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Difference between revisions of "MitoPedia: Substrates and metabolites"

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[[Image:MitoPedia.jpg|left|100px|link=http://www.bioblast.at/index.php/MitoPedia_Glossaries|MitoPedia Glossaries]]
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== '''List of substrates and metabolites''' ==


{{#ask:[[MitoPedia topic::Substrate and metabolite]]
{{#ask:[[MitoPedia topic::Substrate and metabolite]]

Revision as of 11:30, 16 May 2014

Template:Mainpage mitopedia

Bioblast Glossaries


TermAbbreviationDescription
2-Hydroxyglutarate2HGReduction of oxoglutarate (2OG or alpha-ketoglutarate) to 2-hydroxyglutarate (2HG) is driven by NADPH. 2HG is also formed in side reactions of lactate dehydrogenase and malate dehydrogenase. Millimolar 2HG concentrations are found in some cancer cells compared to , whereas side activities of lactate and malate dehydrogenase form submillimolar s-2-hydroxyglutarate (s-2HG). However, even wild-type IDH1 and IDH2, notably under shifts toward reductive carboxylation glutaminolysis or changes in other enzymes, lead to “intermediate” 0.01–0.1 mM 2HG levels, for example, in breast carcinoma compared with nanomolar concentrations in benign cells. 2HG is considered an important player in reprogramming metabolism of cancer cells.
ADPDAdenosine diphosphate is a nucleotide. In OXPHOS core metabolism, ADP is a substrate of ANT and ATP synthase in the phosphorylation system. ADP is the discharged or low-energy counterpart of ATP. ADP can accept chemical energy by regaining a phosphate group to become ATP, in substrate-level phosphorylation (in anaerobic catabolism), at the expense of solar energy (in photosynthetic cells) or chemiosmotic energy (respiration in heterotrophic cells). ADP is added to mitochondrial preparations at kinetically saturating concentrations to induce the active state for evaluation of OXPHOS capacity.
ATPTAdenosine triphosphate is a nucleotid and functions as the major carrier of chemical energy in the cells. As it transfers its energy to other molecules, it looses its terminal phosphate group and becomes adenosine diphosphate (ADP).
Acetyl-CoA
acetyl-CoA
Acetyl-CoA, C23H38N7O17P3S, is a central piece in metabolism involved in several biological processes, but its main role is to deliver the acetyl group into the TCA cycle for its oxidation. It can be synthesized in different pathways: (i) in glycolysis from pyruvate, by pyruvate dehydrogenase, which also forms NADH; (ii) from fatty acids β-oxidation, which releases one acetyl-CoA each round; (iii) in the catabolism of some amino acids such as leucine, lysine, phenylalanine, tyrosine and tryptophan.
In the mitochondrial matrix, acetyl-CoA is condensed with oxaloacetate to form citrate through the action of citrate synthase in the tricarboxylic acid cycle. Acetyl-CoA cannot cross the mitochondrial inner membrane but citrate can be transported out of the mitochondria. In the cytosol, citrate can be converted to acetyl-CoA and be used in the synthesis of fatty acid, cholesterol, ketone bodies, acetylcholine, and other processes.
ActivityaThe activity (relative activity) is a dimensionless quantity related to the concentration or partial pressure of a dissolved substance. The activity of a dissolved substance B equals the concentration, cB [mol·L-1], at high dilution divided by the unit concentration, c° = 1 mol·L-1:
aB = cB/c°

This simple relationship applies frequently to substances at high dilutions <10 mmol·L-1 (<10 mol·m-3). In general, the concentration of a solute has to be corrected for the activity coefficient (concentration basis), γB,

aB = γB·cB/c°

At high dilution, γB = 1. In general, the relative activity is defined by the chemical potential, µB

aB = exp[(µB-µ°)/RT]
AcylcarnitineACAcylcarnitines are esters derivative of carnitine and fatty acids, involved in the metabolism of fatty acids. Long-chain acylcarnitines such as palmitoylcarnitine must be transported in this form, conjugated to carnitine, into the mitochondria to deliver fatty acids for fatty acid oxidation and energy production. Medium-chain acylcarnitines such as octanoylcarnitine are also frequently used for high-resolution respirometry.
Additive effect of convergent electron flowAα&βAdditivity Aα&β describes the principle of substrate control of mitochondrial respiration with convergent electron flow. The additive effect of convergent electron flow is a consequence of electron flow converging at the Q-junction from respiratory Complexes I and II (NS or CI&II e-input). Further additivity may be observed by convergent electron flow through glycerophosphate dehydrogenase and electron-transferring flavoprotein Complex. Convergent electron flow corresponds to the operation of the TCA cycle and mitochondrial substrate supply in vivo. Physiological substrate combinations supporting convergent NS e-input are required for reconstitution of intracellular TCA cycle function. Convergent electron flow simultaneously through Complexes I and II into the Q-junction supports higher OXPHOS capacity and ET capacity than separate electron flow through either CI or CII. The convergent NS effect may be completely or partially additive, suggesting that conventional bioenergetic protocols with mt-preparations have underestimated cellular OXPHOS-capacities, due to the gating effect through a single branch. Complete additivity is defined as the condition when the sum of separately measured respiratory capacities, N + S, is identical to the capacity measured in the state with combined substrates, NS (CI&II). This condition of complete additivity, NS=N+S, would be obtained if electron channeling through supercomplex CI, CIII and CIV does not interact with the pool of redox intermediates in the pathway from CII to CIII and CIV, and if the capacity of the phosphorylation system does not limit OXPHOS capacity (excess E-P capacity factor is zero). In most cases, however, additivity is incomplete, NS < N+S.
Adenine nucleotidesANAdenine nucleotides, which are also sometimes referred to as adenosines or adenylates, are a group of organic molecules including AMP, ADP and ATP. These molecules present the major players of energy storage and transfer.
AerobicoxThe aerobic state of metabolism is defined by the presence of oxygen (air) and therefore the potential for oxidative reactions (ox) to proceed, particularly in oxidative phosphorylation (OXPHOS). Aerobic metabolism (with involvement of oxygen) is contrasted with anaerobic metabolism (without involvement of oxygen): Whereas anaerobic metabolism may proceed in the absence or presence of oxygen (anoxic or oxic conditions), aerobic metabolism is restricted to oxic conditions. Below the critical oxygen pressure, aerobic ATP production decreases.
Amount of substancen [mol]The amount of substance n is a base physical quantity, and the corresponding SI unit is the mole [mol]. Amount of substance (sometimes abbreviated as 'amount' or 'chemical amount') is proportional to the number NX of specified elementary entities X, and the universal proportionality constant is the reciprocal value of the Avogadro constant (SI),
nX = NX·NA-1

nX contained in a system can change due to internal and external transformations,

dnX = dinX + denX

In the absence of nuclear reactions, the amount of any atom is conserved, e.g., for carbon dinC = 0. This is different for chemical substances or ionic species which are produced or consumed during the advancement of a reaction r,

Amount dn.png
A change in the amount of Xi, dni, in an open system is due to both the internal formation in chemical transformations, drni, and the external transfer, deni, across the system boundaries. dni is positive if Xi is formed as a product of the reaction within the system. deni is negative if Xi flows out of the system and appears as a product in the surroundings (Cohen 2008 IUPAC Green Book).
AnaerobicAnaerobic metabolism takes place without the use of molecular oxygen, in contrast to aerobic metabolism. The capacity for energy assimilation and growth under anoxic conditions is the ultimate criterion for facultative anaerobiosis. Anaerobic metabolism may proceed not only under anoxic conditions or states, but also under hyperoxic and normoxic conditions (aerobic glycolysis), and under hypoxic and microxic conditions below the limiting oxygen pressure.
AnaplerosisAnaplerosis is the process of formation of intermediates of the tricarboxylic acid cycle. Malic enzyme (mtME), phosphoenolpyruvate carboxykinase (PEPCK), propionyl-CoA carboxylase, pyruvate carboxylase and proline dehydrogenase play important roles in anaplerosis.
AnoxiaanoxIdeally the terms anoxia and anoxic (anox, without oxygen) should be restricted to conditions where molecular oxygen is strictly absent. Practically, effective anoxia is obtained when a further decrease of experimental oxygen levels does not elicit any physiological or biochemical response. The practical definition, therefore, depends on (i) the techiques applied for oxygen removal and minimizing oxygen diffusion into the experimental system, (ii) the sensitivity and limit of detection of analytical methods of measuring oxygen (O2 concentration in the nM range), and (iii) the types of diagnostic tests applied to evaluate effects of trace amounts of oxygen on physiological and biochemical processes. The difficulties involved in defining an absolute limit between anoxic and microxic conditions are best illustrated by a logarithmic scale of oxygen pressure or oxygen concentration. In the anoxic state (State 5), any aerobic type of metabolism cannot take place, whereas anaerobic metabolism may proceed under oxic or anoxic conditions.
AscorbateAsIn respiratory assays for cytochrome c oxidase activity (Complex IV, CIV), ascorbate is added as regenerating system to maintain TMPD in a reduced state. It has to be titrated into the respiration medium prior to the addition of TMPD, otherwise the autoxidation reaction velocity is permanently elevated.
CHNO-fuel substrateCHNOCHNO-fuel substrates are reduced carbon-hydrogen-nitrogen-oxygen substrates which are oxidized in the exergonic process of cell respiration. Mitochondrial pathways are stimulated by CHNO-fuel substrates feeding electrons into the ETS at different levels of integration and in the presence or absence of inhibitors acting on specific enzymes which are gate-keepers and control various pathway segments.
CarbohydrateCarbohydrates, also known as saccharides, are molecules composed of carbon, hydrogen and oxygen. These molecules can be divided by size and complexity into monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Glucose is a monosaccharide considered the primary source of energy in cells and a metabolic intermediate. This carbohydrate undergoes glycolysis, with the generation of pyruvate, that can enter the TCA cycle. Carbohydrates such as glucose and fructose may also be involved in the Crabtree effect.
CarnitineCarCarnitine is an important factor for the transport of long-chain fatty acids bound to carnitine (carnitine acyltransferase) into the mitochondrial matrix for subsequent β-oxidation. There are two enantiomers: D- and L-carnitine. Only the L-isomer is physiologically active.
Cellular substratesCe; Cm(1) Cellular substrates in vivo, endogenous; Ce.

(2) Cellular substrates in vivo, with exogenous substrate supply from culture medium or serum; Cm.

  • This page needs an update.
Citrate
citrate
citrate, C6H5O7-3, is a tricarboxylic acid trianion, intermediate of the TCA cycle, obtained by deprotonation of the three carboxy groups of citric acid. Citrate is formed from oxaloacetate and acetyl-CoA through the catalytic activity of the citrate synthase. In the TCA cycle, citrate forms isocitrate by the activity of the aconitase. Citrate can be transported out of the mitochondria by the tricarboxylate transport, situated in the inner mitochondrial membrane. The transport occurs as an antiport of malate from the cytosol and it is a key process for fatty acid and oxaloacetate synthesis in the cytosol.
CoenzymeA coenzyme or cosubstrate is a cofactor that is attached loosely and transiently to an enzyme, in contrast to a prosthetic group that is attached permanently and tightly. The coenzyme is required by the corresponding enzyme for its activity (IUPAC definition). A coenzyme is 'a low-molecular-weight, non-protein organic compound participating in enzymatic reactions as dissociable acceptor or donor of chemical groups or electrons' (IUPAC definition).
Coenzyme Q2CoQ2
CoQ2
Coenzyme Q2 or ubiquinone-2 (CoQ2) is a quinone derivate composed of a benzoquinone ring with an isoprenoid side chain consisting of two isoprenoid groups, with two methoxy groups, and with one methyl group. In HRR it is used as a Q-mimetic to detect the redox changes of coenzyme Q at the Q-junction in conjunction with the Q-Module, since the naturally occurring long-chain coenzyme Q (e.g. CoQ10) is trapped within membrane boundaries. CoQ2 can react both with mitochondrial complexes (e.g. CI, CII and CIII) at their quinone-binding sites and with the detecting electrode.
CofactorA cofactor is 'an organic molecule or ion (usually a metal ion) that is required by an enzyme for its activity. It may be attached either loosely (coenzyme) or tightly (prosthetic group)' (IUPAC definition).
Complex II ambiguitiesCII ambiguities
CII-ambiguities Graphical abstract.png
The current narrative that the reduced coenzymes NADH and FADH2 feed electrons from the tricarboxylic acid (TCA) cycle into the mitochondrial electron transfer system can create ambiguities around respiratory Complex CII. Succinate dehydrogenase or CII reduces FAD to FADH2 in the canonical forward TCA cycle. However, some graphical representations of the membrane-bound electron transfer system (ETS) depict CII as the site of oxidation of FADH2. This leads to the false believe that FADH2 generated by electron transferring flavoprotein (CETF) in fatty acid oxidation and mitochondrial glycerophosphate dehydrogenase (CGpDH) feeds electrons into the ETS through CII. In reality, NADH and succinate produced in the TCA cycle are the substrates of Complexes CI and CII, respectively, and the reduced flavin groups FMNH2 and FADH2 are downstream products of CI and CII, respectively, carrying electrons from CI and CII into the Q-junction. Similarly, CETF and CGpDH feed electrons into the Q-junction but not through CII. The ambiguities surrounding Complex II in the literature call for quality control, to secure scientific standards in current communications on bioenergetics and support adequate clinical applications.
Convergent electron flown.a.
Convergent electron flow
Convergent electron flow is built into the metabolic design of the Electron transfer pathway. The glycolytic pathways are characterized by important divergent branchpoints: phosphoenolpyruvate (PEPCK) branchpoint to pyruvate or oxaloactetate; pyruvate branchpoint to (aerobic) acetyl-CoA or (anaerobic) lactate or alanine. The mitochondrial Electron transfer pathway, in contrast, is characterized by convergent junctions: (1) the N-junction and F-junction in the mitochondrial matrix at ET-pathway level 4, with dehydrogenases (including the TCA cycle) and ß-oxidation generating NADH and FADH2 as substrates for Complex I and electron-transferring flavoprotein complex, respectively, and (2) the Q-junction with inner mt-membrane respiratory complexes at ET-pathway level 3, reducing the oxidized ubiquinone and partially reduced semiquinone to the fully reduced ubiquinol, feeding electrons into Complex III.
CreatineCrCreatine is a nitrogenous organic acid that occurs naturally in vertebrates and helps primarily muscle cells to supply energy by increasing the formation of adenosine triphosphate (ATP).
Cytochrome ccCytochrome c is a component of the Electron transfer-pathway (Electron transfer pathway) in mitochondria. It is a small heme protein loosely associated with the outer side of the inner mitochondrial membrane. The heme group of cytochrome c transfers electrons from Complex III to Complex IV. The release of cytochrome c into the cytoplasm is associated with apoptosis. Cytochrome c is applied in HRR to test the integrity of the mitochondrial outer membrane (cytochrome c control efficiency).
Cytochrome c control efficiencyjcyt cThe cytochrome c control efficiency expresses the control of respiration by externally added cytochrome c, c, as a fractional change of flux from substrate state CHNO to CHNOc. These fluxes are corrected for Rox and may be measured in the OXPHOS state or ET state, but not in the LEAK state. In this flux control efficiency, CHNOc is the reference state with stimulated flux; CHNO is the background state with CHNO substrates, upon which c is added: jcyt c = (JCHNOc-JCHNO)/JCHNOc.
DithioniteDit
Dit - (The abbreviation 'Dith' has been used previously and is stepwise replaced by Dit.)
The sodium salt of Dithionite Na2S2O4 (Dit) is the 'zero oxygen solution powder' used for calibration of oxygen sensors at zero oxygen concentration, or for stepwise reduction of oxygen concentrations in instrumental O2 background tests. It is not recommended to use dithionite in experiments with biological samples or several multisensor approaches, for these see Setting the oxygen concentration.
DuroquinolDQET-pathway level 2 is supported by duroquinol DQ feeding electrons into Complex III (CIII) with further electron transfer to CIV and oxygen. Upstream pathways are inhibited by rotenone and malonic acid in the absence of other substrates linked to ET-pathways with entry into the Q-junction.
ET-pathway substrate typesn.a.See Electron-transfer-pathway state
Electron-transfer-pathway stateET-pathway state
SUIT-catg FNSGpCIV.jpg

Electron-transfer-pathway states are obtained in mitochondrial preparations (isolated mitochondria, permeabilized cells, permeabilized tissues, tissue homogenate) by depletion of endogenous substrates and addition to the mitochondrial respiration medium of fuel substrates (CHNO) activating specific mitochondrial pathways, and possibly inhibitors of specific pathways. Mitochondrial electron-transfer-pathway states have to be defined complementary to mitochondrial coupling-control states. Coupling-control states require ET-pathway competent states, including oxygen supply. Categories of SUIT protocols are defined according to mitochondrial ET-pathway states.

» MiPNet article
Energy chargeAECThe energy charge of the adenylate system or adenylate energy charge (AEC) has been defined by Atkinson and Walton (1967) as (ATP + ½ ADP)/(AMP + ADP + ATP). Wheather the AEC is a fundamental metabolic control parameter remains a controversial topic.
Ethanolethanol abs.
Ethanol
Ethanol or ethyl alcohol, C2H6O or EtOH, is widely used in the laboratory, particularly as a solvent and cleaning agent. There are different grades of high purity ethanol. Up to a purity of 95.6 % ethanol can be separated from water by destillation. Higher concentrations than 95% require usage of additives that disrupt the azeotrope composition and allow further distillation. Ethanol is qualified as "absolute" if it contains no more than one percent water. Whenever 'ethanol abs.' is mentioned without further specification in published protocols, it refers to ≥ 99 % ethanol a.r. (analytical reagent grade).
FADH2FADH2FADH2 and FAD: see Flavin adenine dinucleotide.
Fatty acidFAFatty acids are carboxylic acids with a carbon aliphatic chain. The fatty acids can be divided by the length of this chain, being considered as short-chain (1–6 carbons), medium-chain (7–12 carbons) and long-chain and very long-chain fatty acids (>12 carbons).

Long-chain fatty acids must be bound to carnitine to enter the mitochondrial matrix, in a reaction that can be catalysed by carnitine acyltransferase. For this reason, long-chain fatty acids, such as palmitate (16 carbons) is frequently supplied to mt-preparations in the activated form of palmitoylcarnitine. Fatty acids with shorter chains, as octanoate (8 carbons) may enter the mitochondrial matrix, however, in HRR they are more frequently supplied also in the activated form, such as octanoylcarnitine.

Once in the mitochondrial matrix, the fatty acid oxidation (FAO) occurs, generating acetyl-CoA, NADH and FADH2. In the fatty acid oxidation pathway control state electrons are fed into the F-junction involving the electron transferring flavoprotein (CETF). FAO cannot proceed without a substrate combination of fatty acids & malate, and inhibition of CI blocks FAO. Low concentration of malate, typically 0.1 mM, does not saturate the N-pathway; but saturates the F-pathway.
Fatty acid oxidationFAOFatty acid oxidation is a multi-step process by which fatty acids are broken down in β-oxidation to generate acetyl-CoA, NADH and FADH2 for further electron transfer to CoQ. Whereas NADH is the substrate of CI, FADH2 is the substrate of electron-transferring flavoprotein complex (CETF) which is localized on the matrix face of the mtIM, and supplies electrons from FADH2 to CoQ. Before the ß-oxidation in the mitochondrial matrix, fatty acids (short-chain with 1-6, medium-chain with 7–12, long-chain with >12 carbon atoms) are activated by fatty acyl-CoA synthases (thiokinases) in the cytosol. For the mitochondrial transport of long-chain fatty acids the mtOM-enzyme carnitine palmitoyltransferase I (CPT-1; considered as a rate-limiting step in FAO) is required which generates an acyl-carnitine intermediate from acyl-CoA and carnitine. In the next step, an integral mtIM protein carnitine-acylcarnitine translocase (CACT) catalyzes the entrance of acyl-carnitines into the mitochondrial matrix in exchange for free carnitines. In the inner side of the mtIM, another enzyme carnitine palmitoyltransferase 2 (CPT-2) converts the acyl-carnitines to carnitine and acyl-CoAs, which undergo ß-oxidation in the mitochondrial matrix. Short- and medium-chain fatty acids do not require the carnitine shuttle for mitochondrial transport. Octanoate, but not palmitate, (eight- and 16-carbon saturated fatty acids) may pass the mt-membranes, but both are frequently supplied to mt-preparations in the activated form of octanoylcarnitine or palmitoylcarnitine.
Flavin adenine dinucleotideFAD, FADH2Flavin adenine dinucleotide, FAD and FADH2, is an oxidation-reduction prosthetic group (redox cofactor; compare NADH). FMN and FAD are the prosthetic groups of flavoproteins (flavin dehydrogenases). Type F substrates (fatty acids) generate FADH2, the substrate of electron transferring flavoprotein (CETF). Thus FADH2 forms a junction or funnel of electron transfer to CETF, the F-junction (compare N-junction, Q-junction), in the F-pathway control state. In contrast, FADH2 is not the substrate but the internal product of succinate dehydrogenase (CII). FAD is the oxidized (quinone) form, which is reduced to FADH2 (hydroquinone form) by accepting two electrons and two protons.
Free radicalsA free radical is any atom or molecule that contains one or more unpaired electrons in an orbital. The degree of chemical reactivity depends on the localization of unpaired electrons. Free radicals are extremely reactive, and they can either donate or accept an electron from other molecules. Free radicals that include oxygen radicals and derivatives of oxygen are reactive oxygen species (ROS). Likewise, reactive nitrogen species (RNS) are nitric oxide-derived compounds. ROS/RNS include oxygen/nitrogen free radicals and non-radicals that are easily converted into radicals. Mitochondria are a main endogenous source of free radicals in cells and consequently are exposed to oxidative-nitrosative damage. Electron transfer in the electron transfer-pathway (ET-pathway) is not perfect, leading an electron leakage. This electron leakage permits the formation of ROS such as superoxide anion (O2•−), hydrogen peroxide (H2O2) and the hydroxyl radical (HO•).
GlucoseGlcGlucose, also known as D-glucose or dextrose, is a monosaccharide and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate.
GlutamateG
Glutamic acid
Glutamic acid, C5H9NO4, is an amino acid which occurs under physiological conditions mainly as the anion glutamate-, G, with pKa1 = 2.1, pKa2 = 4.07 and pKa3 = 9.47. Glutamate&malate is a substrate combination supporting an N-linked pathway control state, when glutamate is transported into the mt-matrix via the glutamate-aspartate carrier and reacts with oxaloacetate in the transaminase reaction to form aspartate and oxoglutarate. Glutamate as the sole substrate is transported by the electroneutral glutamate-/OH- exchanger, and is oxidized in the mitochondrial matrix by glutamate dehydrogenase to α-ketoglutarate (2-oxoglutarate), representing the glutamate-anaplerotic pathway control state. Ammonia (the byproduct of the reaction) passes freely through the mitochondrial membrane.
Glutamate-aspartate carrierThe glutamate-aspartate carrier catalyzes the electrogenic antiport of glutamate- +H+ for aspartate-. It is an important component of the malate-aspartate shuttle in many mitochondria. Due to the symport of glutamate- + +H+, the glutamate-aspartate antiport is not electroneutal and may be impaired by uncoupling. Aminooxyacetate is an inhibitor of the glutamate-aspartate carrier.
GlycerophosphateGpGlycerophosphate (synonym: α-glycerophosphate; glycerol-3-phosphate; C3H9O6P) is an organophosphate and it is a component of glycerophospholipids. The mitochondrial Glycerophosphate dehydrogenase Complex oxidizes glycerophosphate to dihydroxyacetone phosphate and feeds electrons directly to ubiquinone.
HydrideH-The hydride anion is the species H.
HydrogenH2Molecular hydrogen H2 is a constituent of the air with a volume fraction of 0.00005. It is a colorless and odorless gas with a molecular mass of 2.016. Its pharmacological potential and effects on mitochondrial metabolism are discussed in various publications without complete evidence on the underlying mechanisms.
Hydrogen ionH+The terms hydrogen ion H+ and proton, p or p+, are used synonymously in chemistry. A hydrogen ion is a positively charged molecule. In particle physics, however, a proton is a submolecular and subatomic particle with a positive electric charge. The H+ ion has no electrons and is a bare charge with only about 1/64 000 of the radius of a hydrogen atom. Free H+ is extremely reactive, with an extremely short lifetime in aqueous solutions. There H+ forms the hydronium ion H3O+, which in turn is further solvated by water molecules in clusters such as H5O2+ and H9O4+. The transfer of H+ in an acid–base reaction is referred to as proton transfer. The acid is the H+ donor and the base is the H+ acceptor.
Hydrogen peroxideH2O2
Hydrogen peroxide
Hydrogen peroxide, H2O2 or dihydrogen dioxide, is one of several reactive oxygen intermediates generally referred to as reactive oxygen species (ROS). It is formed in various enzyme-catalyzed reactions (e.g., superoxide dismutase) with the potential to damage cellular molecules and structures. H2O2 is dismutated by catalase to water and oxygen. H2O2 is produced as a signaling molecule in aerobic metabolism and passes membranes more easily compared to other ROS.
Hydrogen sulfideH2SHydrogen sulfide (H2S) is involved in signaling and may have have further biological importance.
HydronH+Hydron is the general name for the cation H+ used without regard to the nuclear mass of the hydrogen entity (H is the hydro group), either for H in its natural abundance or without distinction between the isotopes.
Hydronium ionH3O+H+ forms the hydronium ion H3O+, which in turn is further solvated by water molecules in clusters such as H5O2+ and H9O4+.
Hydroxybutyrateβ-hydroxybutyrate or 3-hydroxybutyrate is a ketone body that can be used as a NADH-linked substrate. The β-hydroxybutyrate dehydrogenase produces acetoacetate while reducing NAD+ to NADH.
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