Fig. 2

Basic structure of molluscan hemocyanin. On protein level, hemocyanin represents a partly hollow cylinder (outer wall: dark blue; inner collar: light blue, PDB-ID: 4BED) with a diameter of 35 nm (i). This quaternary structure is built out of two decamers lying on top of each other. The repetitive unit is a subunit dimer colored in golden (collar: yellow). The primary structure of one subunit (ii) shows the composition of one signal peptide (S) and eight paralogous domains called functional units (FU-a – FU-h). Each functional unit forms a tertiary structure as shown in (iii) and comprises a copper binding site with two copper ions that can bind a dioxygen molecule reversibly (PDB-ID: 1JS8). Within the primary structure (ii) also splice sites are illustrated: blue arrows symbolize splice sites of linker introns that lie between the FUs (conserved at the same position and in phase 1 for all analyzed hemocyanins); dotted black arrows symbolize splice sites of internal introns. Their number and positions vary among the different gene structures