Enzymes: Organic Catalysts

Transcription

1 Enzymes: Organic Catalysts Biology 12 A. Allen What are Enzymes? Enzymes are proteins. Enzymes allow many chemical reactions to occur within the homeostasis constraints of a living system. Enzymes function as organic catalysts. A catalyst is a chemical involved in, but not changed by, a chemical reaction. Many enzymes function by lowering the activation energy of reactions. By bringing the reactants closer together, chemical bonds may be weakened and reactions will proceed faster than without the catalyst. The use of enzymes can lower the activation energy of a reaction (Ea).

2 What are Enzymes? Enzymes can act rapidly, as in the case of carbonic anhydrase (enzymes typically end in the -ase suffix), which causes the chemicals to react 107 times faster than without the enzyme present. Carbonic anhydrase speeds up the transfer of carbon dioxide from cells to the blood. There are over 2000 known enzymes, each of which is involved with one specific chemical reaction. Enzymes are substrate specific. The enzyme peptidase (which breaks peptide bonds in proteins) will not work on starch (which is broken down by human-produced amylase in the mouth). How Do Enzymes Work? The functioning of the enzyme is determined by the shape of the protein (primary, secondary tertiary & quaternary structure) active site-the part of the enzyme onto which the substrate fits. It recognizes, confines and orients the substrate in a particular direction. The induced fit hypothesis: As the substrate binds to the enzyme, functional groups from each react which alters the structure of the enzyme. The altered shape of the enzyme placing some strain on the substrate molecules substrate is joined/broken.

3 Cofactors are nonproteins essential for enzyme activity. Ions such as K + and Ca +2 are cofactors. Coenzymes are nonprotein organic molecules bound to enzymes near the active site. NAD + is made from dietary niacin (vitamin B3) (nicotinamide adenine dinucleotide). Factors that Affect Enzyme Efficacy Temperature Increases in temperature increase in molecular movement will speed up the rate of nonenzyme mediated reactions, and so temperature increase speeds up enzyme mediated reactions, but only to a point. When heated too much, enzymes (since they are proteins dependent on their shape) become denatured. When the temperature drops, the enzyme regains its shape. Thermolabile enzymes, such as those responsible for the color distribution in Siamese cats and color camouflage of the Arctic fox, work better (or work at all) at lower temperatures

4 Factors that Affect Enzyme Efficacy Concentration of substrate and product also control the rate of reaction, providing a biofeedback mechanism. Factors that Affect Enzyme Efficacy Changes in ph will also denature the enzyme by changing the shape of the enzyme. Remember: the shape of a protein (in this case, an enzyme determines its function). Enzymes are also adapted to operate at a specific ph or ph range.

5 Enzyme Inhibition There are a number of substances that can inhibit enzyme activity. Competitive Inhibitors are so similar to the enzyme s substrate that they are able to enter the enzyme s active site and block the normal substrate from binding. A drug, sulfanilamide resembles PABA, a chemical essential to a bacteria that infects animals. sulfanilamide competes with PABA, shutting down an essential bacterial (but not animal) pathway. Enzyme Inhibition Non-competitive Inhibitors - substances that attach to a binding site on an enzyme other than the active site, causing a change in the enzyme s shape and a loss of affinity for its substrate. Lead binds to SH groups in this fashion. Irreversible Inhibition occurs when the chemical either permanently binds to or massively denatures the enzyme so that the tertiary structure cannot be restored. Nerve gas permanently blocks pathways involved in nerve message transmission, resulting in death. Penicillin, the first of the "wonder drug" antibiotics, permanently blocks the pathways certain bacteria use to assemble their cell wall components.

6 Allosteric Regulation Cells must control enzyme activity to coordinate cellular activities. They may accomplish this in two ways: by restricting the production of a particular enzyme, or by inhibiting the action of an enzyme that has already been produced. Some enzymes possess receptor sites, called allosteric sites, that are some distance away from the active site. Substances that bind to the allosteric sites may inhibit or stimulate an enzyme s activity.