Table II.
Substrate specificity and kinetics of the ExGase from developing maize seedlings
| Substrate | Km | Vmax |
|---|---|---|
| μm | μmol mg−1 min−1 | |
| p-Nitrophenyl-β-d-glucoside | 886 | 13.40 |
| p-Nitrophenyl-α-d-glucosidea | n.d.b | 0.04 |
| Sophorose | 66 | 7.75 |
| Laminaribiose | 128 | 9.09 |
| Cellobiose | 53 | 3.96 |
| Gentiobiose | 68 | 6.30 |
| Laminaritriose | n.d. | 6.82 |
| Laminaritetraose | n.d. | 6.25 |
| Laminaripentaose | n.d. | 5.43 |
| Cellotriose | n.d. | 4.49 |
| Cellotetraose | n.d. | 4.33 |
| Cellopentaose | n.d. | 4.41 |
All substrates were tested at initial amounts of saturating concentrations of reducing ends (2 mm). The hydrolyzed Glc was determined by enzymatic assay, whereas the color produced by p-nitrophenyl-β-d-glucoside was corrected for Glc equivalents. Values for Glc in disaccharides was halved, as one hydrolysis event gave rise to two molecules of d-Glc, correction for cleavage of disaccharide products from higher order laminari- and cellodextrin oligosaccharides was based on HPLC of the digestion products as described in Figure 5.
No activity was obtained with p-nitrophenyl-β-d-galactopyranoside, -α-d-galactopyranoside, -β-d-mannopyranoside, -β-d-xylopyranoside, -α-l-arabinopyranoside, -α-l-arabinofuranoside, -β-l-fucopyranoside, or -α-l-rhamnopyranoside.
Not determined.