Pleated sheets catalytic domain

Fig. 7. Diagrammatic representation of the catalytic domain viewed to show the large pleated sheet forming the subunit interface across the P axis (S3).

Figure 17. Ribbons representation of the hPL structure. The upper catalytic domain consists of a central -sheet with helices on both sides. The loop eovering the active site is the two turn helix at the left side. The bottom domain consists of two j8-pleated sheets.

The structural differences of the catalytic domains are also reflected in differences in subunit interactions. For LDH, s-MDH, and GAPDH these have been expressed in terms of an orthogonal, right-handed coordinate system (P,Q,R) deflned by Rossmann et al. (57). In LDH they form the three mutually perpendicular molecular 2-fold axes. These axes can be defined with respect to the direction of the strands in the 3-pleated sheet. 

SI, 9II, and /Sill are the three pleated sheet regions of the catalytic domain schematically illustrated in Fig. 10. The individual strands of these sheets are numbered sequentially from the amino terminal, dk. .. 0F are the strands of the parallel pleated sheet region in the coenzyme binding domain schematically illustrated in Fig. 5. The helices are labeled al. . . a4 in the catalytic domain and oA. . . E in the coenzyme binding domain. Ill. . . R17 are reverse bends defined according to Venkatachalam 117). 

There are no apparent similarities between the structure of this domain and the structures of the catalytic domains of LDH (126) or GAPDH (128). The catalytic domain in LADH is built up by a complicated network of /3 structures. There are three main regions of pleated sheet which have been called ySl, 8ll, and y3lll. Schematic diagrams of... 

Fio. 10. Schematic diagram of the three pleated sheet regions in the catalytic domain. 

There are only four helical segments in the catalytic domain comprising 18% of the residues. In contrast, 35% of the residues are in pleated sheet regions and 14% in reverse bends. Thus, a large number of residues, 33%, have no regular secondary structure. However, a considerable portion of these residues, 95-113, form the lobe which binds the second zinc atom. Apart from this lobe there is only one long continuous string of residues, 114-128, which have no apparent secondary structure. 

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