The pepsin residue glycine-76 contributes to active-site loop flexibility and participates in catalysis.

Glycine residues are known to contribute to conformational flexibility of polypeptide chains, and have been found to contribute to flexibility of some loops associated with enzymic catalysis. A comparison of porcine pepsin in zymogen, mature and inhibited forms revealed that a loop (a flap), consisting of residues 71--80, located near the active site changed its position upon substrate binding. The loop residue, glycine-76, has been implicated in the catalytic process and thought to participate in a hydrogen-bond network aligning the substrate. This study investigated the role of glycine-76 using site-directed mutagenesis. Three mutants, G76A, G76V and G76S, were constructed to increase conformational restriction of a polypeptide chain. In addition, the serine mutant introduced a hydrogen-bonding potential at position 76 similar to that observed in human renin. All the mutants, regardless of amino acid size and polarity, had lower catalytic efficiency and activated more slowly than the wild-type enzyme. The slower activation process was associated directly with altered proteolytic activity. Consequently, it was proposed that a proteolytic cleavage represents a limiting step of the activation process. Lower catalytic efficiency of the mutants was explained as a decrease in the flap flexibility and, therefore, a different pattern of hydrogen bonds responsible for substrate alignment and flap conformation. The results demonstrated that flap flexibility is essential for efficient catalytic and activation processes.

The role of the flap residue, threonine 77, in the activation and catalytic activity of pepsin A.

Flexible loops, often referred to as flaps, have been shown to play a role in catalytic mechanisms of different enzymes. Flaps at the active site regions have been observed in the crystal structures of aspartic proteinases and their residues implicated in the catalytic processes. This research investigated the role of the flap residue, threonine 77, in the activation of pepsinogen and the catalytic mechanism of pepsin. Three mutants, T77S, T77V and T77G, were constructed. Differences in amino acid polarity and hydrogen bonding potential were shown to have an influence on the activation and catalytic processes. T77S activated at the same rate and had similar catalytic parameters as the wild-type pepsin. The activation rates of T77V and T77G were slower and their catalytic efficiencies lower than the wild-type. The results demonstrated that the threonine 77 polar side chain played a role in a proteolysis. The contribution of the side chain to zymogen activation was associated with the proteolytic cleavage of the prosegment. It was postulated that the hydroxyl group at position 77 provided an essential hydrogen bond that contributed to proper substrate alignment and, indirectly, to a catalytically favorable geometry of the transition state.

Redesigning the active site of Geotrichum candidum lipase.

Attempts to engineer enzymes with unique catalytic properties have largely focused on altering the existing specificities by reshaping the substrate binding pockets. Few experiments have aimed at modifying the configuration of the residues essential for catalysis. The difference in the topological location of the triad acids of Geotrichum candidum lipase (GCL) and the catalytic domain of human pancreatic lipase (HPL), despite great similarities in their topologies and 3-D structures, suggest that these are related enzymes whose catalytic triads have been rearranged in the course of evolution (Schrag et al., 1992). In this study we prepared a double mutant GCL in which the catalytic triad acid is shifted to the position equivalent to the location of the triad acid of HPL. The double mutant maintains approximately 10% of the wild type activity against triglycerides and the fluorogenic ester 4-methylumbelliferyl-oleate. The only significant differences between the 3-D structures of the double mutant and wild type GCL are at the mutated sites. Even the water structure in the region of the triad is unchanged. The hydrogen bonding pattern of the catalytic triad of the double mutant is very similar to that of pancreatic lipase. The acid of the double mutant is stabilized by only two hydrogen bonds, whereas three hydrogen bonds are observed in the wild type enzyme. These results strongly support the hypothesis that the pancreatic lipases are evolutionary switchpoints between the two observed arrangements of the catalytic triads supported by the alpha/beta hydrolase fold and suggest that this fold provides a stable protein core for engineering enzymes with unique catalytic properties.