Dissection of an enzyme by protein engineering

A method is described for producing fragments of a protein suitable for studies of protein folding. The codon for a single methionine residue is introduced into the cloned gene of barnase, and the gene product cleaved with cyanogen bromide. The site of mutation was chosen to be at the surface of the protein in a region connecting segments of secondary structure in the native enzyme. The α + β protein was mutated from Val36 → Met, and split into two fragments, B(1–36) containing the α-helical regions and B(37–110), the β-sheet. The fragments were purified by ion exchange chromatography. Neither retains catalytic activity. Fluorescence, circular dichroism, and 1H nuclear magnetic resonance data indicate that their structures are each close to that of random-coil peptides. The two fragments associate to form a tight complex (Kd = 0.2 to 0.6 μm), which displays spectroscopic properties similar to those of the uncleaved protein. The catalytic activity is restored in the complex with a value for Km similar to that for native enzyme but with kcat reduced about three- to fourfold. The second-order rate constant for association on mixing fragments in the concentration range 2.5 to 7.5 μm is 1 × 105 s−1 m−1.