Subunit-directed click coupling via doubly cross-linked hemoglobin efficiently produces readily purified functional bis-tetrameric oxygen carriers

Abstract

While cross-linked hemoglobin (Hb) tetramers can deliver oxygen as a supplement to red cells, they also cause unacceptable increases in blood pressure, presumably from their penetration of the linings of blood vessels (endothelia) where the internal hemes bind endogenous nitric oxide (NO). This penetration would lower the local concentration of NO that normally induces vasodilation. Enlarging the effective size of the oxygen-carrying protein by coupling two Hbs can prevent their extravasation. Efficient and selective protein–protein coupling to produce those species has been a significant challenge. Introduction of an azide within a protein provides a directionally-oriented reaction site for utilization of the Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) in the protein–protein-coupling process based on solubility-directed sequential addition to a bis-alkyne. However, it is known that Hb with an azide-containing cross-link between α-subunits is unreactive in CuAAC. To direct reaction away from the α-subunits of Hb, a specific fumaryl cross-link is installed exclusively between the most reactive sites on those subunits, thereby blocking the α-99 lysyl groups and preventing any further reaction. This modification allows installation of an azide-containing cross-link exclusively between lysine-82 ε-amino groups of the β-subunits of Hb. The multiply interconnected sites establish a geometry that permits initial interfacial interaction of the cross-linked Hb-azide with Cu(I) and a bis-alkyne. After coupling, the protein-linked azide product undergoes CuAAC at the remaining alkyne with a second cross-linked Hb-azide, producing a fully functional cross-linked Hb bis-tetramer whose oxygenation and structural properties include cooperativity and oxygen affinity that should be suitable for testing as an alternative to red cells in transfusions.