Structures of CBM33s and GH61s. The figure shows Ta GH61A, a cellulose-active GH61 from Thermoascus aurantiacus (A), CBP21, a chitin-active CBM33 from Serratia marcescens (B), and details of the active sites of these two enzymes (C & D, respectively). Panels A and B show cartoons of the complete proteins; the side chains of two conserved histidines, which are labeled in panels C & D, are also shown. The grey balls in panels C and D represent metal ions (see text for details); the red balls indicate water molecules. Note that the histidines labeled His22 and His28 in panels C and D, respectively, are the N-terminal residues of the mature proteins (i.e. after removal of the signal peptide for secretion) and that the N-terminal amino group participates in coordination of the metal ion.