tricarboxylic acid cycle

tricarboxylic acid cycle:

see Krebs cycleKrebs cycle,
series of chemical reactions carried out in the living cell; in most higher animals, including humans, it is essential for the oxidative metabolism of glucose and other simple sugars.
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Tricarboxylic Acid Cycle

(also Krebs cycle, citric acid cycle), a pathway commonly involved in the metabolism of animals, plants, microorganisms, and fungi for the oxidative transformation of the dicarboxylic and tricarboxylic acids formed as intermediates in the breakdown of proteins, fats, and carbohydrates. The tricarboxylic acid cycle, or TCA cycle, was discovered by H. A. Krebs and W. Johnson in 1937.

Occurring in the mitochondria, the TCA cycle starts with citric acid and ends with the formation of oxaloacetic acid and CO₂ and reduction of the coenzymes of the dehydrogenases nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). The tricarboxylic acids acting as substrates in the TCA cycle are citric, ci’s-aconitic, isocitric, and oxalosuccinic acids; the dicarboxylic acid substrates are ketoglutaric, succinic, fumaric, malic, and oxaloacetic acids. Acetic acid should also be considered a substrate of the cycle. In its active form, that is, as acetyl coenzyme A (acetyl Co A), acetic acid figures in the condensation with oxaloacetic acid resulting in the formation of citric acid. It is the acetyl group in the structure of citric acid that undergoes oxidation. Here, the carbon atoms are oxidized to CO₂; some of the hydrogen atoms are partially accepted by the dehydrogenase coenzymes, and some enter in proton form into solution, that is, into the surrounding medium.

Usually it is pyruvic acid (pyruvate), which is formed during glycolysis in transamination reactions and which occupies a central position in intersecting metabolic pathways, that is pointed to as the source material for acetyl CoA. Indeed, by the action of a polyenzymatic complex—pyruvate dehydrogenase—pyruvate is oxidized with the formation of CO₂ (first decarboxylation) and acetyl CoA, and NAD is reduced (Figure 1). However, the oxidation of pyruvate is by no means the only pathway for the formation of acetyl CoA, which is a characteristic product of the mitochondrial oxidation of fatty acids (thiolase enzyme), as well as of the reverse condensation reaction of the citric acid. All the enzymes participating in the reactions of the TCA cycle are located in the mitochondria, with most of them bound firmly to membrane structures.

Citric acid, whose transformation marks the start of the TCA cycle, is formed in an endergonic reaction that proceeds by using the energy-rich bond linking the acetyl group with CoA [CH₃(O)C ~ SCoA]. Subsequently, citric acid is isomerized into isocitric acid; here, cis-aconitic acid is formed as an intermediate. The product of the further transformation of isocitric acid by the action of the corresponding dehydrogenase is apparently oxalosuccinic acid, the decarboxylation of which (the second CO₂ molecule) leads to α-ketoglutaric acid. The α-ketoglutaric dehydrogenase system in many ways (high molecular weight, complex multicomponent structure, stepwise reactions, and, partially, the same coenzymes) is similar to the action of the pyruvate dehydrogenase. The reaction products are CO₂ (third decarboxylation), NADH ‧ H ⁺, and succinyl CoA. In this step, succinyl CoA synthetase catalyzes the reversible reaction for the formation of free succinate: succinyl CoA + Pi + GDP ⇄ succinate + CoA + GTP. This reaction effects what is known as substrate level phosphorylation, that is, the formation of the energy-rich guano-sine triphosphate (GTP) or adenosine triphosphate (ATP) from guanosine diphosphate (GDP) and inorganic phosphate (Pi) using the energy of succinyl CoA. After the formation of succinate, the flavoprotein succinic dehydrogenase becomes active,

Figure 1. The tricarboxylic acid cycle. The single-ruled boxes contain the enzymes and the oxidized forms of the coenzymes; CO₂ and the reduced coenzymes appear in double-ruled boxes.

leading to the formation of fumaric acid. Fumarase provides for the equilibration of fumaric acid and malic acid, while the dehy-drogenase of malic acid (coenzyme NAD⁺) completes the TCA cycle by forming oxaloacetic acid. In this step, the condensation (condensing enzyme) reaction between oxaloacetic acid and acetyl CoA forming citric acid is repeated.

The energy efficiency of the processes in the tricarboxylic acid cycle is low. However, the 4 moles of NADH, 1 mole of FADH₂, and 3 moles of CO₂ formed in the oxidation of pyruvate and succeeding reactions are important products of oxidative transformations. This is especially true of the reduced forms of NAD and FAD. The further oxidation of NAD and FAD is accomplished by the enzymes figuring in the respiratory chain and is linked to phosphorylation, that is, the formation of ATP from the esterifi-cation of inorganic phosphate. Each molecule of pyruvate completely oxidized to CO₂ and H₂O results in the formation of at least 15 energy-rich phosphate bonds. The oxidation of NADH and FADH₂ by the enzymes in the respiratory chain is rather efficient; it occurs using atmospheric oxygen, leads to the formation of water, and serves as the major source for the cell’s energy reserves. However, the enzymes of the TCA cycle do not participate directly in this oxidation.

REFERENCES

Krebs, H., and H. Kornberg. Prevrashcheniia energii v zhivykh sistemakh. Moscow, 1959. (Translated from English.)
Filippovich, Iu. B. Osnovy biokhimii. Moscow, 1969.
Lehninger, A. Biokhimiia. Moscow, 1974. Chapter 16. (Translated from English.)

S. E. SEVERIN

tricarboxylic acid cycle

[trī‚kär·bäk′sil·ik ′as·əd ‚sī·kəl] (biochemistry) Krebs cycle

tricarboxylic acid cycle

cycle

[si´k'l] a succession or recurring series of events.cardiac cycle a complete cardiac movement, or heart beat, including systole, diastole, and the intervening pause.

Cardiac cycle. From Applegate, 2000.cell cycle the cycle of biochemical and morphological events occurring in a reproducing cell population; it consists of the S phase, occurring toward the end of interphase, in which DNA is synthesized; the G₂ phase, a relatively quiescent period; the M phase, consisting of the four phases of mitosis; and the G₁ phase of interphase, which lasts until the S phase of the next cycle.citric acid cycle tricarboxylic acid cycle.estrous cycle the recurring periods of estrus in adult females of most mammalian species and the correlated changes in the reproductive tract from one period to another.hair cycle the successive phases of the production and then loss of hair, consisting of anagen, catagen, and telogen.Krebs cycle tricarboxylic acid cycle.menstrual cycle see menstrual cycle.ovarian cycle the sequence of physiologic changes in the ovary involved in ovulation; see also ovulation and reproduction.reproductive cycle the cycle of physiologic changes in the reproductive organs, from the time of fertilization of the ovum through gestation and childbirth; see also reproduction.sex cycle (sexual cycle) 1. the physiologic changes that recur regularly in the reproductive organs of nonpregnant female mammals.2. the period of sexual reproduction in an organism that also reproduces asexually.TCA cycle tricarboxylic acid cycle.tricarboxylic acid cycle the cyclic metabolic mechanism by which the complete oxidation of the acetyl portion of acetyl-coenzyme A is effected; the process is the chief source of mammalian energy, during which carbon chains of sugars, fatty acids, and amino acids are metabolized to yield carbon dioxide, water, and high-energy phosphate bonds. Called also citric acid cycle, Krebs cycle, and TCA cycle.

Central pathways of metabolism: How the body produces energy from the energy-containing nutrients using the tricarboxylic acid cycle. From Davis and Sherer, 1994.urea cycle a cyclic series of reactions that produce urea" >urea; it is a major route for removal of the ammonia produced in the metabolism of amino acids in the liver and kidney.

tri·car·box·yl·ic ac·id cy·cle

together with oxidative phosphorylation, the main source of energy in the mammalian body and the end toward which carbohydrate, fat, and protein metabolism is directed; a series of reactions, beginning and ending with oxaloacetic acid, during the course of which a two-carbon fragment is completely oxidized to carbon dioxide and water with the production of 12 high-energy phosphate bonds. So called because the first four substances involved (citric acid, cis-aconitic acid, isocitric acid, and oxalosuccinic acid) are all tricarboxylic acids; from oxalosuccinate, the others are, in order, α-ketoglutarate, succinate, fumarate, l-malate, and oxaloacetate, which condenses with acetyl-CoA (from fatty acid degradation) to form citrate (citric acid) again. Synonym(s): citric acid cycle, Krebs cycle

tricarboxylic acid cycle

n. See Krebs cycle.

tricarboxylic acid cycle

Citric acid cycle, see there.

tri·car·box·yl·ic ac·id cy·cle

(TCA cycle) (trī-kahr-bok-sil'ik as'id sī'kĕl) Together with oxidative phosphorylation, the main source of energy in the mammalian body and the end toward which carbohydrate, fat, and protein metabolism is directed; a series of reactions, beginning and ending with oxaloacetic acid, during the course of which a two-carbon fragment is completely oxidized to carbon dioxide and water with the production of 12 high-energy phosphate bonds. So called because the first four substances involved (citric acid, cis-aconitic acid, isocitric acid, and oxalosuccinic acid) are all tricarboxylic acids; from oxalosuccinate, the others are, in order, α-ketoglutarate, succinate, fumarate, l-malate, and oxaloacetate, which condenses with acetyl-CoA (from fatty acid degradation) to form citrate (citric acid) again.
Synonym(s): Krebs cycle.

cycle

(si'kel) [Gr. kyklos, circle] A regular, complete series of movements or events.

anovular cycle

Menstrual cycle in which ovulation is absent.

CARDIAC CYCLE (ONE HEARTBEAT, PULSE 75): The outer circle represents the ventricles, the middle circle the atria, and the inner circle the movement of blood and its effect on the heart valves.

cardiac cycle

The period from the beginning of one heartbeat to the beginning of the succeeding beat, including systole and diastole. Normally, the atria contract immediately before the ventricles. The ordinary cycle lasts 0.8 sec with the heart beating approx. 60 to 85 times a minute in the adult at rest. Atrial systole lasts 0.1 sec, ventricular systole 0.3 sec, and diastole 0.4 sec. Although the heart seems to be working continuously, it actually rests for a good portion of each cardiac cycle. A wart, typically found on the genitals, the perineum, the anus, or the mucosal surfaces of the vagina or mouth, usually spread by sexual contact. It is caused by various types of human papilloma virus and may be spread by physical contact with an area containing a wart. The spread of a wart from one labium to the other by autoinoculation is possible. The virus that causes the wart is usually transmitted sexually. Synonym: genital wart

Treatment

Topically applied liquid nitrogen, imiquimod cream, fluorouracil, or podophyllin may prove effective; multiple treatments are usually needed, including occasionally surgery, electrosurgery, or laser ablation. Extremely large lesions (Buschke-Lowenstein tumor) may need radical excision.

See: Cardiac Cycle illustration

cell cycle

The cycle of the growth and development of a cell. The cell cycle consists of mitosis, during which chromosomes actively divide to form two sister cells, and the interphase, during which the cell grows, begins to synthesize DNA, and prepares for chromosomal division. The interphase consists of several gap or G phases and the S (DNA Synthesis) phase. See: interphase; meiosis and mitosis for illus.

cell growth cycle

The order of physical and biochemical events that occur during the growth of cells. In tissue culture studies, the cyclic changes are divided into specific periods or phases: the DNA synthesis or S period, the G₂ period or gap, the M or mitotic period, and the G₁ period.

citric acid cycle

Krebs cycle.

Cori cycle

See: Cori cycle

duty cycle

During chest compressions of a victim of cardiac arrest, the relative amount of time that the chest is compressed compared to the time that the chest is allowed to recoil to its fully inflated position. A cycle of 50% occurs when chest compression equals chest recoil.

estrus cycle

The sequence from the beginning of one estrus period to the beginning of the next. It includes proestrus, estrus, and metestrus, followed by a short period of quiescence called diestrus.

gastric cycle

The progression of peristalsis through the stomach.

genesial cycle

1. The period from puberty to menopause.2. The period of sexual maturity.

glycolytic cycle

The cycle by which glucose is broken down in living tissue.

initiated cycle

In assisted reproduction, any month when a woman is treated with drugs that stimulate the ovary to produce follicles.

Krebs cycle

See: Krebs cycle

life cycle

All of the developmental history of an organism, whether in a free-living condition or in a host (e.g., as a parasite that experiences part of its cycle inside another organism).

PHASES OF THE MENSTRUAL CYCLE

menstrual cycle

The periodically recurrent series of changes occurring in the uterus and associated sex organs (ovaries, cervix, and vagina) associated with menstruation and the intermenstrual period. The human cycle averages 28 days in length, measured from the beginning of menstruation. The menstrual cycle is, however, quite variable in length, even in the same person from month to month. Variations in the length of the cycle are due principally to variation in the length of the proliferative phase. See: illustration

The menstrual cycle is divided into four phases characterized by histological changes that take place in the uterine endometrium. They are:

Proliferative Phase: Following blood loss from the endometrium, the uterine epithelium is restored to normal; the endometrium becomes thicker and more vascular; the glands elongate. During this period, the ovarian follicle is maturing and secreting estrogens; with the estrogen stimulation, the endometrium hypertrophies, thickening and becoming more vascular, and the glands elongate. The phase is terminated by the rupture of the follicle and the liberation of the ovum at about 14 days before the next menstrual period begins. Fertilization of the ovum is most likely to occur in the days immediately following ovulation.

Luteal or Secretory Phase: After releasing the ovum, the corpus luteum secretes progesterone. With the progesterone stimulation, the endometrium becomes even thicker; the glands become more tortuous and produce an abundant secretion containing glycogen. The coiled arteries make their appearance; the endometrium becomes edematous; the stroma becomes compact. During this period, the corpus luteum in an ovary is developing and secreting progesterone. This phase lasts 10 to 14 days.

Premenstrual or Ischemic Phase: If pregnancy has not occurred, the coiled arteries constrict and the endometrium becomes anemic and shrinks a day or two before menstruation. The corpus luteum of the ovary begins involution. This phase lasts about 2 days and is terminated by the opening up of constricted arteries, the breaking off of small patches of endometrium, and the beginning of menstruation with the flow of menstrual fluid.

Menstruation: The functional layer of the endometrium is shed.

The menstrual cycle is altered by pregnancy, the use of contraception, intercurrent illnesses, diet, and exercise.

nitrogen cycle

A series of natural processes in which nitrogen is discharged from animal life into the soil; the nitrogen is taken up from the soil by nitrogen-fixing bacteria and converted to nitrates usable by plants for their nourishment; and in turn nitrogen is taken up by plant-eating animals.

sleep-wake cycle

The amount of time spent asleep and awake and the cycle of that schedule from day to day.

stimulated cycle

A cycle in assisted reproduction in which a woman receives drugs to stimulate her ovaries for production of additional follicles. See: unstimulated cycle

stretch-shortening cycle

An eccentric muscle contraction followed immediately by a concentric contraction of the same muscle group. The elastic potentiation that occurs during the eccentric phase increases the force of output of the concentric contraction. These exercises replicate functional movement patterns and are typically used in the advance phase of rehabilitation, particularly in sports rehab. Exercises incorporating this phenomenon are called plyometrics. See: plyometrics

tricarboxylic acid cycle

Krebs cycle.

unstimulated cycle

A cycle in assisted reproduction in which a woman does not receive drugs to stimulate her ovaries for production of additional follicles. See: stimulated cycle

urea cycle

The complex cyclic chemical reactions in some (ureotelic) animals, including humans, that produce urea from the metabolism of nitrogen-containing foods. This cycle, first described by Sir Hans Krebs, provides a method of excreting the nitrogen produced by the metabolism of amino acids as urea.

Wald cycle

See: Wald cycle

tricarboxylic acid cycle

see KREBS CYCLE.

Krebs,

Sir Hans Adolph, German biochemist in England and Nobel laureate, 1900-1981. Krebs cycle - together with oxidative phosphorylation, the main source of energy in the mammalian body and the end toward which carbohydrate, fat, and protein metabolism are directed. Synonym(s): tricarboxylic acid cycleKrebs-Henseleit cycle - the sequence of chemical reactions, occurring primarily in the liver, that results in the production of urea. Synonym(s): urea cycleKrebs-Ringer solution - a modification of Ringer solution.

cycle, Krebs

A series of reactions in which the intermediate products of carbohydrate, fat and protein metabolism are converted to carbon dioxide and hydrogen atoms (electrons and hydrogen ions). This cycle can only operate in the presence of oxygen. Further oxidation yields carbon dioxide, water and ATP. This cycle occurs in the mitochondria that are found in the cytoplasm of cells of living organisms. It forms one of the processes in the metabolism of glucose providing energy (stored in ATP) to maintain the vital functions of the cells (e.g. mitosis). This cycle represents the principal energy pathway of the corneal endothelium. Syn. citric acid cycle; tricarboxylic acid cycle. See mitosis.

Table C9 Relationship between the minimum angle of resolution, the Snellen fraction and the equivalent spatial frequency of a sine wave
	Snellen fraction
resolution (min of arc)	(m)	(ft)	Spatial frequency (cpd)
0.5	6/3	20/10	60
0.6	6/3.6	20/12	50
0.75	6/4.5	20/15	40
1.0	6/6	20/20	30
1.25	6/7.5	20/25	24
1.5	6/9	20/30	20
2.0	6/12	20/40	15
2.5	6/15	20/50	12
4.0	6/24	20/80	7.5
5.0	6/30	20/100	6
8.0	6/48	20/160	3.8
10.0	6/60	20/200	3
20.0	6/120	20/400	1.5

tricarboxylic acid cycle

tricarboxylic acid cycle

tricarboxylic acid cycle

Krebs′ cy`cle

tricarboxylic acid cycle

tricarboxylic acid cycle:

Tricarboxylic Acid Cycle

REFERENCES

tricarboxylic acid cycle

tricarboxylic acid cycle

cycle

tri·car·box·yl·ic ac·id cy·cle

tricarboxylic acid cycle

tricarboxylic acid cycle

tri·car·box·yl·ic ac·id cy·cle

cycle

anovular cycle

cardiac cycle

Treatment

cell cycle

cell growth cycle

citric acid cycle

Cori cycle

duty cycle

estrus cycle

gastric cycle

genesial cycle

glycolytic cycle

initiated cycle

Krebs cycle

life cycle

menstrual cycle

nitrogen cycle

sleep-wake cycle

stimulated cycle

stretch-shortening cycle

tricarboxylic acid cycle

unstimulated cycle

urea cycle

Wald cycle

tricarboxylic acid cycle

Krebs,

cycle, Krebs

tricarboxylic acid cycle

Synonyms for tricarboxylic acid cycle

noun in all plants and animals: a series of enzymatic reactions in mitochondria involving oxidative metabolism of acetyl compounds to produce high-energy phosphate compounds that are the source of cellular energy

Synonyms

Related Words