Metabolic pathway

Under metabolic pathways ( English metabolic pathways ) is the on - / From - and remodeling processes in the cells . Metabolic pathways are the defined sequence of biochemical reactions , mostly catalyzed by enzymes . A metabolic pathway corresponds to a simple biological process, whereby the definition of this term is fluid. In 1995 Donald Nicholson combined all previously known metabolic pathways in his Metabolic Pathway Chart . The sequence of the temporal activity of the genesfor the enzymes involved in the individual successive biochemical conversions in the metabolic pathway , which form a kind of "enzyme cascade", is referred to as a gene effect chain . It is a result of gene regulation . An example of such a gene chain is blood coagulation .

Metabolic pathways, organized by function

Pathways of cellular respiration

See also : cellular respiration

• Fatty acid oxidations (e.g. α- or β-oxidation )
• Cori cycle
• Glycolysis (formal reversal: gluconeogenesis )
• Anaerobic breathing
• Citric acid cycle
• Glyoxylate cycle
• Respiratory chain ( oxidative phosphorylation )
• Q cycle

Other ways

• Cholesterol biosynthesis
• Pentose phosphate route
• Porphyrin synthesis (or heme synthesis)
• Urea cycle
• Fatty acid synthesis

Biochemical cycles

General scheme of the citric acid cycle

Metabolic pathways are reaction sequences in which metabolites enter and from which they can be extracted. Metabolic pathways are often cyclical; the initial product of the metabolic pathway is then also the end product. The ring closure takes place through regeneration or subsequent delivery of the starter molecule. The best-known example: the citric acid cycle , the “hub of metabolism”, in which anabolic and catabolic pathways of carbohydrates , proteins and fats converge. Many of these cycles were named after their discoverers ( name reactions ) and thus represent a piece of biochemical history.

Calvin cycle

• light-independent reaction ( dark reaction ) of photosynthesis , in which three molecules of the CO ₂ acceptor ribulose-1,5-bisphosphate are regenerated from five molecules of a C ₃ compound ( 3-phosphoglycerate )

Citric acid cycle

• Final degradation of a C ₂ compound (activated acetic acid = acetyl-CoA) with formation of CO ₂ and GTP as well as the reduction equivalents NADH, H ⁺ and FADH ₂ , which are oxidized in the respiratory chain to form H ₂ O

Cori cycle

• Pathways of glucose metabolism involving the liver and skeletal muscle

Urea cycle

• Ammonia detoxification in the liver through a metabolite cycle between mitochondria and cytosol

Hatch-Slack cycle

• C ₄ dicarboxylate pathway via two cell types of the C ₄ plants

Lynen cycle (syn.HMG-CoA cycle)

• Formation of ketone bodies (acetoacetate and secondary products) when hungry and increasingly when the carbohydrate metabolism is disturbed (e.g. diabetes mellitus )

Pentose phosphate cycle

• Build-up and breakdown of pentose phosphates and formation of NADPH. The pentose phosphate cycle plays an important role in red blood cells.

Q cycle

• cyclical electron flow at complex III of the respiratory chain ; Doubling the energy yield

Substrate cycle

• Idle cycle through repeated phosphorylation / dephosphorylation of a metabolite . Regulatory function; possible importance in heat generation

Gene effect chain

Example of a gene chain: metabolism of phenylalanine.

A specific gene is responsible for the formation of each enzyme through protein biosynthesis . The sequence of conversion steps by the substrate-specific enzymes corresponds to the underlying gene chain. If there is a genetic defect in a gene and the enzyme cannot be synthesized, the relevant metabolic step is canceled. The subsequent biochemical conversions cannot take place either, since the required substrate is not provided for them. Some genetic diseases of the metabolism are based on such genetic defects in which the biochemical conversions brought about by the enzymes are not possible as a result of a genetic defect. Well-known examples are phenylketonuria and hemophilia . If a gene can influence its effectiveness by placing it in front of another gene, this is known as epistasis .

Web links

A number of electronic databases with metabolic data are freely available on the WWW. One of the most important is KEGG :

• KEGG PATHWAY Database
• WikiPathways - a wiki of the scientific community dedicated to archiving and improving metabolic pathways

Individual evidence

1. ^ IUBMB-Nicholson: Metabolic Pathways Chart
2. ^ Gerhard Czihak, H. Langer, H. Ziegler: Biology: A textbook . Edition 6, Springer-Verlag, 2013. ISBN 9783642852640 . P. 221.
3. ^ Ulrich Weber: Biology Upper Level Complete Volume. Cornelsen Verlag 2001, page 152
4. ↑ [1]