diseases. A novel method, inside-out PEGylation, has been used here to conjugate a multiarm maleimide-PEG (Mal-PEG) to β-cross-linked (βXL-Hb) hemoglobin (Hb) tetramers through the Cys β93 residues. This method produces a polymer with a single PEG backbone that is surrounded by multiple proteins, rather than coating a single protein with multiple PEG chains. Electrophoresis under denaturing conditions showed a large molecular weight species. Gel filtration chromatography and analytical ultracentrifugation determined the most prevalent species had three βXL-Hb to one Mal-PEG. Thermal denaturation studies showed that the cross-linked and PEGylated species were more stable than native Hb. Cross-linking under oxy-conditions produced a high oxygen affinity Hb species (P₅₀  = 9.18 Torr), but the oxygen affinity was not significantly altered by PEGylation (P₅₀  = 9.67 Torr). Inside-out PEGylation can be used to produce a hemoglobin-based oxygen carrier and potentially for other multiprotein complexes.

Development and Applications of the Copper-Catalyzed Azide-Alkyne Cycloaddition (CuAAC) as a Bioorthogonal Reaction

The emergence of bioorthogonal reactions has greatly broadened the scope of biomolecule labeling and detecting. Of all the bioorthogonal reactions that have been developed, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) is the most widely applied one, mainly because of its relatively fast kinetics and high efficiency. However, the introduction of copper species to in vivo systems raises the issue of potential toxicity. In order to reduce the copper-induced toxicity and further improve the reaction kinetics and efficiency, different strategies have been adopted, including the development of diverse copper chelating ligands to assist the catalytic cycle and the development of chelating azides as reagents. Up to now, the optimization of CuAAC has facilitated its applications in labeling and identifying either specific biomolecule species or on the omics level. Herein, we mainly discuss the efforts in the development of CuAAC to better fit the bioorthogonal reaction criteria and its bioorthogonal applications both in vivo and in vitro.

Enhanced Nitrite Reductase Activity and Its Correlation with Oxygen Affinity in Hemoglobin Bis-Tetramers

The vasoactivity of circulating cross-linked hemoglobin is consistent with the acellular protein penetrating the endothelial lining of blood vessels where hemoglobin can bind nitric oxide, the signal for relaxation of the muscles that surround blood vessels. In an important contrast, derivatives of bis-tetramers that are produced from hemoglobin by chemical coupling do not cause vasoconstriction in animal models. Presumably, they are unable to enter the endothelia where hemoglobin tetramers bind to nitric oxide. In addition, hemoglobin bis-tetramers can produce nitric oxide in circulation through their intrinsic nitrite reductase activity. Examination of this activity for hemoglobin-derived bis-tetramers that are acetylated at lysyl amino groups in their α subunits reveals enhanced activity (k = 2.21 M(-1) s(-1)) compared to that of nonacetylated bis-tetramers (k = 0.70 M(-1) s(-1)). Plots of nitrite reductase activities as a function of the corresponding oxygen affinities of certain allosteric-state-stabilized derivatives reveal a significant correlation, providing a basis for interpretation of the correlated functions.

Strain-promoted azide–alkyne cycloaddition for protein–protein coupling in the formation of a bis-hemoglobin as a copper-free oxygen carrier

Conventional chemical approaches to protein–protein coupling present challenges due to the intrinsic competition between the desired interactions of reagents with groups of the protein as well as reactions with water.