Metabolic pathway

From Wikipedia, the free encyclopedia

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 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.

Various metabolic pathways within a cell form the cell's metabolic network. The metabolic pathway a substrate enters depends on the needs of the cell, i.e. the specific combination of concentrations of the anabolical and catabolical end products (the energetics of the flux-generating step). Metabolic pathways include the principal chemical, mostly enzyme-dependent, reactions that an organism needs to keep its homeostasis.

1 Overview
2 Major metabolic pathways
- 2.1 Cellular respiration
3 See also
4 External links

[edit] Overview

Metabolic pathways often have these properties:

They contain many steps, like a cascade. The first step is usually irreversible. The other steps need not be irreversible and in many cases, the pathway can go in opposite direction depending on the current need of the cell.

Glycolysis features excellent examples of these features:

As glucose enters a cell it is immediately phosphorylated by ATP to glucose 6-phosphate in the irreversible first step. This is to prevent the glucose leaving the cell.
In times of excess lipid or protein energy sources glycolysis may run in reverse (gluconeogenesis) in order to produce glucose 6-phosphate for storage as glycogen or starch.

They are regulated, usually by feedback inhibition, or by a cycle where one of the products in the cycle 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.

[edit] Major metabolic pathways

[edit] Cellular respiration

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)
Gluconeogenesis
HMG-CoA reductase pathway (isoprene prenylation chains, see cholesterol)
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, cyanobacteria)
Chemosynthesis (some bacteria)

[edit] See also

v • d • e</span> Metabolism
Cell metabolism/Metabolism \| Catabolism \| Anabolism
Protein \|	Protein metabolism (Protein synthesis/Amino acid synthesis/Catabolism)
Carbohydrate \|	Carbohydrate metabolism (Anabolism/Catabolism)
Lipid \|	Lipid metabolism (Synthesis/Anabolism/Catabolism)
Metabolic pathway \| Metabolic network
Cellular respiration (Anaerobic/Aerobic)