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ENZYMES. Enzymes An enzyme is a protein catalyst. Enzymes facilitate reactions without being used up. Lower the activation energy of a reaction. Generally, energy added to a reaction to speed it up is in the form of heat. This is not good for cells – Why?
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ENZYMES • Enzymes • An enzyme is a protein catalyst. • Enzymes facilitate reactions without being used up. • Lower the activation energy of a reaction. • Generally, energy added to a reaction to speed it up is in the form of heat. • This is not good for cells – Why? • Because of this, enzymes are critical
Enzymes simply decrease the activation energy of a reaction therefore, they will not affect equilibrium nor will they cause a reaction that would not normally occur spontaneously to do so.
Enzymes act on a substrate (reactant). • The substrate binds to a particular site on an enzyme that is very specific for it. • Most enzymes will not even bind to isomers of reactants. • Enzymes are generally named for their substrate and end in –ase. • eg) amylase binds to amylose, maltase binds to maltose. • The substrate binds to a very small portion of the enzyme. • The active site • Usually a pocket or groove in the quaternary structure of the enzyme.
As the substrate nears the functional groups of the amino acids in the enzyme protein, the enzyme changes it shape to better accommodate the substrate. • Known as the induced-fit model of enzyme-substrate interaction. • The attachment of the substrate to the active site creates the enzyme-substrate complex. • Essentially, the enzyme stresses the bonds of the substrate that would normally break in a reaction. • The heat generated by the cell, in conjunction with the stressed bonds brings the substrate to its transition state.
Some enzymes require cofactors for them to work properly. • Usually either inorganic substances or organic coenzymes • Many functions, including shuttling of molecules from one enzyme to another. • VISIT THE SITE BELOW FOR ANIMATION • http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html
ENZYME INHIBITION • Some compounds reduce the efficacy of an enzyme. These are usually broken down into two groups. • Competitive inhibitors • Similar enough to the substrate to bind to the active site and block the normal substrate from binding. • Can be overcome by increasing the concentration of substrate. • Noncompetitive inhibitors • Do not compete for the active site. • Bind to another site on the enzyme and change the shape of the entire molecule. • The enzyme then loses its affinity for the substrate.
Cells must regulate enzyme activity to coordinate cellular activity. • Can be accomplished two ways: • Restricting production of enzymes • Inhibiting action of enzymes • Some enzymes have allosteric sites that do not relate to substrate activity. • Binding an activator to an allosteric site allows the enzyme to function properly. • Binding an allosteric inhibitor stabilizes the inactive form of the enzyme
FEEDBACK INHIBITION • Feedback Inhibition • Used by cells to control metabolic pathways involving a sequence of reactions. • A product formed later in the series acts as an allosteric inhibitor to an enzyme earlier in the series. • As the product is used up, the inhibitor concentration decreases and the enzyme once again becomes active. • The sequence restarts, resulting in more product which then inhibits the earlier enzyme.
