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In biochemistry, a metabolic pathway is a series of chemical reactions occurring within a cell, catalyzed by enzymes, resulting in either the formation of a metabolic product to be used or stored by the cell, or the initiation of another metabolic pathway (then called a flux generating step). Many of these pathways are elaborate, and involve a step by step modification of the initial substance to shape it into the product with the exact chemical structure desired.
Metabolic pathways often have these common properties:
- They are reversible, usually because the first step is a committed step, such as breakdown for the release of energy, that runs in two directions.
- The pathways are regulated, usually by feedback inhibition, or may be a cycle where the end product starts the reaction again, such as the Krebs Cycle (see below).
- Anabolic and catabolic pathways in eukaryotes are separated by either compartmentation or by the use of different enzymes and cofactors.
Major metabolic pathways
Main article: Cellular respiration
Several distinct but linked metabolic pathways are used by cells to transfer the energy released by breakdown of fuel molecules to ATP. These occur within all living organisms in some forms:
Other pathways occurring in (most or) all living organisms include:
- Fatty acid oxidation (β-oxidation)
- Thin acid oxidation
- HMG-CoA reductase pathway (isoprene prenylation chains, seecholesterol)
- Pentose phosphate pathway (hexose monophosphate shunt)
- Porphyrin synthesis (or heme synthesis) pathway
- Urea cycle
Creation of energetic compounds from non-living matter:
- Photosynthesis (plants, algae)
- Chemosynthesis (some bacteria)
- Open Directory Project: Metabolic Pathways
- Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry and Cell Biology
- KEGG: Kyoto Encyclopedia of Genes and Genomes
- MetaCyc: a database of nonredundant, experimentally elucidated metabolic pathways (700 pathways from more than 600 different organisms).
- aMAZE: a workbench for the representation, management, annotation and analysis of information on networks of cellular processes: genetic regulation, biochemical pathways, signal transductions.
- PathCase Pathways Database System
- Interactive Flow Chart of the Major Metabolic Pathways
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