Increased concentration of the substrate increases enzyme activity until all the enzymes are immediately occupied by substrate molecules so it cannot work any faster as enzyme activity becomes the limiting factor (substrate concentration increasing increases enzyme activity until it is no longer the limiting factor)
All enzymes have an optimum pH depending on the enzyme at optimum pH they work as fast as they can so from a lower pH to this pH enzyme activity increases however after the optimum the enzyme begins to be denatured as the pH breaks apart the hydrogen bonds in the tertiary structure of the proteins (enzymes are proteins) so the enzyme activity decreases after this pH level
