Carbohydrates
Introduction
Carbohydrates are carbon compounds that contain large quantities of hydroxyl groups. The simplest carbohydrates also contain either an aldehyde moiety (these are termed polyhydroxyaldehydes) or a ketone moiety (polyhydroxyketones). All carbohydrates can be classified as either monosaccharides, oligosaccharides or polysaccharides.
Anywhere from two to ten monosaccharide units, linked by glycosidic bonds, make up an oligosaccharide. Polysaccharides are much larger, containing hundreds of monosaccharide units. The presence of the hydroxyl groups allows carbohydrates to interact with the aqueous environment and to participate in hydrogen bonding, both within and between chains. Derivatives of the carbohydrates can contain nitrogens, phosphates and sulfur compounds. Carbohydrates also can combine with lipid to form glycolipids or with protein to form glycoproteins. |
Carbohydrate Nomenclature
The predominant carbohydrates encountered in the body are structurally related to the aldotriose glyceraldehyde and to the ketotriose dihydroxyacetone. All carbohydrates contain at least one asymmetrical (chiral) carbon and are, therefore, optically active. In addition, carbohydrates can exist in either of two conformations, as determined by the orientation of the hydroxyl group about the asymmetric carbon farthest from the carbonyl. With a few exceptions, those carbohydrates that are of physiological significance exist in the D-conformation. The mirror-image conformations, called enantiomers, are in the L-conformation.
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Structures of Glyceraldehyde Enantiomers
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Monosaccharides
The monosaccharides commonly found in humans are classified according to the number of carbons they contain in their backbone structures. The major monosaccharides contain four to six carbon atoms.
Carbohydrate Classifications
The aldehyde and ketone moieties of the carbohydrates with five and six carbons will spontaneously react with alcohol groups present in neighboring carbons to produce intramolecular hemiacetals or hemiketals, respectively. This results in the formation of five- or six-membered rings. Because the five-membered ring structure resembles the organic molecule furan, derivatives with this structure are termed furanoses. Those with six-membered rings resemble the organic molecule pyran and are termed pyranoses
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Such structures can be depicted by either Fischer or Haworth style diagrams. The numbering of the carbons in carbohydrates proceeds from the carbonyl carbon, for aldoses, or the carbon nearest the carbonyl, for ketoses.
The rings can open and re-close, allowing rotation to occur about the carbon bearing the reactive carbonyl yielding two distinct configurations (α and β) of the hemiacetals and hemiketals. The carbon about which this rotation occurs is the anomeric carbon and the two forms are termed anomers. Carbohydrates can change spontaneously between the α and β configurations: a process known as mutarotation. When drawn in the Fischer projection, the α configuration places the hydroxyl attached to the anomeric carbon to the right, towards the ring. When drawn in the Haworth projection, the α configuration places the hydroxyl downward.
The spatial relationships of the atoms of the furanose and pyranose ring structures are more correctly described by the two conformations identified as the chair form and the boat form. The chair form is the more stable of the two. Constituents of the ring that project above or below the plane of the ring are axial and those that project parallel to the plane are equatorial. In the chair conformation, the orientation of the hydroxyl group about the anomeric carbon of α-D-glucose is axial and equatorial in β-D-glucose.
The spatial relationships of the atoms of the furanose and pyranose ring structures are more correctly described by the two conformations identified as the chair form and the boat form. The chair form is the more stable of the two. Constituents of the ring that project above or below the plane of the ring are axial and those that project parallel to the plane are equatorial. In the chair conformation, the orientation of the hydroxyl group about the anomeric carbon of α-D-glucose is axial and equatorial in β-D-glucose.
Chair form of α-D-Glucose
Disaccharides
Covalent bonds between the anomeric hydroxyl of a cyclic sugar and the hydroxyl of a second sugar (or another alcohol containing compound) are termed glycosidic bonds, and the resultant molecules areglycosides. The linkage of two monosaccharides to form disaccharides involves a glycosidic bond. Several physiogically important disaccharides are sucrose, lactose and maltose.
Maltose: the major degradation product of starch, is composed of 2 glucose monomers in an α–(1,4) glycosidic bond.
Maltose
Polysaccharides
Most of the carbohydrates found in nature occur in the form of high molecular weight polymers calledpolysaccharides. The monomeric building blocks used to generate polysaccharides can be varied; in all cases, however, the predominant monosaccharide found in polysaccharides is D-glucose. When polysaccharides are composed of a single monosaccharide building block, they are termed homopolysaccharides. Polysaccharides composed of more than one type of monosaccharide are termed heteropolysaccharides.
Glycogen
Glycogen is the major form of stored carbohydrate in animals. This crucial molecule is a homopolymer of glucose in α–(1,4) linkage; it is also highly branched, with α–(1,6) branch linkages occurring every 8-10 residues. Glycogen is a very compact structure that results from the coiling of the polymer chains. This compactness allows large amounts of carbon energy to be stored in a small volume, with little effect on cellular osmolarity.
Glycogen Structure. Section of a glycogen polymer depicting glucose monomers as colored balls. The blue balls represent glucose linked by α1,4 glycosidic bonds. The red balls represent glucose at branch points where there are both α1,4 and α1,6 glycosidic bonds. The orange balls represent the reducing ends of the polymeric chains of α1,4-linked glucoses. The area in the box is expanded to show the actual structure of the glucose monomers in both α-1,4- and α-1,6 glycosidic linkages. |
Carbohydrate Metabolism
Since all digestible forms of carbohydrates are eventually transformed into glucose, it is important to consider how glucose is able to provide energy in the form of adenosine triphosphate (ATP) to various cells and tissues. Glucose is metabolized in three stages:
- glycolysis
- the Krebs Cycle
- oxidative phosphorylation
- During exercise, hormonal levels shift and this disruption of homeostasis alters the metabolism of glucose and other energy-bearing molecules. Therefore, in this SparkNote the metabolism of carbohydrates will be considered in the context of exercise strategies and hypotheses.
Glycolysis
The breakdown of glucose to provide energy begins with glycolysis. To begin with, glucose enters the cytosol of the cell, or the fluid inside the cell not including cellular organelles. Next, glucose is converted into two, three-carbon molecules of pyruvate through a series of ten different reactions. A specific enzyme catalyzes each reaction along the way and a total of two ATP are generated per glucose molecule. Since ADP is converted to ATP during the breakdown of the substrate glucose, the process is known as substrate-level phosphorylation. During the sixth reaction, glyceraldehyde 3-phosphate is oxidized to 1,3 bisphosphoglycerate while reducing nicotinamide adenosine dinucleotide (NAD) to NADH, the reduced form of the compound. NADH is then shuttled to the mitochondria of the cell where it is used in the electron transport chain to generate ATP via oxidative phosphorylation, which will be described later. The most important enzyme in glycolysis is called phosphofructokinase (PFK)and catalyzes the third reaction in the sequence. Since this reaction is so favorable under physiologic conditions, it is known as the "committed step" in glycolysis. In other words, glucose will be completely degraded to pyruvate after this reaction has taken place. With this in mind, PFK seems as if it would be an excellent site of control for glucose metabolism. In fact, this is exactly the case. When ATP or energy is plentiful in the cell, PFK is inhibited and the breakdown of glucose for energy slows down. Therefore, PFK can regulate the degradation of glucose to match the energy needs of the cell. This type of regulation is a recurring theme in biochemistry. |
Krebs Cycle and Oxidative Phosphorylation/Electron Transport ChainThere are many compounds that are formed and recycled during the Krebs Cycle (Citirc Acid Cycle). These include oxidized forms of nictotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) and their reduced counterparts: NADH and FADH2. NAD+ and FAD are electron acceptors and become reduced while the substrates in the Krebs Cycle become oxidized and surrender their electrons.
The Krebs Cycle begins when the pyruvate formed in the cytoplasm of the cell during glycolysis is transferred to the mitochondria, where most of the energy inherent in glucose is extracted. In the mitochondria, pyruvate is converted to acetyl CoA by the enzyme pyruvate carboxlase. In general, Acetyl-CoA condenses with a four carbon compound called oxaloacetate to form a six carbon acid. This six-carbon compound is degraded to a five and four carbon compound, releasing two molecules of carbon dioxide. At the same time, two molecules of NADH are formed. Finally, the C-4 carbon skeleton undergoes three additional reactions in which guanosine triphosphate (GTP), FADH2 and NADH are formed, thereby regenerating oxaloacetate. FADH2 and NADH are passed on to the electron transport chain (see below) that is embedded in the inner mitochondria membrane. GTP is a high-energy compound that is used to regenerate ATP from ADP. Therefore, the main purpose of the Krebs Cycle is to provide high-energy electrons in the form of FADH2 and NADH to be passed onward to the electron transport chain.
The high-energy electrons contained in NADH and FADH2 are passed on to a series of enzyme complexes in the mitochondrial membrane.
Three complexes work in sequence to harvest the energy in NADH and FADH2 and convert it to ATP: NADH-Q reductase, cytochrome reductase and cytochrome oxidase. The final electron acceptor in the electron transport chain is oxygen. Each successive complex is at lower energy than the former so that each can accept electrons and effectively oxidize the higher energy species. In effect, each complex harvests the energy in these electrons to pump protons across the inner mitochondria membrane, thereby creating a proton gradient. In turn, this electropotential energy is converted to chemical energy by allowing proton flux back down its chemical gradient and through specific proton channels that synthesize ATP from ADP. Approximately two molecules of ATP are produced during the Kreb cycle reactions, while approximately 26 to 30 ATP are generated by the electron transport chain. In summary, the oxidation of glucose through the reduction of NAD+ and FADH is coupled to the phosphorylation of ADP to produce ATP. Hence, the process is known as oxidative phosphorylation.
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Simple Explaination Video:
https://www.youtube.com/watch?v=U7E_Fgk8Q20
https://www.youtube.com/watch?v=U7E_Fgk8Q20
References:
- Carbohydrates. (n.d.). Retrieved December 18, 2015, from http://themedicalbiochemistrypage.org/carbohydrates.php