The reason for the decline and eventual stoppage in reaction rate in increasing substrate concentration, is due to the fact that the enzymes have now become the main limiting factor, their numbers are constant, and they have a maximum rate at which they break down substrates, so it is inevitable, that as the enzymes reach their maximum capacity (where the number of substrate molecules is more than the enzyme can break down), the increase in rate of reaction will decrease and will eventually reach a maximum.
Temperature
As the temperature increases, the enzyme molecules and the substrate molecules gain energy, and as a result, they move about faster (they have a higher kinetic energy). This results in both more frequent collisions between substrate and enzyme molecules, and more successful collisions between substrate and enzyme (more collisions have enough energy to cause a reaction, as they themselves have more energy). Therefore as the temperature the reactants are in increases, the rate of reaction also increases in direct proportion. However, as the temperature approaches a certain point, the increase in reaction rate dramatically decreases, and beyond a certain point, the reaction rate decreases very dramatically, this is shown on the graph below, of rate of reaction to temperature.
The reason for this decline in reaction rate beyond a certain point, is that beyond this temperature, the enzymes denatures, therefore meaning that the shape of its active site is permanently changed, this means that the enzymes cannot accept the substrate molecules and thus cannot break them down, explaining the sharp decline. This is why enzymes have an optimal temperature, which is the maximum working temperature for the enzymes without being too high as to denature it. The enzymes denature, as the molecules or polypeptide chains that make them up vibrate more and more, until they gain sufficient energy to break their bonds, which holds the enzyme in its shape (that makes it able to accept only substrates of certain shapes), therefore id the bonds are broken, the enzymes tertiary and quaternary structures are changed permanently, and it cannot accept substrate molecules.
Effect of pH on enzyme activity
Different enzymes have a different optimal pH, which is the pH in which the enzyme catalyses substrate at its fastest rate. If the pH is lower or higher than this, then the reaction rate dramatically decreases until it is eventually nil. For example, the enzyme pepsin has an optimal pH of 2, any higher than this, and the reaction rate dramatically decreases, as shown below.
Beyond the optimal pH, the enzyme is denatured, which results in the changing of the active site and therefore inability of the enzyme to accept the substrate molecules to break down. The reason the enzyme denatures due to the bonding that determines the tertiary and quaternary structures being affected, namely the hydrogen bonding between the R groups, and also the ionic bonding involved in holding and stabilizing the structures. A change in pH can alter or even break the bonds, resulting in the change in tertiary and quaternary structure, the enzyme is denatured. It is the nature of the bonding and how much it is affected by the pH which determines the optimal pH of the enzyme.
Enzyme Concentration
The higher the concentration of enzymes the reaction rate increases directly proportionally. Assuming that the substrate is always in excess, the reaction rate will always increase in direct proportion, but if the substrate is not in excess, then the increase in reaction rate will decrease and will eventually reach a point where the maximum rate of reaction is reached. This is due to the fact that at higher enzyme concentrations, there will not be enough substrate molecules for all the enzymes to catalyse, therefore adding more enzymes will have no effect on the rate of reaction. The graph shows the relationship between increasing reaction rates and the concentration of enzymes.
Enzyme Inhibitors
Enzyme inhibitors are substances which alter the catalytic action of the enzyme and consequently slow down, or in some cases, stop catalysis. There are three common types of enzyme inhibition - competitive, non-competitive, substrate inhibition and feedback inhibition. Competitive inhibitors have a shape resembling that of the substrate molecules; they can therefore fit into the active site of the enzyme, while there the substrate molecule cannot be catalysed due to the inhibitor being in the way, the reaction rate is therefore reduced. How much the reaction rate is reduced depends on the concentration of inhibitor relative to that of the substrate. This category of competitive inhibitor can be broken down into reversible and non reversible inhibition. Reversible inhibitors spend an interval of time before leaving, so the reaction rate is lowered but not by as much as the non-reversible inhibitors, which permanently bond the enzymes active site, rendering the enzyme useless, therefore lowering the rate of reaction considerably more. Non-competitive inhibitors are molecules that fit into another part of the enzyme, not the active site, and change the shape of the active site there. These are non competitive, as they do not compete with the substrate for the active site. These non-competitive inhibitors decrease the reaction rate considerably due to the fact that no substrate molecules can stop them from bonding to the part of the enzyme that they do. Substrate inhibition occurs where there is such a high number of substrate molecules that they actually block each other from access to the active site, therefore reducing the number of substrate molecules broken down. The concentration of substrate molecules must be extremely high for this to happen, but when it does, it affects the reaction rate considerably. Feedback inhibition occurs where a product of enzyme catalysis is either a non-competitive inhibitor of an enzyme, or is a competitive inhibitor for another enzyme in the chain of enzymes, therefore the reaction rate can reach a maximum, no matter how high the substrate or enzyme concentration, as the increased products that this produces results in more products that are inhibitors made which directly decreases the rate of reaction proportionally.