Enzymes, are organic catalysts that lower the activation energy for chemical reactions to proceed. Enzyme activity is affected by temperature, ph and the amount of substrate available. Temperature is very important to the action of enzymes and there is a narrow range of temperature that enzymes work optimally, usually body temperature. If you observe a curve of temperature vs. enzyme activity, it peaks around body temperature in humans(37 degrees Celsius) and as you go below or above that temperature, the action of the enzyme decreases. It is a bell-shaped curve. Enyzmes become denatured(lose their shape) and their active site which links to the substrate molecule, is unable to "fit" as well, therefore, enzyme action decreases. Ph works in the same way. Enzymes work best under certain conditions of ph--either acid(as in protease enzymes in the stomach), alkaline, or neutral. Any deviation from the optimum ph results in the activity of enzymes decreasing. Finally, the more substrate that is available, the more the active sites of the enzyme molecules will bind to the available substrates. Initially, the reaction rate will increase. However, once all active sites have bound to available substrate, there will be no further increase in the rate of reaction. It will level off.