5.5

• Enzymes affect activation energy by lowering the energy requirement barrier so that the reaction can proceed at normal cell temperatures.
• An enzyme's shape is important to its function because it determines which reactions the enzyme catalyzes.
• Each of your cells is like a miniature chemical factory capable of performing thousands of different reactions.
• As a cell's needs change, some of these reactions speed up, while others slow down.
• Enzymes and catalysts are specialized proteins that coordinate the cell's chemistry.
• To start a chemical reaction, it is first necessary to weaken chemical bonds in the reactant molecules.
• Activation energy is the "start-up" energy that activates the reactants and triggers a chemical reaction.
• One way to provide activation energy is to heat up the mixture of molecules.
• Hotter molecules may collide with enough energy to weaken bonds, whereas cooler molecules collide with less energy.
• Cellular reactions depend on the assistance of catalysts, compounds that speed up chemical reactions.
• The main catalysts of chemical reactions in organisms are specialized proteins called enzymes.
• Enzymes provide a way for reactions to occur at the cell's normal temperature.
• An enzyme doesn't supply activation energy to the reacting molecules, but instead lowers the energy requirement barrier so that the reaction can proceed at normal cell temperatures.
• The activation energy barrier is like a wall between two parts of a pond.
• If an enzyme lowers the wall, more frogs have enough energy to reach the other side.
• Each enzyme catalyzes a specific kind of chemical reaction.
• At any moment in the cell's life, the specific enzymes that are present and active determine which reactions occur.
• Just how does an enzyme catalyze only one type of reaction? The reason is that the shape of each enzyme fits the shape of only particular reactant molecules.
• A specific reactant acted upon by an enzyme is called the enzyme's substrate.
• The substrate fits into a particular region of the enzyme, called the active site.
• The fit between substrate and enzyme is not rigid.
• As the substrate enters, the active site changes shape slightly, fitting the substrate more snugly.
• This places certain functional groups of the active site in position to catalyze the reaction.
• The tighter grip may also bend the substrate, weakening its bonds and making them easier to break.
• Sucrose is slightly distorted as it enters the active site.
• The weakened bond reacts with water.
• The result is two products: a glucose molecule and a fructose molecule.
• Once these products are released, the enzyme's active site is ready to accept another molecule of sucrose.
• In fact, this recycling ability is a key characteristic of enzymes.
• An enzyme can lower activation energy by accepting two reactant molecules (substrates) into adjacent sites.
• Holding the reactants together enables them to react more easily.
• Enzymes can catalyze the formation of larger molecules from smaller molecules.
• An enzyme's structure and shape are essential to its function.
• An enzyme's shape is sensitive to changes in its surrounding environment.
• Factors such as pH and temperature can greatly affect how well an enzyme works or if it can work at all.
• The hydrogen bond is the weakest type of chemical bond, but it plays an important role in stabilizing biological structures.
• Hydrophobic interactions are non-specific attractions between nonpolar groups that occur when they come into contact with one another.
• Amino acids have both acidic (carboxyl) and basic (amino) groups that can form hydrogen bonds with other amino acids.
• Ionic bonds form between oppositely charged atoms or molecules.
• Peptides are short chains of two or three amino acids joined by peptide bonds.