Carbohydrate metabolism
Carbohydrate metabolism is the whole of the biochemical processes responsible for the metabolic formation, breakdown, and interconversion of carbohydrates in living organisms.
Carbohydrates are central to many essential
Humans can consume a variety of carbohydrates,
Metabolic pathways
Glycolysis
Glycolysis consists of ten steps, split into two phases.[2] During the first phase, it requires the breakdown of two ATP molecules.[1] During the second phase, chemical energy from the intermediates is transferred into ATP and NADH.[2] The breakdown of one molecule of glucose results in two molecules of pyruvate, which can be further oxidized to access more energy in later processes.[1]
Glycolysis can be regulated at different steps of the process through feedback regulation. The step that is regulated the most is the third step. This regulation is to ensure that the body is not over-producing pyruvate molecules. The regulation also allows for the storage of glucose molecules into fatty acids.
Gluconeogenesis
Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms.[6] In vertebrates, gluconeogenesis occurs mainly in the liver and, to a lesser extent, in the cortex of the kidneys. It is one of two primary mechanisms – the other being degradation of glycogen (glycogenolysis) – used by humans and many other animals to maintain blood sugar levels, avoiding low levels (hypoglycemia).[7] In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc.[8] In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.
In humans, substrates for gluconeogenesis may come from any non-carbohydrate sources that can be converted to pyruvate or intermediates of
The gluconeogenesis pathway is highly
Glycogenolysis
Glycogenolysis refers to the breakdown of glycogen.[12] In the liver, muscles, and the kidney, this process occurs to provide glucose when necessary.[12] A single glucose molecule is cleaved from a branch of glycogen, and is transformed into glucose-1-phosphate during this process.[1] This molecule can then be converted to glucose-6-phosphate, an intermediate in the glycolysis pathway.[1]
Glucose-6-phosphate can then progress through glycolysis.[1] Glycolysis only requires the input of one molecule of ATP when the glucose originates in glycogen.[1] Alternatively, glucose-6-phosphate can be converted back into glucose in the liver and the kidneys, allowing it to raise blood glucose levels if necessary.[2]
Glucagon in the liver stimulates glycogenolysis when the blood glucose is lowered, known as hypoglycemia.[12] The glycogen in the liver can function as a backup source of glucose between meals.[2] Liver glycogen mainly serves the central nervous system. Adrenaline stimulates the breakdown of glycogen in the skeletal muscle during exercise.[12] In the muscles, glycogen ensures a rapidly accessible energy source for movement.[2]
Glycogenesis
Glycogenesis refers to the process of synthesizing glycogen.[12] In humans, glucose can be converted to glycogen via this process.[2] Glycogen is a highly branched structure, consisting of the core protein Glycogenin, surrounded by branches of glucose units, linked together.[2][12] The branching of glycogen increases its solubility, and allows for a higher number of glucose molecules to be accessible for breakdown at the same time.[2] Glycogenesis occurs primarily in the liver, skeletal muscles, and kidney.[2] The Glycogenesis pathway consumes energy, like most synthetic pathways, because an ATP and a UTP are consumed for each molecule of glucose introduced.[13]
Pentose phosphate pathway
The
Fructose metabolism
Fructose must undergo certain extra steps in order to enter the glycolysis pathway.[2] Enzymes located in certain tissues can add a phosphate group to fructose.[12] This phosphorylation creates fructose-6-phosphate, an intermediate in the glycolysis pathway that can be broken down directly in those tissues.[12] This pathway occurs in the muscles, adipose tissue, and kidney.[12] In the liver, enzymes produce fructose-1-phosphate, which enters the glycolysis pathway and is later cleaved into glyceraldehyde and dihydroxyacetone phosphate.[2]
Galactose metabolism
Lactose, or milk sugar, consists of one molecule of glucose and one molecule of galactose.[12] After separation from glucose, galactose travels to the liver for conversion to glucose.[12] Galactokinase uses one molecule of ATP to phosphorylate galactose.[2] The phosphorylated galactose is then converted to glucose-1-phosphate, and then eventually glucose-6-phosphate, which can be broken down in glycolysis.[2]
Energy production
Many steps of carbohydrate metabolism allow the cells to access energy and store it more transiently in ATP.[15] The cofactors NAD+ and FAD are sometimes reduced during this process to form NADH and FADH2, which drive the creation of ATP in other processes.[15] A molecule of NADH can produce 1.5–2.5 molecules of ATP, whereas a molecule of FADH2 yields 1.5 molecules of ATP.[16]
Pathway | ATP input | ATP output | Net ATP | NADH output | FADH2 output | ATP final yield |
---|---|---|---|---|---|---|
Glycolysis (aerobic) | 2 | 4 | 2 | 2 | 0 | 5-7 |
Citric-acid cycle | 0 | 2 | 2 | 8 | 2 | 17-25 |
Typically, the complete breakdown of one molecule of glucose by aerobic respiration (i.e. involving glycolysis, the
Hormonal regulation
Glucoregulation is the maintenance of steady levels of glucose in the body.
Hormones released from the pancreas regulate the overall metabolism of glucose.[17] Insulin and glucagon are the primary hormones involved in maintaining a steady level of glucose in the blood, and the release of each is controlled by the amount of nutrients currently available.[17] The amount of insulin released in the blood and sensitivity of the cells to the insulin both determine the amount of glucose that cells break down.[4] Increased levels of glucagon activates the enzymes that catalyze glycogenolysis, and inhibits the enzymes that catalyze glycogenesis.[15] Conversely, glycogenesis is enhanced and glycogenolysis inhibited when there are high levels of insulin in the blood.[15]
The level of circulatory glucose (known informally as "blood sugar"), as well as the detection of nutrients in the Duodenum is the most important factor determining the amount of glucagon or insulin produced. The release of glucagon is precipitated by low levels of blood glucose, whereas high levels of blood glucose stimulates cells to produce insulin. Because the level of circulatory glucose is largely determined by the intake of dietary carbohydrates, diet controls major aspects of metabolism via insulin.[18] In humans, insulin is made by beta cells in the pancreas, fat is stored in adipose tissue cells, and glycogen is both stored and released as needed by liver cells. Regardless of insulin levels, no glucose is released to the blood from internal glycogen stores from muscle cells.
Carbohydrates as storage
Carbohydrates are typically stored as long polymers of glucose molecules with
In some animals (such as termites)[20] and some microorganisms (such as protists and bacteria), cellulose can be disassembled during digestion and absorbed as glucose.[21]
Human diseases
- Diabetes mellitus
- Lactose intolerance
- Fructose malabsorption
- Galactosemia
- Glycogen storage disease
See also
- Inborn errors of carbohydrate metabolism
- Hitting the wall (glycogen depletion)
- Second wind (increased ATP from fatty acids after glycogen depletion)
References
- ^ .
- ^ OCLC 824794893.
- ^ ISBN 9780123849533.
- ^ ISBN 978-0323389303.
- ^ “Regulation of Cellular Respiration (Article).” Khan Academy. www.khanacademy.org, https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/variations-on-cellular-respiration/a/regulation-of-cellular-respiration.
- ISBN 978-1-57259-153-0.
- ^ Silva P. "The Chemical Logic Behind Gluconeogenesis". Archived from the original on August 26, 2009. Retrieved September 8, 2009.
- ISBN 978-0801442384.
- PMID 21814506.
- S2CID 53097552.
- ISBN 978-0-07-182537-5.
- ^ PMID 23680095.
- ISBN 978-1-337-51421-7.
- ^ PMID 28041936.
- ^ a b c d Ahern, Kevin; Rajagopal, Indira; Tan, Taralyn (2017). Biochemistry Free for All. Oregon State University.
- ^ a b Energetics of Cellular Respiration (Glucose Metabolism).
- ^ ISBN 9780323189071.
- PMID 647618.
- ^ G Cooper, The Cell, American Society of Microbiology, p. 72
- S2CID 4384555.
- .
External links
- Carbohydrate+metabolism at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
- BBC - GCSE Bitesize - Biology | Humans | Glucoregulation
- Sugar4Kids