It is essential for dental professionals to have a basic understanding of biochemistry because it is the foundation for understanding and applying the concepts of nutrition. An overview of the biochemical concepts relevant to nutrition will serve as a wonderful resource as the learner goes through this textbook. A comprehensive review of chemistry and biochemistry concepts can be found online at Evolve.
Biochemistry is the study of life at the molecular level. The three major areas of biochemistry are structure, metabolism, and information. Structure describes the three-dimensional arrangement of atoms in a molecule, the smallest particle of a substance that retains all the properties of the substance. Important for life, the structure of a biomolecule determines its function. A biomolecule is any molecule that is produced by a living cell or organism, which would include carbohydrates, proteins, nucleic acids, and lipids, as well as other organic compounds found in living organisms; in contrast, a nutrient is a substance required by the body that must be supplied by an outside source which is usually food. Metabolism involves the production and use of energy. In metabolism, energy can be extracted from dietary carbohydrates, proteins, and lipids and used to create the biomolecules required for life. This highly regulated system ensures that energy is not wasted. Information involves the transfer of biological information from deoxyribonucleic acid (DNA) to ribonucleic acid (RNA) to protein. The blueprint for life is stored in DNA and the resulting proteins carry out all the processes required for life.
Atoms in a compound are held together by chemical bonds. There are two types of chemical bonds that form. An ionic bond forms between a positively charged metal ion and a negatively charged nonmetal ion. Hydroxyapatite in tooth enamel is composed of ionic bonds between calcium ions (Ca2+), phosphate ions (PO43−), and hydroxide ions (OH−). A covalent bond forms when electrons are equally shared between two nonmetals. Ultimately, the biomolecules responsible for life are based on carbon (C) because of carbon’s ability to form stable covalent bonds to itself and many other atoms, forming long chains and rings. In addition to carbon, the combination of different atoms, like hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and phosphorus (P), into biomolecules provides for great variety in chemical structure, properties and reactivity in biological systems. One way to organize this variety in chemical structure is the classification of molecules into functional groups. A functional group is a group of atoms that gives a family of molecules its characteristic chemical and physical properties. Molecules that have similar functional groups have similar properties. Figure 2-1 defines and exemplifies a few functional groups found in biochemistry.
Functional groups can be converted into other functional groups via chemical reactions such as oxidation-reduction, condensation, and hydrolysis. Oxidation-reduction reactions are important in metabolism as biomolecules are degraded or synthesized. Oxidation can be defined as a loss of electrons, an increase in charge, a gain of O atoms, or a loss of H atoms. Reduction can be defined as a gain of electrons, a decrease in charge, a loss of O atoms, or a gain of H atoms. In metabolism, energy is extracted from glucose (C6H12O6) by completely oxidizing it to carbon dioxide (CO2). Condensation and hydrolysis reactions are important in digestion and metabolism. In general, a condensation reaction creates a new molecule by forming a bond between two smaller molecules, while a hydrolysis reaction breaks a larger molecule into two smaller molecules. When carbohydrates, proteins, and lipids are digested, these biomolecules are hydrolyzed into smaller building blocks for absorption in the digestive system.
As shown in Table 2-1, the four major classes of biomolecules are carbohydrates, proteins, nucleic acids, and lipids. These biomolecules are characterized by the type of polymer, and monomer they contain as well as by their general function. A polymer is a large molecule containing numerous repeating units called monomers. A monomer is the smallest repeating unit present in a polymer.
|Carbohydrates (polysaccharides)||Amino acids||Structure and biocatalysts (enzymes)|
|Proteins||Monosaccharides||Energy source, energy storage form and structure|
|Nucleic acids||Energy source, energy storage form, and biological membranes|
|Lipids||Nucleotides||Genetic information transfer and energy|
The biological function of carbohydrates involves energy metabolism and storage. As shown in Figure 2-2, plants use photosynthesis to make oxygen (O2) and glucose (C6H12O6), the carbohydrate from which animals acquire the energy required for life. Via the process of respiration, animals degrade the carbohydrate glucose (C6H12O6) into CO2 and water (H2O), and plants use these products for photosynthesis.
Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides, depending on the number of sugar monomers present (one, two, or many). Monosaccharides are composed of a single monomeric unit with the molecular formula Cn(H2O)n, where n is 3 to 8. Figure 2-3 shows the linear structures of the most common monosaccharides. Monosaccharides undergo oxidation-reduction reactions. When a monosaccharide is reduced, the aldehyde functional group changes to a sugar alcohol (e.g., sorbitol).
In aqueous solution, linear monosaccharides spontaneously form cyclic structures. When two monosaccharides combine, a disaccharide is formed. This involves the formation of a glycosidic bond. As shown in Figure 2-4, two glucose monomers can combine via a condensation reaction to form maltose. Maltose is a disaccharide that results from the degradation of starch and is used in brewing alcoholic beverages. When glucose and fructose combine, the disaccharide sucrose is formed (Figure 2-5). This disaccharide is table sugar and one of the sweetest carbohydrates. When the two monosaccharides galactose and glucose combine, the disaccharide lactose is formed (Figure 2-5). This disaccharide is found in milk and dairy products.
Many monosaccharides combine to form a polysaccharide. As shown in Table 2-2, polysaccharides can be characterized by the monosaccharide monomer present and overall function. One of the most important dietary polysaccharides is starch, the storage form of energy in plants. Starch is composed of two different polysaccharides (α-amylose and amylopectin). Figure 2-6 shows the linear structure of the polysaccharide α-amylose. Figure 2-7 shows the branched structure of the polysaccharide amylopectin. Important for the storage of energy in animals, glycogen also is a branched polysaccharide containing glucose monomers. The highly branched structure of glycogen allows its rapid degradation into glucose when energy is needed.
|Amylose (in starch)||Glucose||Nutrient storage (plants)|
|Amylopectin (in starch)||Glucose||Nutrient storage (plants)|
|Glycogen||Glucose||Nutrient storage (animals)|
|Dextran||Glucose||Nutrient storage (yeast and bacteria)|
|Inulin||Fructose||Nutrient storage (plants)|
|Cellulose||Glucose||Structure in plants|
|Pectin||Galacturonic acid||Structural rigidity in plants and gelling agent in yogurt and jelly|
|Lignin||Coniferyl alcohol||Structural rigidity in plant cell walls|