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4/4/11 Energy • Energy can take a huge number of forms A Bit About Enzymes by Genghis Khan Example: A nuclear power plant uses radiation energy to heat water and turn it into steam, which moves a turbine, which generates electricity. – motion – radiant energy (light, UV, radio waves, etc.) – heat – chemical energy – radioactivity • The First Law of Theormodynamics states that: Energy cannot be created or destroyed; it can only be changed from one form to another. Example: An athlete eats Wheaties (which contain chemical energy stored in the matter that makes them up) and converts their energy into heat and motion— and also into chemical energy (e.g. for making the proteins used in building muscle tissue). 1 4/4/11 Energy Another example: A falling basketball hits the ground and converts kinetic energy to elastic potential energy. This is immediately converted back into kinetic energy, which forces the ball back upwards. • For our purposes here, all energy can essentially be classified into two types: potential energy and kinetic energy – Potential energy is "stored" in a system – Kinetic energy is the energy that is acting on a system – Example: If you compress a bedspring, you exert kinetic energy on it as you push. When the spring is fully compressed, it stores potential energy. Release the spring, and, with a loud "boing", its potential energy is converted into kinetic energy of motion. The reason the ball doesn't bounce eternally is that energy is constantly "leaking" out of the system—in the form of sound waves, friction with the air and the floor, etc. Chemical Energy • Potential energy is also stored in chemical bonds —in the bonds that hold atoms together in a molecule. • Kinetic energy is released when chemical bonds are broken. • However, kinetic energy is needed to form chemical bonds in the first place. 2 4/4/11 So a plant captures kinetic energy (light) from the Sun, and is able to use that energy, in a beautiful and complex way, to stick atoms together, form chemical bonds, and build up more plant tissue, as it grows. If that plant burns, then the potential energy that was stored in all those chemical bonds is released, as kinetic energy (light, heat, motion). Reactions Reactions • So a chemical reaction both consumes energy and releases energy. • If it produces more than it consumes, the reaction is self-propagating, and is said to be exothermic. – Simplest example: Fire. • If it consumes more than it produces, the reaction must have a source of energy to proceed, and is said to be endothermic. • Exothermic reactions, once started, will continue until completion. – When gasoline is burned, for example, the chemical bonds that make it up break apart and reform with oxygen molecules. We write it as 2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O. • However, even an exothermic reaction requires an input of energy to get it started—such as the spark that sets off a fire, or that starts the burning of gasoline in your car's engine • This is the activation energy. 3 4/4/11 Catalyst • A catalyst is a substance that lowers the activation energy of a chemical reaction. • A catalyst is not affected by the chemical reaction—in other words, it's not "burned up" or otherwise consumed by the reaction. Enzymes • Enzymes are proteins that act as catalysts in living systems. – Small organic molecules that aren't proteins but that play various "helping roles" in driving chemical reactions are known as coenzymes. – A number of vitamins are coenzymes, such as B1 (thiamine), B2 (riboflavin), B9 (folic acid), and others. • One enzyme generally catalyzes only one chemical reaction. • The molecule(s) that an enzyme acts on is/are called the substrate. We can graph the amount of potential energy in a chemical system over time. Notice that the activation energy appears as a "hump" that has to be "gotten over" for the reaction to "roll downhill". If an enzyme is present, it lowers the activation energy—making it far more likely that the reaction will proceed to completion. 4 4/4/11 Enzyme Shapes Diagram of how the "lock and key" model works. . . • "Lock and key" model – An enzyme molecule has a single region called the active site, which the substrate fits into like a key in a lock – Once the substrate molecule is bound to the active site, the enzyme molecule may put stress on it, or may bring it together with another molecule. . . – . . . and thus the substrate may be either broken up, or joined together, as the case may be. The enzyme beta-amylase. . . The enzyme beta-amylase with a substrate molecule in the active site. . . 5 4/4/11 Another real example: phenylalanine hydroxylase, shown here, with a substrate molecule in the active site. Another real example: betalactamase, shown here with a substrate molecule (penicillin) in the active site. Bacteria that can make beta-lactamase are resistant to penicillin, because they can break it down! 6