|
Aminocyclopropane-1-carboxylic acid synthase (ACC synthase, ACS) () 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 acitvated 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. In enzymology, a 1-aminocyclopropane-1-carboxylate synthase is an enzyme that catalyzes the chemical reaction :S-adenosyl-L-methionine 1-aminocyclopropane-1-carboxylate + methylthioadenosine 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. ==Enzyme mechanism== The reaction catalyzed by 1-aminocyclopropane-1-carboxylic acid synthase (ACS) is the committed and rate-limiting step in the biosynthesis of ethylene (), 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). Proposed steps of the reaction mechanism: # Formation of the ACS-PLP Schiff Base # Imine Exchange # Formation of the Quinonoid Intermediate # Tyrosine and PLP stabilized 3C-Ring formation ;Formation of the ACS-PLP Schiff Base The aldehyde of coenzyme PLP reacts to form an imine (Schiff base) linkage with the catalytic domain lysine (278) residue of ACS. ;Imine exchange 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). ;Formation of the Quinonoid Intermediate 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. ;Tyrosine and PLP stabilized 3C-Ring formation 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. * *Note inhibitors AVG and AMA bind PLP to form a ketimine and oxime respectively (whose reverse reactions are much less favorable) and prevent the ACC synthase catalyzed reaction with SAM. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「1-aminocyclopropane-1-carboxylate synthase」の詳細全文を読む スポンサード リンク
|