# Structural Biochemistry/Carbohydrates/Carbohydrate Metabolism
## Overview

All cells require energy for continual survival and operation. This energy comes from energy-containing compounds such as sugars, starch or lipids. The breakdown and interconversion of these energy-containing compounds in living organisms is a biochemical process coined Carbohydrate metabolism.

## Humans

Carbohydrate metabolism is carried out by aerobic respiration where glucose and oxygen are metabolized releasing water and carbon dioxide. In cellular respiration metabolic reactions in order to convert the energy stored in the carbohydrate into ATP (adenosine triphosphate). ATP is created and is often referred to as "the molecular unit of currency" for intracellular energy transfer. ATP stores the now broken down energy and transports it to different areas of the cell when needed.
Carbohydrates are stored as polysaccharides consisting of longer polymers of glucose(monosaccharides) by glycosidic bonds. When energy is needed or to be stored, these polysaccharides are cleaved into their smaller monosaccharides units in preparation for catabolism. Carbohydrate catabolism is this breakdown of larger carbohydrates into smaller pieces in order to retrieve the energy within the bonds.
There are also other types of Carbohydrate metabolism such as glycolysis, anaerobic respiration, glycogenesis and more.

### Glycolysis

Glycolysis metabolic pathway is used by most microorganisms such as yeast, bacteria, animals, and humans. Glycolysis means the dissolution of sugar. Glycolysis begins with a single molecule of glucose (C$_6$H$_{12}$O$_6$) and ends with the production of pyruvic (CH3COCOO$^-$ + H$^+$). The pathway is catabolic (producing energy by converting complex molecules into simpler ones). The energy produced during glycolysis comes from the degradation of glucose and if stored as a molecule called adenosine triphosphate (ATP). The six-carbon glucose is reduced to two molecules of the three-carbon pyruvic acid. ATP synthesis is said to be coupled to glycolysis because the glycolytic sequence is produced by utilizing two reactions. Even though glycolysis is the primary system for forming energy, some organisms do not require oxygen, such as organisms like yeast, aerobic organisms. Hundreds of biochemical reactions in our bodies require the participation of ATP as a source of energy. These organisms can only gain a small amount of energy needed to function from this process. Glycolysis occurs two major steps, the first step involves the conversion of sugar to, glucose-6-phosphate. The second step is the conversion of the glucose-6-phosphate to pyruvate. Then the products of glycolysis are further metabolized and completely break down glucose. In some microorganisms lactic acid is a final product produced from pyruvic acid. This process is called homolactic fermentation. In some bacteria and yeast lactic acid is not produced in abundant quantities, instead pyruvic acid is made into ethanol and carbon dioxide. This system is called alcoholic fermentation. In tissues some organisms glycolysis is an introduction to complex metabolic machinery. This system converts pyruvic acid to carbon dioxide and water by using oxygen. The most common type of glycolysis is the Embden-Meyerhof-Parnas (EMP pathway), which was discovered by Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas.

#### Embden-Meyerhof-Parnas

The Embden-Meyerhof- Parnas (EMP pathway) is a sequence of chemical reactions that breaks down glucose and releases energy that is captured and stored as ATP. One molecule of glucose makes two molecules of pyruvate and two molecules of ATP. The pyruvate then enters into the tricarboxylic acid cycle if oxygen is present it is further fermented into lactic acid. If not enough oxygen is present in the cell pyruvate is fermented into ethanol. Overall, glycolysis produces ATP and act as building blocks for other synthesis of other cellular products.

### Anaerobic Respiration

Anaerobic respiration is one way of respiration that uses electron acceptors and oxygen. In anaerobes, compounds such as nitrate (NO$_3$), sulfur (S), and sulfate (SO$_4$$^{2-}$) are used. For the electron transport chain to work, a final electron acceptor must be present to allow electrons to pass through the system. In aerobic organisms, the final electron acceptor is oxygen. Anaerobic respiration is mainly used by prokaryotes that live in environments that do not have a lot of oxygen. Anaerobic respiration is energetically less efficient than aerobic respiration. Many anaerobic organisms will die in in oxygen and therefore can only use anaerobic respiration.

### Aerobic Respiration

Anaerobic respiration is one way of respiration that uses electron acceptors and oxygen. In anaerobes, compounds such as nitrate (NO$_3$), sulfur (S), and sulfate (SO$_4$$^{2-}$ are used. For the electron transport chain to work, a final electron acceptor must be present to allow electrons to pass through the system. In aerobic organisms, the final electron acceptor is oxygen. Anaerobic respiration is mainly used by prokaryotes that live in environments that do not have a lot of oxygen. Anaerobic respiration is energetically less efficient than aerobic respiration. Many anaerobic organisms will die in in oxygen and therefore can only use anaerobic respiration.

## Plants

Plants utilize many of the same metabolic reactions as Humans to metabolize Carbohydrates. However, the cell walls chemical and physical properties restricts enzyme attack. Cell walls contain polysaccharides mostly comprised of cellulose and hemicellulose that store energy for the plant cells. Because of the complexities of cell walls, the complete deconstruction of these polysaccharides is very difficult due to the restriction of enzymatic attack upon them. In turn, the biochemical process of recycling of the energy gathered from photosynthesis is relatively inefficient.
Cellulases and hemicellulases are very complex and intricate enzymes that are composed of molecular structures where catalytic modules rapidly increase and contribute to protein-carbohydrate or protein-protein interactions. Cellulosomes are cellulases and hemicellulases that have been synthesized by anaerobes and assemble in a multienzyme complex. It is still unclear as to how the formation of these enzyme complexes are carried out, but it is suspected that anaerobic environments impose selective pressures which drives the formation.
Cellulosomes breakdown and deconstruct plant polysaccharides more efficiently than traditional methods (enzymatic aerobic respiration, etc.) hindered by the cell wall complexities. For example, C. thremocellum utilizes cellulose very rapidly and thus requires a dubious supply of energy. Cellulosomes have a specific activity against cellulose that is 50-fold higher than the traditional energy breakdown method(cellulolysis which uses anaerobic bacteria that produces cellulases enzymes). It is proposed that this increase in efficiency is due to the multienzyme macromolecular complex that potentiates and increases synergistic interactions between the catalytic units and the enzyme-substrate target. Although this hypothesis is reasonable and plausible, the actual reasoning behind this phenomenon is still unclear because of the numerous variable factors associated with cellulose hydrolysis. .

## References

* ↑ G Cooper, The Cell, American Society of Microbiology, p 72
* ↑ Stetten, DeWitt Jr. and Topper, Yale J. Seminars on Carbohydrate Metabolism: "The Metabolism of Carbohydrates, A Review", American Journal of Medicine. Bethesda, Maryland
* ↑ Fontes, C M, & Gilbert, H J. (2010). Cellulosomes: highly efficient nanomachines designed to deconstruct plant cell wall complex carbohydrates. Annual review of biochemistry, 79, 655-81.