The homotetrameric R67 dihydrofolate reductase (R67 DHFR) is illustrated as a ternary complex with its substrates dihydrofolate (green) and NADPH (yellow) modelled into the active site channel. We performed an exploration of the active site of R67 DHFR by combinatorial mutagenesis and selection. The 16 principal active site residues (pink) were combinatorially mutated, creating a library of modified active-site environments within a constant framework. Three functional variants with highly altered active site compositions were identified and characterized. For further details, see Schmitzer et al., pages 809-819.
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