Glycolysis

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(Redirected from Anaerobic metabolism)

Glycolysis is a term referring to a series of biochemical reactions by which a molecule of Glucose (Glc) is oxidized to two molecules of Pyruvic Acid (Pyr) with the concomitant:

1. reduction of two molecules of NAD+ to two molecules of NADH;

2. phosphorylated of two molecules of ADP to two molecules of ATP;

3. release of two molecules of water and two protons (H+).

Glycolysis is the initial process of many pathways of carbohydrate catabolism, and serves two principal functions: generation of high-energy ATP molecules, and production of a variety of 6- or 3-carbon intermediate metabolites which may be removed at various steps in the process for other intracellular purposes. Glycolysis is one of the most universal metabolic processes known, and occurs (with minor variations) in many types of cells in nearly all types of organisms from bacteria to plants to animals and humans. Although glycolysis produces less energy per Glc molecule than complete aerobic oxidation, it can occur at great speed and is anaerobic (i.e., it does not require oxygen).

The most common and well-known form of glycolysis is the Embden-Meyerhof pathway, initially elucidated by Gustav Embden and Otto Meyerhof. The term can be taken to include alternative pathways, such as the Entner-Doudoroff Pathway. However, glycolysis will be used here as a synonym for the Embden-Meyerhof pathway.

Image:Glycolysis10steps.gif

Contents

Output

The overall reaction of glycolysis is:

Glc + 2 NAD+ + 2 ADP + 2 Pi → 2 NADH + 2 Pyr + 2 ATP + 2 H2O + 2 H+

So, for simple fermentations, the metabolism of 1 molecule of Glc has a net yield of 2 molecules of ATP. Cells performing respiration synthesize much more ATP but this is not considered part of glycolysis. Eukaryotic aerobic respiration produces an additional 34 molecules (approximately) of ATP for each Glc molecule oxidized. Unlike those molecules of ATP produced by aerobic respiration, those of glycolysis are produced by substrate-level phosphorylation.

Location

In eukaryotes glycolysis takes place within the cytosol of the cell (as opposed to the mitochondria, where reactions more closely connected to aerobic metabolism occur). Glc enters the cell through facilitated diffusion. In many animal tissues, including skeletal muscle, INS stimulates this process.

Follow up

The ultimate fate of Pyr and NADH produced in glycolysis depends upon the organism and the conditions, most notably the presence or absence of oxygen or other external electron acceptors.

In fermentation, Pyr and NADH are anaerobically metabolized to yield any of a variety of products with an organic molecule acting as the final electron acceptor. For example, the bacteria involved in making yogurt simply reduce the Pyr to lactic acid, whereas yeast produces ethanol and carbon dioxide.

In aerobic organisms, Pyr typically enters the citric acid cycle (also known as the TCA or Krebs cycle), and the NADH is ultimately oxidized by oxygen during oxidative phosphorylation. Although human metabolism is primarily aerobic, under anaerobic conditions, for example in over-worked muscles that are starved of oxygen, Pyr is converted to lactate, as in many microorganisms.

Evolution

Glycolysis is highly conserved in evolution, being common to nearly all living organisms. This suggests great antiquity; it may have originated with the very first prokaryotes, 3.5 billion years ago or more.

Pathway

The first step in glycolysis is phosphorylation of Glc by a family of enzymes called hexokinases to form G6P. In the liver an isozyme of hexokinase called GCK is used, which differs primarily in regulatory properties. This reaction consumes 1 ATP, but the energy is well spent. The cell membrane is permeable to Glc for two reasons - Glc transporters move Glc across the membrane, and neutrally charged Glc is able to passively diffuse through. G6P is negatively charged and this is repelled by the plasma membrane, so this phosphorylation effectively traps Glc in the cell. G6P is then rearranged into F6P by GPI. Fru can also enter the glycolytic pathway via phosphorylation at this point.

Image:Glycolysis.png

Phosphofructokinase-1 then consumes 1 ATP to form F1,6bP. The energy expenditure in this step is justified in 2 ways: the glycolytic process (up to this step) is now irreversible, and the energy supplied destablises the molecule, allowing the ring to be split by ALDO into DHAP and GADP. TPI rapidly interconverts these, with GADP proceeding further into glycolysis. Each molecule of GADP is then oxidized by a molecule of NAD+ in the presence of GAP, forming 1,3-bisphosphoglycerate.

In the next step, PGK generates a molecule of ATP while forming 3-phosphoglycerate. At this step glycolysis has reached the break-even point: 2 molecules of ATP were consumed, and 2 new molecules have been synthesized. This step, one of the two substrate-level phosphorylation steps, requires ADP; thus, when the cell has plenty of ATP (and little ADP) this reaction does not occur. Because ATP decays relatively quickly when it is not metabolized, this is an important regulatory point in the glycolytic pathway. PGAM then forms 2-phosphoglycerate; ENO then forms phosphoenolpyruvate; and another substrate-level phosphorylation then forms a molecule of Pyr and a molecule of ATP by means of the enzyme PK. This serves as an additional regulatory step.

After the formation of F1,6bP, many of the reactions are energetically unfavorable. The only reactions that are favorable are the 2 substrate-level phosphorylation steps that result in the formation of ATP. These two reactions pull the glycolytic pathway to completion.

Etymology

From Greek glyk meaning sweet and lysis meaning dissolving.

See also

External links

References

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