Globin structure

To answer the first question, Lesk and Chothia examined in detail residues at structurally equivalent positions that are involved in helix-heme contacts and in packing the a helices against each other. After comparing the nine globin structures then known, the 59 positions they found that fulfilled these criteria were divided into 31 positions buried in the interior of the protein and 28 in contact with the heme group. These positions are the principal determinants of both the function and the three-dimensional structure of the globin family. 

The so-called globin proteins are an important group of a-helical proteins. These include hemoglobins and myoglobins from many species. The globin structure can be viewed as two layers of helices, with one of these layers perpendicular to the other and the polypeptide chain moving back and forth between the layers. 

Figure 89 illustrates two different tries at simplified representation of the globin structure. For reference, Fig. 89a shows the hemoglobin /3 chain in stereo. Figure 89b shows the globin structure schematically as two layers of helices with the elements in one layer approximately perpendicular to those in the other layer this can be contrasted with a possible description of the up-and-down helix bundles as two layers with their elements approximately parallel to each other. The perpendicular layers provide a rather successful simple schema for the globin structure, but unfortunately there are no other proteins that can be adequately described as two perpendicular layers of helices. Also, specification of the topology in this scheme is cumbersome, since the chain skips back and forth between layers. 

Figure 89c schematizes the globin structure as a twisted cylinder of helices, analogous to the antiparallel /8 barrels to be discussed in Section III,D. The up-and-down helix bundle structures are of course also readily described as cylinders, so that this schema makes the... 

Investigations of static structures include architectural descriptions, comparisons and classifications, and identifications of recurrent patterns such as supersecondary structures. Examples include the classification of types of proteins by Levitt and Chothia (28), the hierarchical analysis of protein structures by Rose (29), or the comparison of globin structures by Lesk and Chothia (30). 

Myoglobin is a classic example of a protein with a single Fe " /Fe redox centre that exhibits a reversible Nernstian response. The kinetics of homogeneous electron transfer are reasonably rapid in a myoglobin system despite the tertiary globin structure surrounding the heme iron. Additionally, the porphyrin... 

Fig. 6. Top Schematic representation ofthe assembly of phycobiliprotein trimers and hexamers from the a- and p-subunits (same as Fig. 2). (A) Stereogram ofthe C-PC p-subunit (B) stereogram of the C-PC(ap)-monomer. Helices are represented by cylinders those ofthe p-subunit are labeled with uppercase letters and those ofthe a-subunit with lowercase letters. Chromophores in (A) and (B) are represented by wire models. Figure source (A) and (B) Schirmer, Bode, Huber, Sidler and Zuber (1985) X-ray crystallographic structure of the light-harvesting biliprotein C-phycocyanin from the thermophilic cyanobacterium Mastigocladus laminosus and its resemblance to globin structures. J Mol Biol 184 268,272.

Oxygen is bound to the iron atom in heme, which remains in the ferrous state because the globin structure prevents interaction of the oxygen with more than one heme at a time. 

In multi-subunit Hbs, the globin structure prevents the close approach of hemes in adjacent subunits, and, thus, prevents //-oxo dimer formation via reaction (2). However, another irreversible reaction, referred to as autoxidation, reaction (3), invariably occurs for all Mb/Hbs ... 

The ferry 1-Fe" is slowly reduced to the met-form. If the globin structures of deoxy-, oxy-or met-forms of myoglobin are disturbed, they are reversibly or irreversibly converted into hemochrome-Fe + or hemichrome-Fe. The role of these denatured species in lipid oxidation of meat is not clear. When ferric hemin is detached from the globin, the ferryl ion can initiate lipid oxidation by hydrogen abstraction to produce a lipid radical plus a proton. Iron can also be released from hemin in the presence of hydroperoxides to participate in lipid oxidation processes. Ascorbic acid and other reducing compounds in muscle cytosol effectively inhibit lipid oxidation promoted by ferryl ions in membranes. 

Fetal hemoglobin has a different globin structure ai a higher affinity for oxygen than adult hemoglobin. 

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