A Webpage by Chul-Won Park and Erik Zipp
for Introduction to Biochemical Engineering, Fall 2000.
What are ENZYMES?

An enzyme is a protein-based substance which serves as a catalyst in living organisms by regulating the rate of spontaneously chemical reactions. The rate of reaction does not solely depend on the free energy difference between the initial and final states, but instead on the actual path through which the reactants are transformed into products. Without these catalysts, simple tasks that occur within living organisms cannot take place at a rate beneficial to the species. The enzymes increase the rate of reaction without effecting the reactions difference in free energy. Natural mechanisms such as the digestion of food, the conservation and transformation of chemical energy, and the construction of cellular macromolecules from smaller precursors are all supported with the use of enzymes. In fact, if a living organism is deficient in a particular enzyme, it could result in serious diseases and possibly death.

Enzymes also act in industrial and medical applications. The fermenting of wine, leavening of bread, the curdling of cheese, and the brewing of beer have been practiced for centuries, but it wasn't until recently that all these processes have been discovered to be possible with the use of enzymes. Once discovered to be beneficial to industrial use, enzymes have found their way into many applications, with none more important than the production of man-made pharmacutical drigs which can kill disease-causing microorganisms, promote wound healing, and even diagnosing certain potentiaklly life-threateneing diseases.


How do ENZYMES work?

Enzymes accelerate biochemical reactions by physically interacting with the reactants and products to provide a more favorable pathway for the transformation of one to the other. They increase the rates of reactions by increasing the probability that the reactant can interact properly. But no matter how enzymes can ioncrease the rate of a reaction, they cannot promote reactions where the DG is positive (non-spontaneous reactions).

As DG is harvested, stored, or used to perform cellular work, it can be transferred to other molecules. It can be transferred into heat, electrical work, or mechanical work depending on the needs of the organism.



General Properties of ENZYMES.

Enzymes promote the rate of reactions in several ways.

  • Higher reaction rates
    Reactions that take advantage of an enzymes ability to speed up the reaction often have rates which are 106 to 1012 times faster than the corresponding uncatalyzed reactions. The reaction rate with enzymes are still several times greater than those using standard chemical agents as catalysts.

  • Milder reaction conditions
    Instead of the harsh environment needed for uncatalysed or chemically catalysed reactions to occur, enzymes often work in temperatures below 100oC, at atmospheric pressure, and at nearly neutral pH.

  • Greater reaction specifity
    Unlike chemically catalyzed reactions, those that use enzymes as catalysts often have no or few side products.

  • Capacity for regulation.
    The rate of a reaction in enzyme catalysis methods vary in responce to the concentration of only the substances themselves, rather than the substrate. This allows allosteric control, covalent modification of enzymes, and variation of the amounts of enzymes synthesized.

However, all these properties of enzymes are dependent on the external environment of the reaction. Temperature and pH both effect the rates at which a reaction takes place, and there exists an optimum value for each.


Effects of pH on ENZYMES.

Most proteins, and therefore enzymes, are active only within a narrow pH range usually between 5 and 9. Several factors are influenced directly by the pH in which the reaction takes place.


The graph of pH against the reaction rate is a bell shaped curve. The curve reprecents the ionization of certain amino acid residue that must be in a specific ionization state for enzyme activity. The inflection point of the curve is called the pK of the reaction and can identify amino acid residues essential to enzymatic activity.


Effects of Temperature on ENZYMES.

Temperature can effect an enzyme in two ways. One is a direct influence on the reaction rate constant, and the other is in thermal denturization of the enzyme at elevated temperatures.

To relate the effect of temperature to the reaction rate constant, the Arrhenius equation is used:

,

where k is the rate constant, R is the gas law constant, A is the frequency factor and Ea is the activation energy of the reaction.

The temperature range over which enzymes show activity is limited between the melting point (0oC) and bioling point (100oC) of water. If a temperature is too low, there can be no noticable reaction rate since the enzyme is operating at a temperature far below its optimum. If the temperature at which the enzyme is operating at is well above 100oC, then thermal deactivization can occur.

Thermal deactivization of enzymes limits their useful lifetime in processing environments. Therefore, it is important in many preocess design and manufacturing levels to have the correct temperature of reaction. If the reaction temperature is too high, the enzymes will eventually deactivate in an irreversible way and thus halting the reaction from taking place. For many enzymes found within mammals, the optimum temperature is 37oC, but deactivization can occur as low as 45 to 55oC. Deactivization of enzymes may be irreversible or reversible.




All pictures (C) Voet, Voet and Pratt, Fundamentals of Chemistry, John Wiley & Sons, Inc. 1999
References: Voet, Voet and Pratt, Fundamentals of Chemistry, John Wiley & Sons, Inc. 1999
Blanch and Clark, Biochemical Engineering, Marcel Dekker, Inc. 1997.