6.1 Metabolism Basics
6.1 Metabolism Basics
Metabolism consists of all the chemical processes that occur in living cells. These processes/reactions can generally be classified as either anabolic or catabolic. Anabolic means to build, catabolic means to breakdown. If you have trouble remembering the difference between the two, remember that anabolic steroids are what are used to build enormous muscle mass.
Figure 6.11 One of these two is taking anabolic steroids, which one would be your guess?
An anabolic reaction/pathway requires energy to build something. A catabolic reaction/pathway generates energy by breaking down something. This is shown in the example below of glucose and glycogen. The same is true for other macronutrients.
Figure 6.12 The breakdown of glycogen to glucose is catabolic. The glucose can then be used to produce energy. The synthesis of glycogen from glucose is anabolic and requires energy.
Anabolic and catabolic can also be used to describe conditions in the body. For instance, after a meal there is often a positive energy balance, or there is more energy and macronutrients than the body needs at that time. Thus, some energy needs to be stored and the macronutrients will be used for synthesis, such as amino acids being used for protein synthesis. However, after a fast, or a prolonged period without energy intake, the body is in negative energy balance and is considered catabolic. In this condition, macronutrients will be mobilized from their stores to be used to generate energy. For example, if prolonged enough, protein can be broken down, then the released amino acids can be broken down to be used as an energy source.
A number of the metabolic reactions either oxidize or reduce compounds. A compound that is being oxidized loses at least one electron, while a compound that is reduced gains at least one electron. To remember the difference, a mnemonic device such as OIL (oxidation is lost), RIG (reduction is gained) is helpful. Oxidation reactions and reduction reactions are “coupled” reactions, one cannot exist without the other. For example, a reduction reaction requires an electron. Where does that electron come from? It comes from an oxidation reaction. Scientists commonly refer to oxidation reactions and reduction reactions as oxidation-reduction reactions, or as redox reactions. Oxidation-reduction reactions are illustrated in the figure below.
Figure 6.13 The purple compound is being oxidized, the orange compound is being reduced1 Another way to remember oxidation versus reduction is LEO goes GER (like a lion)
Lose Elections = Oxidation
Gain Elections = Reduction (YES, gaining electrons is considered reduction)
Iron is a good example we can use to illustrate oxidation-reduction reactions. Iron commonly exists in two states (Fe3+ or Fe2+). It is constantly oxidized/reduced back and forth between the two states. The oxidation/reduction of iron is shown below.
Fe3+ loses an e- → Fe2+ (Oxidation) Fe2+ gains an e- → Fe3+ (Reduction)
Interestingly, the oxidation states of iron (mentioned above) are critical to our ability to use the iron present in our diet. Fe2+ (also known as ferrous iron) is easily absorbed in the small intestine. Fe3+ (also known as ferric iron) is not so easily absorbed. Gastric acid (produced by the stomach) and vitamin C promote the conversion of Fe3+ to Fe2+ so we can maximize iron absorption in the small intestine.
However, some oxidation reduction reactions are not as easy to recognize. There are some simple rules to help you recognize less-obvious oxidation/reduction reactions that are based upon the gain or loss of oxygen or hydrogen. These are as follows:
Oxidation: gains oxygen or loses hydrogen Reduction: loses oxygen or gains hydrogen
Remembering how this applies to hydrogen will be very helpful later in this chapter.
References
1. http://en.wikipedia.org/wiki/Image:Gulf_Offshore_Platform.jpg
6.11 Cofactors
A number of enzymes require cofactors to function. Cofactors can be either organic or inorganic molecules that are required by enzymes to function. Many organic cofactors are vitamins or molecules derived from vitamins. Most inorganic cofactors are minerals. Cofactors can be oxidized or reduced for the enzymes to catalyze the reactions.
Two common cofactors that are derived from the B vitamins, niacin and riboflavin, are NAD
(nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide), respectively.
Both of these cofactors can be reduced (remember that reduction is a process by which electrons, as part of H in this case, are gained); NAD is reduced to form NADH, while FAD is reduced to form FADH2 as shown in the 2 figures below.
Figure 6.111 The reduction of NAD (left) to form NADH (right)3
Figure 6.112 The reduction of FAD (left) to FADH2 (right) 4
NADH and FADH2 are molecules that are critical to our cells’ ability to process the energized electrons obtained through the catabolism (digestion) of food molecules, like glucose. The energized electrons, which are highly reactive and potentially destructive, are temporarily managed by NADH and FADH2 until they can be processed by the Electron Transport Chain step of Cellular Respiration (see Section 6.26 below).
An example of a mineral that serves as a cofactor is Fe2+ for proline and lysyl hydroxylases. Proline and lysine are two amino acids that must be hydroxylated (the addition of an OH group) in order to be used as building blocks for collagen, perhaps the most important structural protein in the body. We will discuss later in detail why vitamin C (ascorbic acid) is needed to reduce iron to Fe2+ so that it can serve as a cofactor for proline and lysyl hydroxylases.
Figure 6.115 Iron (Fe2+) is a cofactor for proline and lysyl hydroxylases
References & Links
- http://en.wikipedia.org/wiki/File:NAD%2B_phys.svg
- http://en.wikipedia.org/wiki/File:Flavin_adenine_dinucleotide.png
- http://en.wikipedia.org/wiki/File:NAD_oxidation_reduction.svg
- http://en.wikipedia.org/wiki/File:FAD_FADH2_equlibrium.png
