Biochemistry: Encyclopedia II - Biochemistry - Carbohydrates

Biochemistry - Carbohydrates

The function of carbohydrates includes energy storage and providing structure. Sugars are carbohydrates, although there are carbohydrates that are not sugars. There are more carbohydrates on Earth than any other type of biomolecule. The simplest type of carbohydrate is a monosaccharide, which among other properties contains carbon, hydrogen, and oxygen in a ratio 1:2:1 (generalized formula C_nH_2nO_n, where n is at least 3). Glucose, one of the most important carboyhydrates, is an example of a monosaccharide. So is fructose, the sugar that gives fruits their sweet taste.

Two monosaccharides can be joined together using dehydration synthesis, in which a hydrogen atom is removed from the end of one molecule and a hydroxyl group (—OH) is removed from the other; the remaining residues are then attached at the sites from which the atoms were removed. The H—OH or H₂O is then released as a molecule of water, hence the term dehydration. The new molecule, consisting of two monosaccharides, is called a disaccharide and is conjoined together by a glycosidic or ether bond. The reverse reaction can also occur, using a molecule of water to split up a disaccharide and break the glycosidic bond; this is termed hydrolysis. The most well-known disaccharide is sucrose, ordinary sugar (in scientific contexts, called table sugar or cane sugar to differentiate it from other sugars). Sucrose is made up of a glucose molecule and a fructose molecule joined together. Another important disaccharide is lactose, made up of a glucose molecule and a galactose molecule. As most humans age, the production of lactase, the enzyme that hydrolyzes lactose back into glucose and galactose, typically decreases. This results in lactase deficiency, also called lactose intolerance.

When a few (around three to six) monosaccharides are joined together, it is called an oligosaccharide (oligo- meaning "few"). These molecules tend to be used as markers and signals, as well as having some other uses.

Many monosaccharides joined together make a polysaccharide. They can be joined together in one long linear chain, or they may be branched. Two of the most common polysaccharides are cellulose and glycogen, both consisting of repeating glucose monomers. Cellulose is made by plants and is an important structural component of their cell walls. Humans can neither manufacture nor digest it. Glycogen, on the other hand, is an animal carbohydrate; humans use it as a form of energy storage.

Image:Glycolysis10steps.gif Glucose is the major energy source in most life forms; a number of catabolic pathways converge on glucose. For instance, polysaccharides are broken down into their monomers (glycogen phosphorylase removes glucose residues from glycogen). Disaccharides like lactose or sucrose are cleaved into their two component monosaccharides. Glucose is metabolized by a very important and ancient ten-step pathway called glycolysis, the net result of which is to break down one molecule of glucose into two molecules of pyruvate; this also produces a net two molecules of ATP, the energy currency of cells, along with two reducing equivalents in the form of converting NAD to NADH. This does not require oxygen; if no oxygen is available (or the cell cannot use oxygen), the NAD is restored by converting the pyruvate to lactate (in humans, for instance) or to ethanol in yeast. Other monosaccharides like galactose and fructose can be converted into intermediates of the glycolytic pathway. In aerobic cells with sufficient oxygen, like most human cells, the pyruvate can be further metabolized. It is irreversibly converted to acetyl-CoA, giving off one carbon atom as the waste product carbon dioxide, generating another molecule of ATP, and generating another reducing equivalent as NADH. The two molecules acetyl-CoA (from one molecule of glucose) then enter the citric acid cycle, producing two more molecules of ATP, six more NADH molecules and two of a related molecule FADH₂, and releasing the remaining carbon atoms as carbon dioxide. The reduced NADH and FADH₂ then enter the electron transport system, where the electrons are transferred to a molecule of oxygen, producing water, and the original NAD⁺ and FAD are regenerated. This is why humans breath in oxygen and breath out carbon dioxide. The energy in transferring the electrons from high-energy states in NADH and FADH₂ is used to generate an additional 28 molecules of ATP (only two had been produced in glycolysis), for a total of 32 molecules of ATP. It is clear that using oxygen to completely oxidize glucose provides an organism with far greater energy, and it is why complex life appeared only after Earth's atmosphere accumulated large amounts of oxygen.

In vertebrates, vigorously contracting skeletal muscle (during weightlifting or sprinting, for example) does not receive enough oxygen to meet the energy demand, and so it shifts to anaerobic metabolism, converting glucose to lactate (lactic acid). The liver can regenerate the glucose, using a process called gluconeogenesis. This process is not quite the opposite of glycolysis, and actually requires three times the amount of energy gained from glycolysis (six molecules of ATP are used, compared to the two gained in glycolysis). Analogous to the above reactions, the glucose produced can then undergo glycolysis in tissues that need energy, be stored as glycogen (or starch in plants), or be converted to other monosaccharides or joined into di- or oligosaccharides.

Adapted from the Wikipedia article "Carbohydrates", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki