5.1: Introduction
In this chapter, we look at the properties and mechanism of action of enzymes. These include allosteric change ( induced fit , enzyme regulation ), energetic events (changes in activation energy ), and how enzymes work in open and closed (experimental) systems. Any catalyst, by definition, accelerates a chemical reaction. But enzymes and inorganic catalysts differ in important ways (blue and red in the table below).
Enzymes are long polymers that can fold into intricate shapes. As a result, they can be more specific than inorganic catalysts in which substrates they recognize and bind to. Finally, enzymes are flexible and can be regulated in ways that rigid, inflexible, metallic inorganic catalysis cannot. The specificity of an enzyme lies in the structure and flexibility of its active site . We will see that the active site of enzymes undergo conformational change during catalysis. The flexibility of enzymes also explains the effects of enzymes to cellular metabolites that indicate the biochemical status of the cell. When such metabolites bind to an enzyme, they force a conformational change in the enzyme that change the catalytic rate of the reaction, a phenomenon called allosteric regulation . As you might imagine, changing the rate of a biochemical reaction can change the rate of an entire biochemical pathway…, and ultimately the steady state concentrations of products and reactants in the pathway.
To understand the importance of allosteric regulation, we’ll look at how we measure the speed of enzyme catalysis. As we consider the classic early 20th century enzyme kinetic studies of Leonor Michaelis and Maud Menten, we’ll focus on the significance of the Km and Vmax values that they derived from their data. But, before we begin our discussion here, remember that chemical reactions are by definition, reversible. The action of catalysts, either inorganic or organic, depends on this concept of reversibility.
Finally, let’s give a nod to recent human ingenuity that enabled enzyme action to turn an extracellular profit ! You can now find enzymes in household cleaning products like detergents, where they digest and remove stains caused by fats and pigmented proteins. Enzymes added to meat tenderizers also digest (hydrolyze) animal proteins down to smaller peptides. Enzymes can even clean a clogged drain!
Learning objectives
When you have mastered the information in this chapter, you should be able to:
1. describe how the molecular flexibility of protein and RNA molecules make them ideal biological catalysts.
2. compare and contrast the properties of inorganic and organic catalysts.
3. explain why catalysts do not change equilibrium concentrations of a reaction conducted in a closed system.
4. compare the activation energies of catalyzed and un-catalyzed reactions and explain the roles of allosteric effectors in enzymatic reactions.
5. discuss how allosteric sites interact with an enzyme's active site and explain the concept of the rate limiting reaction in a biochemical pathway.
6. write simple rate equations for chemical reactions.
7. write the possible rate equations for equations for catalyzed reactions.
8. distinguish between Vmax and Km in the Michaelis-Menten kinetics equation.
9. state what Vmax and Km say about the progress of an enzyme catalyzed reaction.
10. interpret enzyme kinetic data and the progress of an enzyme-catalyzed reaction from this data.
11. more accurately identify Leonor Michaelis and Maud Menten!