Protein myohemerythrin

Figure 18.11 Electron-density maps at different resolution show more detail at higher resolution, (a) At low resolution (5.0 A) individual groups of atoms are not resolved, and only the rodlike feature of an

At least some hemerythrin-containing animals also have a monomeric hemerythrin present in their muscles. This protein, myohemerythrin, is homologous with the subunits of octameric hemerythrin (Hendrickson and Ward, 1977 Ward and Hendrickson, 1977), though its primary structure is substantially different from coelomic hemerythrins (Klippenstein et al, 1976). 

Although semi-met forms of myohemerythrin (the monomeric protein from Themiste zostericola muscle) can be obtained in a similar manner to the octamer (32), the intramolecular disproportionation path is obviously now unavailable to them. In... 

Pancreatic ribonuclease Staphylococcal nuclease Peroxidases Glutathione peroxidase Cytochrome c peroxidase Oxygen carriers Myoglobin, hemoglobin Myohemerythrin, hemerythrin Hormone-binding proteins Uteroglobin Pre albumin Lectins... 

The best characterized and thus prototypical of the binuclear iron-oxo proteins is hemerythrin (Hr), an oxygen carrier from some classes of marine invertebrates, such as sipunculids. The protein consists of several identical 13 kDa subunits which contain two iron atoms and reversibly bind one molecule of O2. Myohemerythrins contain a single 13 kDa subunit (25,26,27). 

A representative sampling of non-heme iron proteins is presented in Fig. 3. Evident from this atlas is the diversity of structural folds exhibited by non-heme iron proteins it may be safely concluded that there is no unique structural motif associated with non-heme iron proteins in general, or even for specific types of non-heme iron centers. Protein folds may be generally classified into several categories (i.e., all a, parallel a/)3, or antiparallel /8) on the basis of the types and interactions of secondary structures (a helix and sheet) present (Richardson, 1981). Non-heme iron proteins are found in all three classes (all a myohemerythrin, ribonucleotide reductase, and photosynthetic reaction center parallel a/)8 iron superoxide dismutase, lactoferrin, and aconitase antiparallel )3 protocatechuate dioxygenase, rubredoxins, and ferredoxins). This structural diversity is another reflection of the wide variety of functional roles exhibited by non-heme iron centers. 

The four-helix bundle is a common motif in which (usually) antiparallel a helices are packed side by side. It is found in myohemerythrin, various cytochromes, and a number of other proteins. A viral example is the coat protein of tobacco mosaic virus (TMV) (Bloomer et al, 1978). TMV represents the most common type, in which the helix axes are nearly antiparallel, off by 18°, coiled with a left-handed superhelical twist (Fig. 5 see Color Insert). The slight misalignment of the individual helix axes allows the side chains to interdigitate efficiently, burying internal hydrophobic side chains. 

Chapman and Liljas, Fig. 5. The four-helix bundle, (a) The prototypical conformation of myohemerythrin (Sheriff et al, 1987) (b) the tobacco mosaic virus coat protein (Namba et al, 1989). 

Hemerythrin and myohemerythrin have roles as oxygen carriers in marine invertebrates, a function similar to that of the heme proteins hemoglobin and myoglobin in mammals and the copper-protein hemo-cyanin in mollusks and arthropods 1-7, 13). 

Figure 1. Schematic representations of protein structures (a) myohemerythrin, an a-helical protein with antiparallel helices ( >) V2 domain of an immunoglobulin, a (3-sheet protein (c) triose phosphate isomerase, a parallel a-ff protein with a central (3 barrel (d) carboxypepti-dase, a parallel a- 3 protein with a central ( -sheet structure (e)para-hydroxybenzoate hydrolase, a complex protein structure with more than one domain. (From Ref. S3 courtesy of J. Richardson.)...

J. Kuriyan, A. T. Briinger, M. Karplus, and W. A. Hendrickson, Acta Crystallogr., Sect. A, 45, 396 (1989). X-Ray Refinement of Protein Structures by Simulated Annealing Test of the Method on Myohemerythrin. 

Hemerythrin, being the best characterized of the group, is the prototype of this class of proteins (5,6). The circulatory protein consists of oligomers of an 17 kDa subunit that contains the dinuclear site. The number of subunits that constitute the holoprotein varies from 3 to 8 among the species studied (6). A monomeric form, myohemerythrin, which has been isolated from muscle tissue (18), is presumably related to hemerythrin in its function just as myoglobin is to hemoglobin. 

The helices are represented by the regularly coiled sections of the ribbon diagram. Myohemerythrin is an oxygen-carrying protein in invertebrates. Sign in at www.thomsonedu.com/login to see an animated version of this figure. 

Three general types of 02-carrying or -storage proteins are known hemoglobin (Hb)/myoglobin (Mb), which contains a non-covalently attached heme, hemerythrin (Hr)/myohemerythrin (myoHr), which contains a non-heme di-iron site held to the protein via ligands furnished by... 

D. Rojewska and R. Elber, Proteins Struct. Funct. Genet., 7, 265 (1990). Molecular E -namics Study of Secondary Structure Motions in Proteins Application to Myohemerythrin. 

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