Evolution of Proteins: Ontogeny and Phylogeny of Hemoglobin

Ontogeny and Phylogeny of Hemoglobin:

Functionally and structurally similar proteins exist in diverse animal and plant species.

Many of these are believed to have a common evolutionary origin. The picture of protein evolution is probably clearest in the case of the hemoglobin’s, where it is possible to make accurate comparisons of hemoglobin structure in many different animal species.

Moreover, in humans (and also in other vertebrates), different kinds of he­moglobin molecules are present in the blood at differ­ent stages of development, and this ontogeny also sheds light on the phylogenetic picture.

In addition to hemoglobin A, several other normally occurring hemoglobin’s are present in humans. In the very early stages of embryonic development, a hemo­globin called Gower-1 appears. This hemoglobin con­sists of two alpha family chains called zeta (ζ) chains and two beta family chains called epsilon (ε) chains; the Gower-1 tetramer is thus represented as ζ₂ε₂.

Be­ginning at about the eighth week of development, the zeta and epsilon chains are gradually replaced by the adult alpha chains and also two different beta family chains that are designated gamma^G (γ^G) and gamma^A (γ^A); the two types of gamma chains are identical ex­cept that in γ^G the amino acid glycine occurs at posi­tion H13 and in γ^A this position is occupied by alanine. Alpha chain production, once begun, continues throughout embryonic, fetal, and adult life.

The next hemoglobin to appear is called Gower-2 and consists of two alpha and two epsilon chains (i.e., ζ₂ε₂). This is quickly followed by the appearance of hemoglobin Portland, which consists of two zeta and two gamma chains (i.e., ζ₂γ₂). Both Gower-2 and Portland are soon replaced by hemoglobin F (designated HbF and called fetal hemoglobin), consisting of two alpha and two gamma chains (i.e., α₂γ₂).

HbF predominates in the blood through the remainder of fetal development. Beginning soon after the twelfth week of develop­ment, the adult beta chains begin to appear and there is a progressive increase in HbA (i.e., α₂/β₂) and an ac­companying decrease in HbF.

Shortly before birth, yet another beta family chain appears called the delta (8) chain; together with alpha chains, the delta chains form a hemoglobin known as A₂ (i.e., α₂δ₂). By about six months after birth, little if any HbF can be found in the blood. At this time, about 98% of the hemoglo­bin is HbA and the remaining 2% is HbA₂. In a normal individual, this ratio persists through adult life. The various human hemoglobin’s and their subunit compo­sitions are listed in Table 4-8. Their differential tem­poral appearance is shown in Figure 4-33.

A consistent feature of the human hemoglobin’s is that two members of each tetramer are always alpha family chains and the other two are beta family chains. Conditions similar to this have been found in all other mammals studied so far and also in other ver­tebrates. Except in the case of the lowest vertebrates (e.g., lampreys and hagfishes), hemoglobin molecules are always tetramers and are formed from one pair of alpha family chains and one pair of beta family chains.

When adult organisms have two or more hemoglobin’s, they share a common alpha family chain. Transition from one developmental stage to another, as in the metamorphosis of amphibians, is accompanied by a change in hemoglobin in which one pair of chains (i.e., the alpha family chains) is conserved.

In the lowest vertebrates, hemoglobin is a monomer (i.e., one globin chain and one heme group), and in this regard is simi­lar to all of the vertebrate myoglobin’s. All of these ob­servations play a critical role in the development of the current concept concerning the evolution of the he­moglobin molecule.

The primary structures of the alpha family and beta family chains of the human hemoglobin’s reveal a high degree of homology. An examination of Table 4-9 re­veals that there are 75 differences .between the alpha and beta chains, implying that about one-half of all po­sitions in each chain are occupied by the same amino acid. A common evolutionary origin for the alpha and beta chains is suggested in view of their extensive ho­mology.

Still higher degrees of homology exist among the al­pha family and among the beta family chains (Table 4- 9). The alpha and zeta chains have 61 differences; the beta and gamma chains have only 39 differences; beta and epsilon chains have only 36 differences; beta and delta chains have only 10 differences; and so on. Thus, the beta, gamma, delta, and epsilon chains are more closely related to each other than any of them are to the alpha family chains; the alpha and zeta chains are more closely related to each other than either one is to the beta family chains.

Just as striking are the similarities between human alpha chains and the alpha chains of other mammals and between human beta chains and the beta chains of other mammals. Table 4-10 lists these differences for a number of mammals in order of diminishing phylogenetic relationship to man. No differences exist between the primary structures of human and chim­panzee alpha and beta globin chains.

Gorilla and hu­man globin chains differ in only two positions. Al­though the more distantly related mammals reveal increasingly large numbers of differences, it is con­sistently observed that the alpha chains or beta chains of the different species are more closely related than are the alpha and beta chains within a species (e.g., human alpha chains and kangaroo alpha chains have a higher degree of homology than do human alpha and human beta chains).

The latter class of observations is basic to contemporary theory on hemoglobin evolu­tion. It is interesting (but not surprising) that a com­mon revelation of these hemoglobin studies is that spe­cies taxonomically more closely related have greater similarities (i.e., fewer differences) in the primary structures of their globin chains.

The secondary and tertiary structure of myoglobin is almost identical to that of the globin chains of hemo­globin, and myoglobin’s primary structure reveals an especially high degree of homology with the alpha family globin chains (see Table 4-9).

In view of their similarity, myoglobin and hemoglobin are believed to have a common evolutionary origin. This notion is also supported by observations that the hemoglobin of the lowest vertebrates (e.g., lampreys and hagfishes) con­sists of a single polypeptide chain and not a tetramer. On the basis of the numerous studies of vertebrate myoglobin’s and hemoglobin’s, it is generally agreed that all the globin chains have a common evolutionary origin. The proposed scheme for the evolution of hu­man myoglobin and hemoglobin is shown in Figure 4- 34.

According to this scheme, the structural genes for the various globin chains arose through a series of gene duplications, and sub­sequently these genes underwent separate and inde­pendent evolution. The degree of similarity that exists between the primary structures of the globin chains (Table 4-9) is used to establish the relative phylogenetic positions of the structural genes.

Thus, the greater degree of homology that exists between the beta and delta globin chains than exists between the beta and gamma chains is presumably the conse­quence of the more recent duplication of the primitive beta/delta gene, and so on.

Based on the extent of homology that exists be­tween the primary structures of polypeptide chains from different species and the presumed frequencies with which gene mutations occur, it is possible to esti­mate the age (and even suggest the primary struc­ture) of the ancestral polypeptide. Such studies are encompassed in a field of biology called molecular pale genetics. Phylogenetic relationships between species suggested on the basis of protein similarities are remarkably consistent with those that were pre­dicted many decades ago using more conventional taxonomic criteria.