Permissible discontinuity region of the alpha-chain of hemoglobin: noncovalent interaction of heme and the complementary fragments alpha 1-30 and alpha 31-141

A Novel α2-Globin Gene Mutation: Hb Debao [α31(B12)Arg→Trp; HBA2: c.94A>T]

We report a novel mutation on the α2-globin gene, Hb Debao [α31(B12)Arg→Trp; HBA2: c.94A>T] detected in a Chinese family. This mutation gives rise to a previously undescribed hemoglobin (Hb) variant that was undetectable by electrophoretic or chromatographic methods. Hb Debao was associated with an α⁺-thalassemia (α⁺-thal) deletion [-α^3.7 (rightward)] producing a mild phenotype with significant microcytosis and hypochromia, while the combination of this mutation with an α⁰-thal deletion (--^SEA) resulting in a severe form of Hb H (β4) disease, which is consistent with a thalassemic phenotype associated with the novel mutation.

Generation of peptides by human erythrocytes: Facts and artifacts

Previously reported data on peptide composition of human erythrocyte lysate were obtained under conditions that did not exclude proteolytic degradation of hemoglobin in the process of peptide isolation. Comparative chromatographic analysis of the diluted erythrocyte lysate incubated in acidic conditions with or without proteolytic enzyme inhibitors showed that several peptides earlier identified as intraerythrocyte ones in fact result from hemoglobin degradation by erythrocyte acidic protease(s) during incubation of the lysate. A rational scheme excluding postlysis proteolysis was developed for isolation of peptide fraction. Further analysis resulted in determination of structure and content of about 50 endogenous intraerythrocyte hemoglobin fragments. A primary endopeptidase splitting of alpha- and beta-globin chains followed by consecutive exopeptidase trimming of primary fragments is suggested as a degradation mechanism. The intraerythrocyte peptides were shown to differ from peptides excreted by the erythrocytes to the extracellular medium in the primary culture. It was also found that intraerythrocyte peptides cannot play the role of precursors of hemoglobin fragments present in tissue extracts.

Hemoglobin Einstein: semisynthetic deletion in the B-helix of the alpha-chain

The influence of the deletion of the tetra peptide segment alpha(23-26) of the B-helix of the alpha-chain of hemoglobin-A on its assembly, structure, and functional properties has been investigated. The hemoglobin with the deletion, ss-Hemoglobin-Einstein, is readily assembled from semisynthetic alpha(1-141) des(23-26) globin and human betaA-chain. The deletion of alpha(23-26) modulates the O2 affinity of hemoglobin in a buffer/allosteric effector specific fashion, but has little influence on the Bohr effect. The deletion has no influence on the thermodynamic stability of the alpha1beta1 and the alpha1beta2 interface. The semisynthetic hemoglobin exhibits normal intersubunit interactions at the alpha1beta1 and alpha1beta2 interfaces as reflected by 1H-NMR spectroscopy. Molecular modeling studies of ss-Hemoglobin-Einstein suggest that the segment alpha(28-35) is in a helical conformation, while the segment alpha(19-22) is the nonhelical AB region. The shortened B-helix conserves the interactions of alpha1beta1 interface. The results demonstrate a high degree of plasticity in the hemoglobin structure that accommodates the deletion of alpha(23-26) without perturbing its overall global conformation.

Characterization of glycated hemoglobin in diabetic patients: usefulness of electrospray mass spectrometry in monitoring the extent and distribution of glycation

A combination of chromatographic and mass spectrometric techniques was used to evaluate the extent and distribution of glycation within the glycated hemoglobin (GHb) molecule. Studies on quantification of hemoglobin (Hb) glycation by electrospray ionization mass spectrometry (ES-MS) of intact globins employed specimens from 10 diabetic individuals and five normal controls. Detailed structural analysis of the phenylboronate affinity chromatography/ion-exchange (IE) HPLC-separated sub-populations of GHb was performed on a specimen carrying 13.7% GHb. An efficient protocol for mapping glycation sites within alpha and beta globins was developed, e.g., Glu-C/Asp-N proteolytic digestion followed by LC-ES-MS. Relative site occupancy within discrete components of GHb was evaluated. A correlation between the degree of glycation measured at Hb level (by affinity chromatography) and at globin level (measured by ES-MS) was carried out. The above studies led us to conclude that during the process of phenylboronate chromatography GHb dimers, rather than tetramers, are bound to the affinity resin so a fraction of glycated dimers rather than tetramers is measured. This finding implies that a process of glycation affects a much higher number of native Hb tetramers than was previously contemplated. No glycation sites appear to be missed by phenylboronate affinity chromatography. We have found no evidence of the presence of multiple glycations within a single globin chain. While glycation of both globins within a dimer cannot be excluded, it is unlikely to be a significant phenomenon. According to ES-MS data, an equivalent of about one globin per alphabeta dimer of the affinity chromatography-isolated GHb carried glycation.

Chemistry of the "molecular trap" of protease-catalyzed splicing reaction of complementary segments of alpha-subunit of hemoglobin A

The complementary fragments of human Hb alpha, alpha1-30, and alpha31-141 are spliced together by V8 protease in the presence of 30% n-propanol to generate the full-length molecule (Hb alpha-semisynthetic reaction). Unlike the other protease-catalyzed protein/peptide splicing reactions of fragment complementing systems, the enzymic condensation of nonassociating segments of Hb alpha is facilitated by the organic cosolvent induced alpha-helical conformation of product acting as the "molecular trap" of the splicing reaction. The segments alpha24-30 and alpha31-40 are the shortest complementary segments that can be spliced by V8 protease. In the present study, the chemistry of the contiguous segment (product) alpha24-40 has been manipulated by engineering the amino acid replacements to the positions alpha27 and alpha31 to delineate the structural basis of the molecular trap. The location of Glu27 and Arg31 residues in the contiguous segment alpha24-40 (as well as in other larger segments) is ideal to generate (i, i + 4) side-chain carboxylate-guanidino interaction in its alpha-helical conformation. The amino acid residue replacement studies have confirmed that the side chains at alpha27 and alpha31 facilitate the semisynthetic reaction. The relative influence of the substitute at these sites on the splicing reaction depends on the chemical nature of the side chain and the location. The gamma-carboxylate guanidino side-chain interaction appears to contribute up to a maximum of 85% of the thermodynamic stability of the molecular trap. The studies also demonstrate that the thermodynamic stability of the molecular trap is determined by two interdependent conformational aspects of the peptide. One is an amino acid-sequence-specific event that facilitates the induction of an alpha-helical conformation to the contiguous segment in the presence of organic cosolvent that imparts some amount of protease resistance to Glu30-Arg31 peptide bond. The second structural aspect is a site-specific event, an i, i + 4 side-chain interaction in the alpha-helical conformation of the peptide which imparts an additional thermodynamic stability to the molecular trap. The results suggest that conformationally driven "molecular traps" of protease-mediated ligation reactions of peptides could be designed into products to facilitate the modular assembly of peptides/proteins.

High-efficiency synthesis of human alpha-endorphin and magainin in the erythrocytes of transgenic mice: a production system for therapeutic peptides

Chemical synthesis of peptides, though feasible, is hindered by considerations of cost, purity, and efficiency of synthesizing longer chains. Here we describe a transgenic system for producing peptides of therapeutic interest as fusion proteins at low cost and high purity. Transgenic hemoglobin expression technology using the locus control region was employed to produce fusion hemoglobins in the erythrocytes of mice. The fusion hemoglobin contains the desired peptides as an extension at the C end of human alpha-globin. A protein cleavage site is inserted between the C end of the alpha-globin chain and the N-terminal residue of the desired peptide. The peptide is recovered after cleavage of the fusion protein with enzymes that recognize this cleavage signal as their substrate. Due to the selective compartmentalization of hemoglobin in the erythrocytes, purification of the fusion hemoglobin is easy and efficient. Because of its compact and highly ordered structure, the internal sites of hemoglobin are resistant to protease digestion and the desired peptide is efficiently released and recovered. The applicability of this approach was established by producing a 16-mer alpha-endorphin peptide and a 26-mer magainin peptide in transgenic mice. Transgenic animals and their progeny expressing these fusion proteins remain health, even when the fusion protein is expressed at > 25% of the total hemoglobin in the erythrocytes. Additional applications and potential improvements of this methodology are discussed.

Restriction in the conformational flexibility of apoproteins in the presence of organic cosolvents: a consequence of the formation of "native-like conformation"

The influence of n-propanol on the overall alpha-helical conformation of beta-globin, apocytochrome C, and the functional domain of streptococcal M49 protein (pepM49) and its consequence on the proteolysis of the respective proteins has been investigated. A significant amount of alpha-helical conformation is induced into these proteins at pH 6.0 and 4 degrees C in the presence of relatively low concentrations of n-propanol. The induction of alpha-helical conformation into the proteins increased as a function of the propanol concentration, the maximum induction occurring around 30% n-propanol. In the case of alpha-globin, the fluorescence of its tryptophyl residues also increased as a function of n-propanol concentration, the midpoint of this transition being around 20% n-propanol. Furthermore, concomitant with the induction of helical conformation into these proteins, the proteolysis of their polypeptide chain by V8 protease also gets restricted. The alpha-helical conformation induced into alpha- and beta-globin by n-propanol decreased as the temperature is raised from 4 to 24 degrees C. In contrast, the alpha-helical conformation of both alpha- and beta-chain (i.e., globin with noncovalently bound heme) did not exhibit such a sensitivity to this change in temperature. However, distinct differences exist between the n-propanol induced "alpha-helical conformation" of globins and the "alpha-helical conformation" of alpha- and beta-chains. A cross-correlation of the n-propanol induced increase in the fluorescence of beta-globin with the corresponding increase in the alpha-helical conformation of the polypeptide chain suggested that the fluorescence increase represents a structural change of the protein that is secondary to the induction of the alpha-helical conformation into the protein (i.e., an integration of the helical conformation induced to the segments of the polypeptide chain to influence the microenvironment of the tryptophyl residues). Presumably, the fluorescence increase is a consequence of the packing of the helical segments of globin to generate a "native-like structure." The induction of alpha-helical conformation into these proteins in the presence of n-propanol and the consequent generation of "native-like conformation" is not unique to n-propanol. Trifluoroethanol, another helix-inducing organic solvent, also behaves in the same fashion as n-propanol. However, in contrast to the proteins described above, n-propanol could neither induce an alpha-helical conformation into performic acid oxidized RNAse-A nor restrict its proteolysis by proteases.(ABSTRACT TRUNCATED AT 400 WORDS)

Proteosynthetic activity of immobilized Staphylococcus aureus V8 protease: application in the semisynthesis of molecular variants of alpha-globin

The proteosynthetic activity of Staphylococcus aureus V8 protease (endoproteinase Glu-C) immobilized onto cross-linked agarose beads by reductive alkylation procedure has been investigated. The overall substrate specificity of the enzyme, as judged by peptide mapping of performic acid oxidized RNase A, as well as the high propensity of the protease to slice selectively the alpha-chain of hemoglobin (Hb) A at the Glu(30)-Arg(31) peptide bond at pH 4.0 and 37 degrees C was essentially unperturbed by the immobilization process. This high susceptibility of Glu(30) of the alpha-chain for proteolysis appears to be a consequence of the conformational aspects of the polypeptide in this region. The proteolysis of two mutant forms of alpha-chain, namely, those of Hb I (K16E) and Hb Sealy (D47H) by immobilized V8 protease at the Glu(30)-Arg(31) peptide bond proceeds with the same selectivity. The immobilized protease also retained the proteosynthetic activity, i.e., the ability to ligate the unprotected alpha-globin fragments at the Glu(30)-Arg(31) peptide bond in the presence of 30% 1-propanol. The use of the insoluble enzyme simplifies the procedures for the construction of new semisynthetic, molecular variants of alpha-globin. The general applicability of the immobilized enzyme for protein semisynthesis has been demonstrated by the construction of a doubly mutated alpha-globin. The complementary fragments from two natural mutant forms of alpha-globin, viz., alpha 1-30 (K16E) from Hb I and alpha 31-141 (D47H) from Hb Sealy, are readily ligated to form the double mutant alpha 1-141 (K16E;D47H).

Conformational properties of the isolated 1-23 fragment of human hemoglobin alpha-chain

With the purpose of establishing whether, as a general rule, regions of a protein chain that are helical in the native structure maintain, at least partially, the same helical structure when isolated in solution, we have prepared the 1-23 fragment of human hemoglobin alpha-chain, and studied its conformational properties in aqueous solution by CD and 1H-NMR. From the analysis of CD and NMR spectral changes with temperature, salt and addition of trifluoroethanol (TFE) it can be concluded that the 1-23 peptide forms a measurable population (18% at 22 degrees C (pH 5.6) TFE/H2O, 30:70 (v/v)) of an alpha-helix structure that spans the same residues that are helical in the native protein (namely, 6 to 17). These results, taken together with similar ones obtained previously in the 1-19, 21-42 and 50-61 RNAase fragments, support the idea that no helices other than the native ones are actually formed in solution by protein fragments. This implies that the final helical structure of a protein is present from the very beginning of the folding process, and also that such elements of secondary structure can act as primary nucleation centers.

Conformational studies of alpha-globin in 1-propanol: propensity of the alcohol to limit the sites of proteolytic cleavage

Selective condensation of the unprotected fragments of alpha-globin--namely, alpha 1-30 and alpha 31-141--is catalyzed by Staphylococcus aureus V8 protease in the presence of 25% 1-propanol. The propensity of 1-propanol to induce the alpha-helical conformation and to generate a "native-like" topology for the polypeptide chain has been now investigated in an attempt to understand the molecular basis of this enzyme-catalyzed stereospecific condensation. Removal of heme from the alpha-chain decreases the overall alpha-helical conformation of the protein considerably. A significant amount of the alpha-helical conformation is restored in the presence of 25% 1-propanol and the digestion of alpha-globin by V8 protease becomes more selective concomitant with the increase in helicity. V8 protease digestion of alpha-globin at pH 6.0 and 4 degrees C occurs at Glu-30, Asp-47, Glu-27, and Glu-23 in the absence of 1-propanol. In the presence of 25% 1-propanol, the digestion is selective to the peptide bond of Glu-30. This selectivity appears to be a characteristic feature of the native conformation of alpha-chain (polypeptide chain with bound heme). 1-Propanol induces the alpha-helical conformation into RNase S peptide also. However, this increased helical conformation did not protect the RNase S peptide from V8 protease digestion at the Glu-9-Arg-10 peptide bond. RNase S peptide is an alpha-helical conformation in RNase S, an interacting fragment-complementing system of S protein and S peptide. S peptide is resistant to V8 protease hydrolysis in this conformation. Thus, the resistance of a peptide bond in a segment of a protein to protease digestion appears to be a consequence of the secondary structure as well as the tertiary interactions of this segment with the rest of the molecule. The results suggest that the 1-propanol induces alpha-helical conformation into segments of alpha-globin as well as packing of these helices in a native-like topology.