Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Energy, Enzymes, and Metabolism A. Enzymes 1. Definitions Enzyme: Biological Catalyst Catalyst: A substance that speeds up a chemical reaction, but does not affect the final equilibrium concentrations of reactants and products. Substrate(s): The starting material(s) (initial reactants) that an enzyme binds to Product(s): The end products that the substrate is converted into by the reaction. 2. Mechanism of Action: Enzymes act by lowering the Activation Energy of a chemical reaction Refer to: Progress of Reaction Diagram Enzymes lower the activation energy by creating a stabilized intermediate state known as an “Enzyme-Substrate Complex” A typical enzymatic reaction follows steps that are similar to these: An enzyme molecule binds to a substrate molecule to form a noncovalently bound enzyme-substrate complex. (The substrate binds with great specificity at the enzyme’s active site.) One or more amino acids in the active site interact with the substrate, altering it. At this time, some products may dissociate, and other (different) substrate molecules may enter the active site. The substrate is converted into the final product(s), which dissociate from the enzyme. The enzyme molecule is now free to bind to another substrate molecule. Refer to: The example of the hydrolysis of sucrose by sucrase. 3. Cofactors and Coenzymes These are substances that are not part of the amino acid structure of proteins, but which are required for the activity of certain enzymes. Most notable of the enzymes that need cofactors are the enzymes that catalyze oxidation-reduction reactions. “Cofactor” usually refers to a substance that is noncovalently bound to the enzyme; “coenzyme” usually refers to a covalently bound substance. 4. Enzyme Inhibitors Competitive Inhibitors: Are molecules that are chemically similar to the substrate and can compete with the substrate for access to the active site (like putting the wrong key in a lock, blocking the correct key but not destroying the keyhole) Noncompetitive Inhibitors: Are substances that chemically interact with the protein that the enzyme is made of, altering the enzymes chemical structure and destroying the active site’s ability to bind to its substrate. A noncompetitive inhibitor may acts on a part of the enzyme other than its active site (like hitting a lock with a sledgehammer, bending the keyhole out of shape so that the key doesn’t fit any more) B. Metabolism: Basic Principles 1. Definitions Metabolism: The processes of catabolism and anabolism Catabolism: The processes by which a living organism obtains its energy and raw materials from nutrients Anabolism: The processes by which energy and raw materials are used to build macromolecules and cellular structures (biosynthesis) 2. Reduction and Oxidation An atom becomes more reduced when it undergoes a chemical reaction in which it Gains electrons By bonding to a less electronegative atom And often this occurs when the atom becomes bonded to a hydrogen An atom becomes more oxidized when it undergoes a chemical reaction in which it Loses electrons By bonding to a more electronegative atom And often this occurs when the atom becomes bonded to an oxygen In metabolic pathways, we are often concerned with the oxidation or reduction of carbon. Reduced forms of carbon (e.g. hydrocarbons, methane, fats, carbohydrates, alcohols) carry a great deal of potential chemical energy stored in their bonds. Oxidized forms of carbon (e.g. ketones, aldehydes, carboxylic acids, carbon dioxide) carry very little potential chemical energy in their bonds. Reduction and oxidation always occur together. In a reduction-oxidation reaction (redox reaction), one substance gets reduced, and another substance gets oxidized. The thing that gets oxidized is called the electron donor, and the thing that gets reduced is called the electron acceptor. 3. Enzymatic Pathways for Metabolism Metabolic reactions take place in a step-wise fashion in which the atoms of the raw materials are rearranged, often one at a time, until the formation of the final product takes place. Each step requires its own enzyme. The sequence of enzymatically-catalyzed steps from a starting raw material to final end products is called an enzymatic pathway (or metabolic pathway) 4. Cofactors for Redox Reactions Enzymes that catalyze redox reactions typically require a cofactor to “shuttle” electrons from one part of the metabolic pathway to another part. There are two main redox cofactors: NAD and FAD. These are (relatively) small organic molecules in which part of the structure can either be reduced (e.g., accept a pair of electrons) or oxidized (e.g., donate a pair of electrons) NAD(oxidized) + H + Pair of electrons NADH(reduced) (Empty of electrons) (Carrying electrons) FAD(oxidized) + H + Pair of electrons FADH(reduced) (Empty of electrons) (Carrying electrons) In humans, NAD is derived from niacin, and FAD is derived from riboflavin NAD and FAD are present only in small (catalytic) amounts – they cannot serve as the final electron acceptor, but must be regenerated (reoxidized) in order for metabolism to continue 5. ATP: A “currency of energy” for many cellular reactions ATP stands for adenosine triphosphate. It is a nucleotide with three phosphate groups linked in a small chain. The last phosphate in the chain can be removed by hydrolysis (the ATP becomes ADP, or adenosine diphosphate). This reaction is energetically favorable – it has a G' of about – 7.5 kcal/mol ATP + H2O ADP + Phosphate + Energy (-7.5 kcal/mol) ATP hydrolysis is used as an energy source in many biological reactions that require energy – for example, active transport in the sodiumpotassium pump (see last section) During catabolism, energy released from the oxidation of carbon is captured and used to synthesize ATP from ADP and phosphate. ADP + Phosphate + Energy ATP + H2O