Anything that disrupts this structure will reduce the enzyme's catalytic power. If we increase the temperature of the solution the enzyme is operating in, we will typically see an increase in the reaction rate until a point is reached at which the enzyme starts to unfold.
More catalyst means a faster reaction, so the reaction rate increases.
More substrate doesn't increase the rate any more. To increase the rate again, we'd need more enzyme. In a complex biochemical system, it is necessary to control the rate of biochemical reactions. For example, if we have a meal with lots of carbohydrate, we need to control the rate at which that is converted to glucose so that enough is present to provide the energy for our activities, but not so much as to put us into a diabetic condition. One of the ways in which this is done is by enzyme inhibition.