Human embryonic, fetal, and adult hemoglobins have different subunit interface strengths. Correlation with lifespan in the red cell

Abstract

The different types of naturally occurring, normal human hemoglobins vary in their tetramer-dimer subunit interface strengths (stabilities) by three orders of magnitude in the liganded (CO or oxy) state. The presence of embryonic z-subunits leads to an average 20-fold weakening of tetramer-dimer interfaces compared to corresponding hemoglobins containing adult a-subunits. The dimer-monomer interfaces of these hemoglobins differ by at least 500-fold in their strengths; such interfaces are weak if they contain z-subunits and exchange with added b-subunits in the form of b 4 (HbH) significantly faster than do those with a-subunits. Subunit exchange occurs at the level of the dimer, although tetramer formation reciprocally influences the amount of dimer available for exchange. Competition between subunit types occurs so that pairs of weak embryonic hemoglobins can exchange subunits to form the stronger fetal and adult hemoglobins. The dimer strengths increase in the order Hb Portland-2 (z 2 b 2 ) < Hb Portland-1 (z 2 g 2 ) ffi Hb Gower-1 (z 2 e 2 ) < Hb Gower-2 (a 2 e 2 ) < HbF 1 < HbF (a 2 g 2 ) < HbA 2 (a 2 d 2 ), i.e., from embryonic to fetal to adult types, representing maturation from weaker to stronger monomermonomer subunit contacts. This increasing order recapitulates the developmental order in which globins are expressed (embryonic ! fetal ! adult), suggesting that the intrinsic binding properties of the subunits themselves regarding the strengths of interfaces they form with competing subunits play an important role in the dynamics of protein assemblies and networks.

Hemoglobin Kirklareli (α H58L), a New Variant Associated with Iron Deficiency and Increased CO Binding

“…As controls, we examined the gel filtration elution profiles of the reduced CO forms of HbA and rHb Kirklareli as a function of total protein concentration using the methods described by Manning et al (19). The K 4,2 for HbCO Kirklareli was estimated to be ϳ0.3 M, which is ϳ4-fold smaller than the value estimated for HbA in our experiments, 1.1 M. The latter value is very similar to previously reported K 4,2 values (20,21). Thus, the rapid rates of degradation of Hb Kirklareli are not due to increased dissociation into dimers.…”

Understanding the roles of intrinsic disorder in subunits of hemoglobin and the disease process of sickle cell anemia

“…Before hemoglobin is able to develop a-subunits, it must have "combinations of z-with e-or g-subunits to form embryonic hemoglobins." 54 Their order of expression is determined by their relative positions on the gene, "i.e., z ! a (2 copies) on chromosome 16 and e !…”

“…b on chromosome 11." 54 During normal development, the embryo is normally z 2 g 2 , z 2 e 2 , or a 2 e 2 , the fetus is typically a 2 g 2 , and, finally, the adult stage hemoglobin consists of either a 2 b 2 or a 2 d 2 . 54 Since the hemoglobin is a hetero-tetramer consisting of several combinations of these 7 different types of subunits (a, b, g1, g2, d, e, and z), in order to understand the potential role of intrinsic disorder in hemoglobin function, the predisposition of all these subunits to intrinsic disorder were evaluated.…”

Developmental expression of human hemoglobins mediated by maturation of their subunit interfaces

“…The preferential assembly of a 2 b 2 over a 2 c 2 has been demonstrated by in vitro studies involving mixing of isolated Hbs. 12,13,22 Thus, when the weak embryonic Hbs f 2 b 2 , f 2 c 2 , and a 2 e 2 are mixed, they slowly dissociate and reassemble to form a 2 b 2 (HbA), the strongest tetramer, through subunit exchange, that is,…”

“…12,13 In this article, these equilibria have been studied together in a single measurement as linked events. Information on individual equilibria enabled us to choose a Hb concentration where differences in their subunit interface bonding strengths would be manifested; if too high a Hb concentration had been studied, the equilibria of all the Hbs would have been shifted toward the tetrameric state thus obscuring any differences between them.…”