Starch Binding Domain

    The overall topology of the SBD of G1 from A. niger shows eight b- 

strands forming two major b-sheets ( Jacks et al., 1995). One sheet 

consists of five strands(2, 3, 5, 6, and 7) arranged in antiparallel 

fashion while the second sheet(1, 4, and 8)has one parallel and one 

antiparallel strand pair. Residues A523, P561 and L562 represent

 additional b-sheet residues to those previously identified

( Jacks et al., 1995) due to an extension of the antiparallel 

interaction of strand 1 and 4. The b-strands are well stabilised as 

evidenced by the slow amide exchange rates for these residues, which

in some cases have half-lives of weeks or months at 313k and pH 5.2, 

and form the core of the SBD. The longer strands(1,4 ,5 and 8) show 

a degree of curvature and twist which allows better packing. 

In addition, two hydrogen bonds between strand 3 and 4 involve residues 

S552 and Y564 in antiparallel fashion(Sorimachi et al., 1996).Thus the

 SBD forms an open-sided, distorted, b-barrel structure.

Possible binding site residues

    The substrate binding sites in domain E of CGTase(cyclodextrin 

glycosyltransferase, EC2.4.1.19) is homologous to the SBD of glucoamylase

from A. niger showing aprroximately 37% amino acids sequence identity,and 

it have been studied previously in detail by Lawson et al. (1994) in the 

crystal structure of the CGTase/maltose complex.Thus we can use the 

knowlege of the SBD disulfide bridge, CGTase binding site residues and 

homology between SBD and CGTase residues amongst other criteria to predict

the possible binding site residues. The proposed SBD residues were for site 1,

W543, E576, K578, W590, N595 and for site 2, T525, T526, Y527, G528, E529,

N530, D554, K555, D560, W563.