Factors affecting enzyme activity

Temperature

Temperature affects enzyme-catalysed reactions by influencing the rate at which they occur. For a reaction to occur between an enzyme and substrate the two molecules need to collide.

At lower temperatures the reaction rate is low as the enzymes and substrates do not collide frequently. As temperature increases, reaction rates generally rise due to more frequent collisions between enzyme and substrate molecules.

Once the temperature exceeds an enzyme's optimal range, the enzyme can denature, losing its structure and function, which significantly decreases or stops the reaction.

The graph below shows this relationship between temperature and rate of reaction. Human enzymes have an optimal activity at about 37 degrees Celsius.

A graph with the x-axis labeled temperature in degrees Celsius, and the y-axis is labeled rate of reaction. The temperature range depicted on the x-axis ranges from zero on the far left side to 70 and beyond on the far right side. A curved line rises from zero and peaks at 40 degrees, and then descends from there. This line is labeled optimal temperature and there is a dotted line which rises from the 40 degree mark on the x-axis to the peak of the curved line

pH

Each enzyme has an optimal pH range in which it functions most effectively, and deviations from this range can lead to reduced activity or denaturation of the enzyme.

Extremely high or low pH levels can cause enzymes to denature, meaning they lose their three-dimensional structure, changing the shape of the active site and resulting in the enzyme being ineffective. This loss of structure can prevent the substrate from binding properly.

This relationship between pH and rate of reaction is shown in the graph below. At a low pH the rate of reaction is low. It is high around the enzyme's optimal pH and then decreases as the pH above the optimal pH.

A graph with the x-axis labeled ph, and the y-axis is labeled rate of reaction. The temperature range depicted on the x-axis ranges from 4 on the far left side to 11 and beyond on the far right side. A curved line rises from zero and peaks at 8, and then descends from there. This line is labeled optimal ph and there is a dotted line which rises from the 8 mark on the x-axis to the peak of the curved line.

The diagram below shows the activity of three enzymes that are found in different parts of our body, showing their optimum pH.

A graph with the x-axis labeled ph, and the y-axis is labeled rate of reaction. On the left side of the x-axis this is a line, labeled pepsin, that curves sharply up and then sharply down. From the peak of this curved line there is a dotted line running back down to the x-axis which is labeled optimum ph of pepsin. In the centre of the x-axis this is a line, labeled salivary amylase, that curves sharply up and then sharply down. From the peak of this curved line there is a dotted line running back down to the x-axis which is labeled optimum ph of salivary amylase. On the right side of the x-axis this is a line, labeled pancreatic lipase, that curves sharply up and then sharply down. From the peak of this curved line there is a dotted line running back down to the x-axis which is labeled optimum ph of pancreatic lipase.

  • Pepsin is found in the stomach, which has a low pH due to the hydrochloric acid.
  • Salivary amylase is found in the mouth, which is neutral.
  • Pancreatic lipase acts in the small intestine, which has a high pH due to bicarbonate, which neutralises the stomach acid.

Substrate concentration

As substrate concentration increases, the rate of an enzyme-catalysed reaction initially rises as more substrate molecules are available to bind to enzyme active sites.

However, if the amount of enzyme is limited, then the reaction will proceed until all enzymes become saturated with substrate. At this point the rate of reaction will plateau, as all active sites are occupied and unable to process additional substrate molecules any faster.

alt-text: A graph with the x-axis labeled substrate concentration and the y-axis labeled rate of reaction. A line curves upward from the base of the x and y axes until it plateaus approximately one third of the way along the x-axis

This plateau in reaction rate can continue until the substrate is exhausted and rate falls again, eventually to zero.

Enzyme concentration

Increasing enzyme concentration can increase the rate of reaction, as there are more enzyme molecules available to catalyse the reaction, leading to more frequent enzyme-substrate interactions. This is shown in the graph below by the dotted line.

alt-text: A graph with the x-axis labeled enzyme concentration and the y-axis labeled rate of reaction. From the base of the x-axis a line curves upward initially sharply, and then about half way along the x-axis continues to rise, but at a lower angle. At the point the angle of rise is lower a dotted line continues the initial sharp rise.

However, this rarely occurs as and more often something such as the substrate concentration becomes the limiting factor. This leads to first a plateau in reaction rate and, when all the substrate molecules are consumed, an eventual zero reaction rate.

This can also be viewed another way.

Enzyme concentration directly affects the rate of reaction. More enzyme molecules mean more active sites for substrate binding, which leads to an increase in reaction rate, provided there is sufficient substrate. However, if the amount of substrate is limited, increasing enzyme concentration beyond a certain point will have little to no effect on enzyme activity, since there will be a surplus of unnecessary enzyme molecules.

The effect of enzyme concentration and substrate concentration on reaction rate

A graph with the x-axis labeled substrate concentration and the y-axis labeled rate of reaction. A line rises sharply from the x-axis which then splits into two lines. One lower line that plateaus, labeled lower enzyme concentration, and a line which rises higher and then similarly plateaus, labeled higher enzyme concentration.

Note that in this case, the amount of enzyme is the limiting factor, so as the enzyme concentration increases, the reaction rate also increases until all the enzymes are saturated.  At this point, the reaction rate plateaus, unless more enzymes are added to the reaction.

Presence of inhibitors

Enzyme function can be regulated by inhibitors in a number of ways:

  • Competitive inhibitors that bind to the active site and block the substrate from binding
  • Non-competitive inhibitors that bind to a different site on the enzyme and alter its shape, reducing its activity
  • Feedback inhibition is a regulatory mechanism where the end product of a metabolic pathway binds to an enzyme involved early in the pathway, reducing its activity to prevent overproduction.

Competitive inhibition

Competitive inhibitors are molecules that bind to the active site of an enzyme, directly competing with the substrate and preventing it from attaching.

The shape of the competitive inhibitor is complementary to the active site of the enzyme.

A graphic depicting competitive inhibition. On the left-hand side of the graphic a circle with a wedge-shaped piece removed is labeled enzyme. The gap in the circle left by the wedge-shaped piece is labeled the active site. Above this gap sits a separate wedge shaped piece labeled competitive inhibitor. To the right of this circle is an arrow pointing to the right. To the right of the arrow is another circle, this one with the wedge labeled competitive inhibitor filling the gap in the previous circle. Above the circle is another separate piece labeled substrate.

Non-competitive inhibition

Non-competitive inhibitors bind to a site other than the enzyme's active site, called the allosteric site, causing a change in the enzyme's shape and reducing its activity regardless of substrate concentration.

A graphic depicting three consecutive circles. In the first circle, on the left side, the circle itself is labeled enzyme. Two chunks have been removed from the circle, with one labeled active site and the other labeled allosteric site. To the right of this circle is an arrow pointing to the right. To the right of this arrow is the second, centre, circle. This circle is the same as the first and next to the allosteric site is a piece designed to fit the space in the circle carved out as the allosteric site. The matching piece is labeled allosteric inhibitor. To the right of this circle is an arrow pointing to the right. To the right of this arrow is the final circle, on the right side of the graphic. This circle depicts the allosteric inhibitor filling the gap caused by the allosteric site. On the other side of the circle there are two further chunks labeled distorted active site. Above these chunks another piece is depicted labeled substrate.

Feedback inhibition

Feedback inhibition is a regulatory mechanism where the end product of a biochemical pathway inhibits an enzyme involved early in the pathway, preventing overproduction and conserving resources.

Example

In bacteria, feedback inhibition occurs in the production of the amino acid isoleucine from the amino acid threonine in a series of enzyme catalysed reactions.

When isoleucine levels become sufficiently high, it acts as an inhibitor by binding to the first enzyme in the pathway, threonine deaminase.

By binding allosterically to threonine deaminase, isoleucine changes the enzyme's shape, reducing its activity and thereby slowing down or stopping the production of more isoleucine.

This ensures that the cell does not produce excess isoleucine, conserving energy and resources.

alt-text: A graphic depicting a cycle. To the left-hand side of the cycle there is a wedge-shaped piece labeled threonine. An arrow at the bottom of this wedge points toward an image of a circle with a wedge-shaped cut-out and the threonine wedge about to fit into the space. An arrow from this image points to the next in the series, which is an image of the circle with the threonine wedge fitting in the cut-out to make the circle whole. This image is labeled enzyme one. From this circle an arrow points to an squiggly oval shaped image labeled intermediate substrate A. An arrow from this intermediate substrate A points to the next image in the cycle which is a circle with the intermediate substrate A positioned alongside it, overlapping with one edge. This circle is labeled enzyme two. From enzyme tow another arrow points to another image which looks like a squiggly oval shape cut in half lengthwise. This shape is labeled intermediate substrate B. From this image another arrow points to the next image in the cycle, which depicts another circle with the intermediate substrate B shape positioned alongside it, overlapping with one edge. This circle is labeled enzyme 3. From this circle another arrow points toward a rectangular shape labeled isoleucine. From the isoleucine shape another arrow points toward another shape. This shape consists of a circle with a wedge-shaped chunk removed. On the opposite side from this space the isoleucine rectangular shape is placed alongside, overlapping partly with the circle. From the wedge-shaped space an arrow, which splits in two points outward. One point of the arrow points outward from the cycle, and the other point connects to the next image of the cycle. This circle is labeled inhibition of the pathway. The next image is again that of the threonine wedge depicted at the start of the cycle. The arrow pointing up from the original threonine image connects to this threonine wedge. At this point the cycle is complete.