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alpha decay

Type of radioactive disintegration (see radioactivity) in which some unstable atomic nuclei dissipate excess energy by spontaneously ejecting an alpha particle. Alpha particles have two positive charges and a mass of four atomic mass units; they are identical to helium nuclei. Though they are emitted at speeds about one-tenth that of light, they are not very penetrating and have ranges in air of about 1–4 in. (2.5–10 cm). Alpha decay commonly occurs in elements with atomic numbers greater than 83 (bismuth), but can occur in some rare-earth elements in the atomic-number range of 60 (neodymium) to 71 (lutetium). Alpha decay half-lives range from about a microsecond (10⁻⁶ second) to billions of years (10¹⁷ seconds).

Learn more about alpha decay with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Alpha Centauri

Triple star in the constellation Centaurus, the faintest component of which, Proxima Centauri, is currently the closest star to the Sun (about 4.2 light-years away). The two brighter components are about 0.2 light-year farther from the Sun. As seen from Earth, the system is the third brightest star (after Sirius and Canopus); Proxima is indistinguishable as a separate star to the unaided eye. Alpha Centauri can be seen from Earth's surface only from points south of about 40° N.

Learn more about Alpha Centauri with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Alpha-ketoglutaric acid

Alpha-ketoglutaric acid is one of two ketone derivatives of glutaric acid. (The term "ketoglutaric acid," when not further qualified, almost always refers to the alpha variant. Beta-ketoglutaric acid varies only by the position of the ketone functional group, and is much less common.)

Its anion, Alpha-ketoglutarate (also called oxo-glutarate) is an important biological compound. It is the keto acid produced by de-amination of glutamate, and is an intermediate in the Krebs cycle.

Functions

Krebs cycle

It is a key intermediate in the Krebs cycle, coming after isocitrate and before succinyl CoA. Anaplerotic reactions can replenish the cycle at this juncture by synthesizing alpha-ketoglutarate from transamination of glutamate, or through action of glutamate dehydrogenase on glutamate.

Formation of amino acids

Glutamine is synthesized from glutamate by glutamine synthase, which utilizes an ATP to form glutamyl phosphate; this intermediate is attacked by ammonia as a nucleophile giving glutamine and inorganic phosphate.

Nitrogen transporter

Another function is to combine with nitrogen released in the cell, therefore preventing nitrogen overload.

Alpha-ketoglutarate is one of the most important nitrogen transporter in metabolic pathways. The amino groups of amino acids are attached to it by transamination and carried to the liver where the urea cycle takes place.

Alpha-ketoglutarate is transaminated, along with glutamine, to form the excitatory neurotransmitter glutamate. Glutamate can then be decarboxylated (requiring vitamin B₆) into the inhibitory neurotransmitter GABA.

It is reported that high ammonia and/or high nitrogen levels may occur with high protein intake, excessive aluminum exposure, Reye's syndrome, cirrhosis, and urea cycle disorder.

Relationship to molecular oxygen

Acting as a co-substrate, it also plays important function in oxidation reactions involving molecular oxygen.

Molecular oxygen (O₂) directly oxidizes many compounds to produce useful products in an organism, such as antibiotics, etc., in reactions catalyzed by oxygenases. In many oxygenases, alpha-ketoglutarate helps the reaction by being oxidized together with the main substrate. In fact, one of the alpha-ketoglutarate-dependent oxygenases is an O₂ sensor, informing the organism the oxygen level in its environment.