1-aminocyclopropane-1-carboxylate synthase | |||||||||
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Identifiers | |||||||||
EC no. | 4.4.1.14 | ||||||||
CAS no. | 72506-68-4 | ||||||||
Databases | |||||||||
IntEnz | IntEnz view | ||||||||
BRENDA | BRENDA entry | ||||||||
ExPASy | NiceZyme view | ||||||||
KEGG | KEGG entry | ||||||||
MetaCyc | metabolic pathway | ||||||||
PRIAM | profile | ||||||||
PDB structures | RCSB PDB PDBe PDBsum | ||||||||
Gene Ontology | AmiGO / QuickGO | ||||||||
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Aminocyclopropane-1-carboxylic acid synthase (ACC synthase, ACS) (EC 4.4.1.14) is an enzyme that catalyzes the synthesis of 1-Aminocyclopropane-1-carboxylic acid (ACC), a precursor for ethylene, from S-Adenosyl methionine (AdoMet, SAM), an intermediate in the Yang cycle and activated methyl cycle and a useful molecule for methyl transfer. ACC synthase, like other PLP dependent enzymes, catalyzes the reaction through a quinonoid zwitterion intermediate and uses cofactor pyridoxal phosphate (PLP, the active form of vitamin B6) for stabilization. [1] [2] [3]
In enzymology, a 1-aminocyclopropane-1-carboxylate synthase is an enzyme that catalyzes the chemical reaction
Hence, this enzyme has one substrate, S-adenosyl-L-methionine, and two products, 1-aminocyclopropane-1-carboxylate and methylthioadenosine.
This enzyme belongs to the family of lyases, specifically carbon-sulfur lyases. The systematic name of this enzyme class is S-adenosyl-L-methionine methylthioadenosine-lyase(1-aminocyclopropane-1-carboxylate-forming). Other names in common use include 1-aminocyclopropanecarboxylate synthase, 1-aminocyclopropane-1-carboxylic acid synthase, 1-aminocyclopropane-1-carboxylate synthetase, aminocyclopropanecarboxylic acid synthase, aminocyclopropanecarboxylate synthase, ACC synthase, and S-adenosyl-L-methionine methylthioadenosine-lyase. This enzyme participates in propanoate metabolism. It employs one cofactor, pyridoxal phosphate.
The reaction catalyzed by 1-aminocyclopropane-1-carboxylic acid synthase (ACS) is the committed and rate-limiting step in the biosynthesis of ethylene [20], a gaseous plant hormone that is responsible for the initiation of fruit ripening, shoot and root growth and differentiation, leaf and fruit abscission, flower opening, and flower and leaf senescence. (source) It is a pyridoxal phosphate (PLP) dependent gamma-elimination (?). In the gamma elimination, PLP acts as a sink twice (absorbing electrons from two deprotonations). [4] [5]
Proposed steps of the reaction mechanism:
The aldehyde of coenzyme PLP reacts to form an imine (Schiff base) linkage with the catalytic domain lysine (278) residue of ACS.
An imine exchange occurs, and the amine nitrogen of the substrate, S-Adenosyl methionine, replaces Lys (278) in the imine linkage. (Stabilized by H bonding).
PLP acts as an 'electron sink' absorbing delocalized electron density during the reaction intermediates (countering the excess electron density on the deprotonated a-carbon). PLP facilitates the enzyme activity, increasing the acidity of the alpha carbon by stabilizing the conjugate base. The PLP-stabilized carbanion intermediate formed is the quinonoid intermediate.
PLP and Tyrosine stabilize negative charges during deprotonation. Tyrosine attacks the sulfur bound carbon, allowing S(CH3)(Ado) to leave, and during ring formation, Tyrosine leaves.
ACC synthase reaches optimal activity in conditions of pH 8.5 and with Km = 20 um relative to its substrate, SAM.
ACC Synthase and ethylene biosynthesis are regulated by a whole host of stimuli. Stresses such as wounding, noxious chemicals, auxin, flooding, and indole-3-acetic acid (IAA) promote ethylene synthesis, creating a positive feedback cycle with ACC synthase, up-regulating its activity.
However, it is also inhibited by a number of compounds as well. S-Adenosylethionine can bind as a substrate for ACC synthase (with higher affinity than SAM) and therefore inhibit any reaction with SAM. ACC Synthase is also competitively inhibited by aminoethoxyvinylglycine (AVG) and aminooxyacetic acid (AOA), inhibitors to many pyridoxal phosphate-mediated enzymic reactions. They are natural toxins that cause slow binding inhibition by interfering with the coenzyme pyridoxal phosphate. ACC synthase activity is also inhibited by intermediates of the activated methyl cycle and the methionine-recycling pathway: 5′-methylthioadenosine, α-keto-γ-methylthiobutyric acid, and S-adenosylhomocysteine. [8] [9] [10]
ACC Synthase is 450-516 amino acid long sequence depending on the species of plant from which it is extracted. Though it is comparable in the species in which it is found, its COOH terminal domain is more variable, leading to differences such as oligomerization. The COOH terminal domain is responsible for oligomerization. In most ACC Synthase producing cells, ACC Synthase exists as a dimer. However, in some we find a monomer ("which is more active and efficient [than its dimer counterpart"). [11]
The structure of ACS has been largely determined via X-ray crystallography. [12] Conservation of the residues in ACS's catalytic domain and sequence homology suggest that ACS catalyzes the synthesis of ACC in a similar fashion as other enzymes that require PLP as a cofactor. However, unlike many other PLP-dependent enzymes, Lys (278) is not the only residue that interacts with the substrate. The proximity of the electronegative oxygen from Tyr (152) to the C-γ-S bond suggests a crucial role in the formation of ACC. [13] X-ray crystallography with aminoethoxyvinylglycine (AVG) a competitive inhibitor confirmed Tyrosine's role in the γ elimination. [14]
As of late 2007, 6 structures have been solved for this class of enzymes, with PDB accession codes 1B8G, 1IAX, 1IAY, 1M4N, 1M7Y, and 1YNU.
The main functional groups in the catalytic domains are the Nitrogen from the Lys 278 residue and the Oxygen from the Tyrosine 152 residue.
ACC Synthase is the key, rate limiting step in ethylene synthesis. Because the up-regulation of ACC-Synthase is what induces fruit ripening and often spoilage there is more research being done on the regulatory mechanisms and biosynthetic pathways of ethylene to avoid spoilage. [15] [16]
Methionine is an essential amino acid in humans. As the substrate for other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical role in the metabolism and health of many species, including humans. It is encoded by the codon AUG.
1-Aminocyclopropane-1-carboxylic acid (ACC) is a disubstituted cyclic α-amino acid in which a cyclopropane ring is fused to the Cα atom of the amino acid. It is a white solid. Many cyclopropane-substituted amino acids are known, but this one occurs naturally. Like glycine, but unlike most α-amino acids, ACC is not chiral.
Aminolevulinic acid synthase (ALA synthase, ALAS, or delta-aminolevulinic acid synthase) is an enzyme (EC 2.3.1.37) that catalyzes the synthesis of δ-aminolevulinic acid (ALA) the first common precursor in the biosynthesis of all tetrapyrroles such as hemes, cobalamins and chlorophylls. The reaction is as follows:
Pyridoxal phosphate (PLP, pyridoxal 5'-phosphate, P5P), the active form of vitamin B6, is a coenzyme in a variety of enzymatic reactions. The Enzyme commission has catalogued more than 140 PLP-dependent activities, corresponding to ~4% of all classified activities. The versatility of PLP arises from its ability to covalently bind the substrate, and then to act as an electrophilic catalyst, thereby stabilizing different types of carbanionic reaction intermediates.
Lysine 2,3-aminomutase is a radical SAM enzyme that facilitates the conversion of the amino acid lysine to beta-lysine. It accomplishes this interconversion using three cofactors and a 5'-deoxyadenosyl radical formed in a S-Adenosyl methionine (SAM) activated radical reaction pathway.[1] The generalized reaction is shown below:
Cystathionine-β-synthase, also known as CBS, is an enzyme (EC 4.2.1.22) that in humans is encoded by the CBS gene. It catalyzes the first step of the transsulfuration pathway, from homocysteine to cystathionine:
Cystathionine gamma-lyase is an enzyme which breaks down cystathionine into cysteine, α-ketobutyrate, and ammonia. Pyridoxal phosphate is a prosthetic group of this enzyme.
The transsulfuration pathway is a metabolic pathway involving the interconversion of cysteine and homocysteine through the intermediate cystathionine. Two transsulfurylation pathways are known: the forward and the reverse.
Shang Fa Yang was a Taiwanese-American plant scientist and a professor at the University of California, Davis. He was awarded the Wolf Prize in Agriculture and elected a member of the US National Academy of Sciences.
In enzymology, an aminocyclopropanecarboxylate oxidase (EC 1.14.17.4) is an enzyme that catalyzes the chemical reaction
In enzymology, an alanine racemase is an enzyme that catalyzes the chemical reaction
Cystathionine beta-lyase, also commonly referred to as CBL or β-cystathionase, is an enzyme that primarily catalyzes the following α,β-elimination reaction
In enzymology, a 1-aminocyclopropane-1-carboxylate deaminase (EC 3.5.99.7) is an enzyme that catalyzes the chemical reaction
In enzymology, a discadenine synthase (EC 2.5.1.24) is an enzyme that catalyzes the chemical reaction
In enzymology, an adenosylmethionine-8-amino-7-oxononanoate transaminase is an enzyme that catalyzes the chemical reaction
Aminooxyacetic acid, often abbreviated AOA or AOAA, is a compound that inhibits 4-aminobutyrate aminotransferase (GABA-T) activity in vitro and in vivo, leading to less gamma-aminobutyric acid (GABA) being broken down. Subsequently, the level of GABA is increased in tissues. At concentrations high enough to fully inhibit 4-aminobutyrate aminotransferase activity, aminooxyacetic acid is indicated as a useful tool to study regional GABA turnover in rats.
Cobalamin biosynthesis is the process by which bacteria and archea make cobalamin, vitamin B12. Many steps are involved in converting aminolevulinic acid via uroporphyrinogen III and adenosylcobyric acid to the final forms in which it is used by enzymes in both the producing organisms and other species, including humans who acquire it through their diet.
In molecular biology, the Cys/Met metabolism PLP-dependent enzyme family is a family of proteins including enzymes involved in cysteine and methionine metabolism which use PLP (pyridoxal-5'-phosphate) as a cofactor.
Alpha-ketoglutarate-dependent hydroxylases are a major class of non-heme iron proteins that catalyse a wide range of reactions. These reactions include hydroxylation reactions, demethylations, ring expansions, ring closures, and desaturations. Functionally, the αKG-dependent hydroxylases are comparable to cytochrome P450 enzymes. Both use O2 and reducing equivalents as cosubstrates and both generate water.
Ethylene (CH
2=CH
2) is an unsaturated hydrocarbon gas (alkene) acting naturally as a plant hormone. It is the simplest alkene gas and is the first gas known to act as hormone. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, the abscission (or shedding) of leaves and, in aquatic and semi-aquatic species, promoting the 'escape' from submergence by means of rapid elongation of stems or leaves. This escape response is particularly important in rice farming. Commercial fruit-ripening rooms use "catalytic generators" to make ethylene gas from a liquid supply of ethanol. Typically, a gassing level of 500 to 2,000 ppm is used, for 24 to 48 hours. Care must be taken to control carbon dioxide levels in ripening rooms when gassing, as high temperature ripening (20 °C; 68 °F) has been seen to produce CO2 levels of 10% in 24 hours.