Nonsequential unfolding of the alpha/beta barrel protein indole-3-glycerol-phosphate synthase

The folding of the enzyme indole-3-glycerol-phosphate synthase (IGPS), a member of the (alpha/beta)(8) fold family, has been studied. At least two folding intermediates have been detected using spectroscopic and activity measurements in combination with gel filtration chromatography. These two intermediates are produced by parallel pathways of a nonsequential unfolding mechanism rather than being consecutive steps in a sequential process. One intermediate can be detected in unfolding experiments because it is kinetically trapped in that conformation, but it is not observed in refolding experiments. It has spectroscopic and hydrodynamic properties very similar to those of the native protein, but it is inactive. The other intermediate could not be characterized because it either aggregates or unfolds under our experimental conditions and could not be isolated chromatographically.