B.3 Factors Affecting the Activity of Enzymes

• The ability of an enzyme to function depends on the unique structural configuration of the apoenzyme (protein) and the availability of the correct cofactors and substrates.

• -  The reactions are also subject to environmental factors that can alter their shape or otherwise affect their ability to form activated E-S complexes.

• -  As a result, the metabolism and homeostasis are at the mercy of these same factors.

TEMPERATURE

• The hydrogen bonds that contribute to the tertiary structure of enzymes are weak and sensitive to temperature.

• -  If temperatures rise too much, these bonds break, and as with any other protein, an enzyme loses its tertiary structure, allowing it to take on a new shape and possibly a new set of properties.

• -  An optimum temperature exists where the enzyme and its substrate have the best shape to combine and form an activated E-S complex.

• The active site can become distorted by deviations from this temperature.
  o As a result, the ability of the enzyme to function maximally can be compromised with variations in temperature, and the reaction slows down or stops altogether.

  o Slight temperature fluctuations causes light deformations in the shape of enzymes. In many cases when normal conditions are restored, these enzymes will renature, or recover their optimal conformations.

• The temperature of the human body is normally about 37oC. Should this temperature increase (as occurs when one has a fever), many enzymes will continue to function but at a decreased rate.

• -  If body temperature reaches 40oC, the condition becomes severe and is known as hyperthermia.

• Homeostatic responses of the body include increased delivery of blood to the surface and sweating, which has a cooling effect as heat is converted into the kinetic energy of the water molecules that evaporate from the skin surface.

  o The higher the body temperature goes, the more distorted the enzymes become, and important reactions and body functions can be jeopardized.

  o If the temperature increases too much, enzymes will be denatured (change shape; lose tertiary structure) and cease to function as they normally would.

  o Many enzymes are denatured before body temperature rises to 50oC –fevers at this temperature are lethal.

• As body temperatures drop lower than 37oC, reactions slow down because of decreased kinetic energy (less molecular movement), not because enzymes get denatured.

  o When enzymes cool down, their molecular structures strengthen and they become less able to be distorted and form complexes with substrates, therefore decreasing their activity, as proposed by the Induced Fit Hypothesis.

  o Hypothermia occurs when the body temperature falls below 35oC. The body’s reaction are to shiver, which generates a little heat; form “goose bumps” on the skin (elevated hair follicles) providing a little more insulation, and deliver less blood to the surfaces to reduce heat loss by radiation.

  pH

• -  As with temperatures, enzymes function best at a particular pH.

• -  If the pH of a system deviates too far from an enzyme’s optimum pH, the H-bonds that contribute to the tertiary structure of the enzyme will be disrupted, allowing it to denature.

• -  Even though the majority of human tissues function at the slightly alkaline (basic) pH of about 7.3-7.4, there are some tissues and organs in the body whose enzymes depend on the maintenance of a significantly different pH in order to function.

• Ex.The stomach secretes a peptidase enzyme called pepsin, which functions best at a pH of about 2.5. This low pH results from the production and release of HCl by cells in the stomach lining.

• o In sharp contrast, the enzymes in the next organ along the digestive tract, the small intestine, function at a pH of close to 8.3. This significantly large pH change is achieved by the release of bicarbonate ions into the beginning region of the small intestine from the nearby pancreas.

  o These ions bond to free hydrogen ions in the food materials coming from the stomach. This sharply increases the pH of the material as it moves into the small intestine.

  o This example of pH requirements not only illustrates the significance of pH as a critical factor governing enzyme activity, but also stresses the importance of buffers in the body.

HEAVY METAL IONS

• Heavy metal ions can be very toxic in biological systems.

• -  Specifically mercury, thallium, and lead poisoning.

• -  The potential of these ions of these metals to be reactive in the body is what makes them so dangerous. They each have large positively charged nuclei giving them high affinities or attractive powers towards electrons.

• -  When they associate with other molecules, they tend to draw electrons toward themselves, away from their normal bonding patterns.

o In the case of enzymes, this re-arrangement of electrons leads to distortion of the

shape of the enzyme.
o The presence of heavy metal ions in the body lessens the chance that activated

enzyme-substrate complexes will form thereby decreasing enzymatic activity.

o Normal body function can be compromised should the level of anyone of these

types of ions be elevated for any length of time.

INHIBITORS

• The Lock and Key Model demonstrates how the presence of an inhibitor can affect the ability of an enzyme to bond with its substrate.

  o A greater concentration of inhibitors relative to the concentration of substrates will have a significant impact on the rate at which enzymes can function.

• -  There are two general categories of inhibitors:

o Competitive inhibitors have similar shapes to substrates and compete with them for occupancy of active sites. As long as an inhibitor occupies (“plugs up”) an active site, the enzyme is prevented from functioning because no activated enzyme-substrate complex can be formed.

o The intended reaction will not occur.
- The other class of inhibitors is called allosteric or noncompetitive inhibitors.

These substances combine with the enzymes in a location other than the active sites.

o These associations affect the electron distributions in the enzymes, which distort the shapes of the active sites as a result.

o The formation of activated enzymes-substrate complexes is prevented because the substrates can no longer be properly accommodated.

SUBSTRATE AND ENZYME CONCENTRATION

• The substrates for enzymatic reactions in a biological system are either for molecular products of another metabolic process or they have been introduced into the system, perhaps as food substances.

• -  Individual cells obtain some substrates by transporting them across their membranes into their interiors.

• -  Regardless of how they are acquired, the amount (or concentration) of them in a system is significant. If they are limited, then any reaction involving them will be limited and the rate of product formation may be minimal.

  o At higher substrate concentrations the rate of reactions involving them will be increased as long as the correct enzymes are available.

• -  Using a cell as an example, increasing the availability of substrates will increase the rate of their metabolism to the point where the cell is conducting the reactions as fast as it can because all the enzymes are “in use”.

• -  Cells do not synthesize unlimited amounts of enzymes therefore the rate of the enzymatic reactions becomes dependent on the concentration of the enzymes.

• Above this concentration, where there is an excess of substrates, the reaction will continue at a constant rate.

  o Similar to the effect of increasing substrate concentrations, altering the concentration of the enzymes will impact the reaction rate.

  o Obviously, if there are few enzymes available, little product can be formed.

  • -  In a biological system, the introduction of substrates can stimulate enzyme production.

  • -  As enzyme concentrations are increased, reaction rates will increase as long as substrates are available.