Hemocyanin subunits

Primary structure of hemocyanins. Many hemocyanin subunits were sequenced and analyzed (see latest reviews of Burmester and coworkers ). However, only for a few Hes are all subunits composing the native structure sequenced. In most cases, a sequence identity of only about 30% is found between the subunit types of native hemocyanins within one species as well as when compared with those from different... 

The 4 X 6-meric tarantula He from Eurypelma califor-nicum consists of 7 different subunit types. In contrast to crustacean hemocyanin, the chelicerate hemocyanin subunit lost the alpha 1.2 helix (Figure 4, 5d, e), which then allows the assembly of larger hemocyanins than hexamers. Each hexamer contains subunit a, d, e, f and g. The sixth position is taken by subunit b or c. Thus, the smallest structurally identical units are the 2 x 6-meric half-molecules, which dimerize... 

Figure 4 Structures of a crustacean hemocyanin subunit and the hexamer P. interruptus). The three domains are colored differently (green I red II blue III). Alpha-helix 1,2 will he missing in the chelicerate subimit compared to a crustacean suhimit (a). The top view shows the threefold symmetry along the central channel (b), the side view (c) shows that the two trimers are isologously dimerized...

Figure 5 Comparison of the tertiary structures of a catecholoxidase from a sweet potato Ipomea sp. (a), functional units of molluscau hemocyauins (O. dofleini (b) and R. thomasiana (c)) and arthropod hemocyanin subunits from L. polyphemus (d) and P. interruptm (e). All structures are centered with respect to domain 11 (red) carrying the active site, domain 1 is colored green, domain 111 blue. The histidines are colored gray. In the Limulus structure (d) the alpha 1,3 helix is missing compared to the Panulirus (e) structure (ellipse)...

Comparison of the arrangements of the domains within an arthropod hemocyanin subunit and the functional unit of a molluscan hemocyanin also suggests two different types of phenoloxidases, the a-phenoloxidase and the m-phenoloxidase. Cleavages of the N-terminal domain of a-phenoloxidase and the C-terminal domain of m-phenoloxidase open the entrances to the active sites for bulky substrates. The key amino acids are indicated in Figure 9. 

The hemocyanin subunits from Limulus polyphemus are also subdivided into three domains, which consist of the amino acids 1-154 (domain 1), 155-380 (domain 2), and 381-628 (domain 3) (Fig. 29). Domains 1 and 2 are primarily a-helical, whereas domain 3 consists mainly of /(-strands. Domain 2 is situated between the other two domains and contains the copper center. Each copper is coordinated by three histidine residues, which come from two helices. Subunit 2 is primarily responsible for the contact between the hexamer subunits. The chloride-binding site is situated between domains 1 and 2, and the calcium-bin-ding site is located in domain 3, which has a seven-ribbon /(-meander structure [37],... 

Fig. 29a-d. Ribbon model of the three domains of hemocyanin subunit II from Limuluspoly-phemus A whole subunit B domain 1 C domain 2 D domain 3. From Hazes et al. 1993 [37] with permission. Secondary structure nomenclature according to [246]... 

K. A. Magnus H. Ton-That J. A. Carpenter, Three-Dimensional Structure of the Oxygenated Form of the Hemocyanin Subunit II of Umulus Polyphemus at Atomic Resolution. In Bioinorganic Chemistry of Copper,K. D. Karlin, Z. Tyeklar, Eds. Chapman Hall New York, 1993 pp 143-150. 

Figure 3 Hierarchies in the structures of hemocyanins from arthropods and molluscs. A kidney shaped arthropod subunit (Mr 72 kDa) binds one molecule of dioxygen. Depending on the species, the subunits associate to 1 x 6, 2 x 6, 4 x 6, 6 x 6 and 8 X 6-meric hemocyanins found freely dissolved in the hemolymph. A molluscan hemocyanin subunit (Mr 400 kDa) folds into eight functional units each carrying one active site (two cannot be seen being hidden in the interior of the cylinder). Ten of these subunits form cylinders. Depending on the species, decamers or didecamers are found. Thus, hemocyanins can bind up to 160 dioxygen molecules with a cooperativity h of over 11...

Liu, S. and Magnus, KA. (2002) Preliminary crystallographic studies of Limulus polyphemus hemocyanin subunits Ilia, Illb and IV. Biochim. Biophys. Acta, 15%, 177-181. 

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