Ethylene biosynthesis: processing of a substrate analog supports a radical mechanism for the ethylene-forming enzyme

Biological formation of ethylene

This review summarizes the structures, biochemical properties, and mechanisms of two major biological sources of ethylene, the ethylene-forming enzyme (EFE) and 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO). EFE is found in selected bacteria and fungi where it catalyzes two reactions: (1) the oxygen-dependent conversion of 2-oxoglutarate (2OG) to ethylene plus three molecules of CO2/bicarbonate and (2) the oxidative decarboxylation of 2OG while transforming l-arginine to guanidine and l-Δ1-pyrroline-5-carboxylic acid. ACCO is present in plants where it makes the plant hormone by transforming ACC, O2, and an external reductant to ethylene, HCN, CO2, and water. Despite catalyzing distinct chemical reactions, EFE and ACCO are related in sequence and structure, and both enzymes require Fe(ii) for their activity. Advances in our understanding of EFE, derived from both experimental and computational approaches, have clarified how this enzyme catalyzes its dual reactions. Drawing on the published mechanistic studies of ACCO and noting the parallels between this enzyme and EFE, we propose a novel reaction mechanism for ACCO.

Ethylene-independent functions of the ethylene precursor ACC in Marchantia polymorpha

The plant hormone ethylene has many roles in growth and development¹. In seed plants, the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) is converted into ethylene by ACC oxidase (ACO), and treatment with ACC induces ethylene responses². However, non-seed plants lack ACO homologues^3-8, which led us to examine the relationship between ACC and ethylene in the liverwort Marchantia polymorpha. Here, we demonstrate that ACC and ethylene can induce divergent growth responses in Marchantia. Ethylene increases plant and gemma size, induces more gemma cups and promotes gemmae dormancy. As predicted, Mpctr1-knockout mutants display constitutive ethylene responses, whereas Mpein3-knockout mutants exhibit ethylene insensitivity. Compared with the wild type, Mpctr1 gemmae have more and larger epidermal cells, whereas Mpein3 gemmae have fewer and smaller epidermal cells, suggesting that ethylene promotes cell division and growth in developing gemmae. By contrast, ACC treatment inhibits gemma growth and development by suppressing cell division, even in the Mpein3-knockout alleles. Knockout mutants of one or both ACC SYNTHASE (ACS) gene homologues produce negligible levels of ACC, have more and larger gemma cups, and have more-expanded thallus branches. Mpacs2 and Mpacs1 Mpacs2 gemmae also display a high frequency of abnormal apical notches (meristems) that are not observed in ethylene mutants. These findings reveal that ethylene and ACC have distinct functions, and suggest that ACC is a signalling molecule in Marchantia. ACC may be an evolutionarily conserved signal that predates its efficient conversion to ethylene in higher plants.

The Ethylene Precursor ACC Affects Early Vegetative Development Independently of Ethylene Signaling

The plant hormone ethylene plays a pivotal role in virtually every aspect of plant development, including vegetative growth, fruit ripening, senescence, and abscission. Moreover, it acts as a primary defense signal during plant stress. Being a volatile, its immediate biosynthetic precursor, 1-aminocyclopropane-1-carboxylic acid, ACC, is generally employed as a tool to provoke ethylene responses. However, several reports propose a role for ACC in parallel or independently of ethylene signaling. In this study, pharmacological experiments with ethylene biosynthesis and signaling inhibitors, 2-aminoisobutyric acid and 1-methylcyclopropene, as well as mutant analyses demonstrate ACC-specific but ethylene-independent growth responses in both dark- and light-grown Arabidopsis seedlings. Detection of ethylene emanation in ethylene-deficient seedlings by means of laser-based photoacoustic spectroscopy further supports a signaling role for ACC. In view of these results, future studies employing ACC as a proxy for ethylene should consider ethylene-independent effects as well. The use of multiple knockout lines of ethylene biosynthesis genes will aid in the elucidation of the physiological roles of ACC as a signaling molecule in addition to its function as an ethylene precursor.

Thermodynamic investigation of the binary and ternary complexes involving 1-aminocyclopropane carboxylic acid with reference to plant hormone

Complex formation equilibria of 1-aminocyclopropane carboxylic acid (ACC) and 3,3-bis(1-methylimidazol-2-yl) propionic acid (BIMP) with metal ions Cu2+, Ni2+, Co2+, Zn2+, Mn2+ and Fe2+ were investigated. ACC forms 1:1 and 1:2 complexes in addition to the hydrolysed form of the 1:1 complex, except in the case of Mn2+ and Fe2+, where the hydrolysed complex is not formed. BIMP forms 1:1 and 1:2 complexes in addition to the hydrolsed form of the 1:1 complex in the case of Mn2+ and Cu2+, however the hydrolysed complex is not detected for Ni2+, Co2+, Zn2+ and Fe2+. The concentration distribution diagrams of the complexes were determined. The Fe2+-complex with BIMP is exothermic and the thermodynamic parameters were calculated. The effect of organic solvent on the acid dissociation constants of 1-aminocyclopropane carboxylic acid (ACC) and 3,3-bis(1-methylimidazol-2-yl) propionic acid (BIMP) and the formation constants of Fe2+ complexes were investigated. Fe2+ forms a mixed-ligand complex with ACC and BIMP with stoichiometric coefficients 1:1:1. The formation constant was determined. The ternary complex is enhanced by back donation from the negatively charged 1-aminocyclopropane carboxylate to the π-system of BIMP. From the concentration distribution diagram, the ternary complex prevails in the physiological pH range.

DFT and docking studies of rhodostreptomycins A and B and their interactions with solvated/nonsolvated Mg²⁺ and Ca²⁺ ions

The interactions of L-aminoglucosidic stereoisomers such as rhodostreptomycins A (Rho A) and B (Rho B) with cations (Mg(2+), Ca(2+), and H(+)) were studied by a quantum mechanical method that utilized DFT with B3LYP/6-311G. Docking studies were also carried out in order to explore the surface recognition properties of L-aminoglucoside with respect to Mg(2+) and Ca(2+) ions under solvated and nonsolvated conditions. Although both of the stereoisomers possess similar physicochemical/antibiotic properties against Helicobacter pylori, the thermochemical values for these complexes showed that its high affinity for Mg(2+) cations caused the hydration of Rho B. According to the results of the calculations, for Rho A-Ca(2+)(H2O)6, ΔH = -72.21 kcal mol(-1); for Rho B-Ca(2+)(H2O)6, ΔH = -72.53 kcal mol(-1); for Rho A-Mg(2+)(H2O)6, ΔH = -72.99 kcal mol(-1) and for Rho B-Mg(2+)(H2O)6, ΔH = -95.00 kcal mol(-1), confirming that Rho B binds most strongly with hydrated Mg(2+), considering the energy associated with this binding process. This result suggests that Rho B forms a more stable complex than its isomer does with magnesium ion. Docking results show that both of these rhodostreptomycin molecules bind to solvated Ca(2+) or Mg(2+) through hydrogen bonding. Finally, Rho B is more stable than Rho A when protonation occurs.

1-Aminocyclopropane-1-carboxylic acid oxidase: insight into cofactor binding from experimental and theoretical studies

1-Aminocyclopropane-1-carboxylic acid oxidase (ACCO) is a nonheme Fe(II)-containing enzyme that is related to the 2-oxoglutarate-dependent dioxygenase family. The binding of substrates/cofactors to tomato ACCO was investigated through kinetics, tryptophan fluorescence quenching, and modeling studies. α-Aminophosphonate analogs of the substrate (1-aminocyclopropane-1-carboxylic acid, ACC), 1-aminocyclopropane-1-phosphonic acid (ACP) and (1-amino-1-methyl)ethylphosphonic acid (AMEP), were found to be competitive inhibitors versus both ACC and bicarbonate (HCO(3)(-)) ions. The measured dissociation constants for Fe(II) and ACC clearly indicate that bicarbonate ions improve both Fe(II) and ACC binding, strongly suggesting a stabilization role for this cofactor. A structural model of tomato ACCO was constructed and used for docking experiments, providing a model of possible interactions of ACC, HCO(3)(-), and ascorbate at the active site. In this model, the ACC and bicarbonate binding sites are located close together in the active pocket. HCO(3)(-) is found at hydrogen-bond distance from ACC and interacts (hydrogen bonds or electrostatic interactions) with residues K158, R244, Y162, S246, and R300 of the enzyme. The position of ascorbate is also predicted away from ACC. Individually docked at the active site, the inhibitors ACP and AMEP were found coordinating the metal ion in place of ACC with the phosphonate groups interacting with K158 and R300, thus interlocking with both ACC and bicarbonate binding sites. In conclusion, HCO(3)(-) and ACC together occupy positions similar to the position of 2-oxoglutarate in related enzymes, and through a hydrogen bond HCO(3)(-) likely plays a major role in the stabilization of the substrate in the active pocket.

Mechanistic studies of 1-aminocyclopropane-1-carboxylic acid oxidase: single turnover reaction

The final step in the biosynthesis of the plant hormone ethylene is catalyzed by the non-heme iron-containing enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO). ACC is oxidized at the expense of O(2) to yield ethylene, HCN, CO(2), and two waters. Continuous turnover of ACCO requires the presence of ascorbate and HCO(3)(-) (or an alternative form), but the roles played by these reagents, the order of substrate addition, and the mechanism of oxygen activation are controversial. Here these issues are addressed by development of the first functional single turnover system for ACCO. It is shown that 0.35 mol ethylene/mol Fe(II)ACCO is produced when the enzyme is combined with ACC and O(2) in the presence of HCO(3)(-) but in the absence of ascorbate. Thus, ascorbate is not required for O(2) activation or product formation. Little product is observed in the absence of HCO(3)(-), demonstrating the essential role of this reagent. By monitoring the EPR spectrum of the sample during single turnover, it is shown that the active site Fe(II) oxidizes to Fe(III) during the single turnover. This suggests that the electrons needed for catalysis can be derived from a fraction of the initial Fe(II)ACCO instead of ascorbate. Addition of ascorbate at 10% of its K(m) value significantly accelerates both iron oxidation and ethylene formation, suggesting a novel high-affinity effector role for this reagent. This role can be partially mimicked by a non-redox-active ascorbate analog. A mechanism is proposed that begins with ACC and O(2) binding, iron oxidation, and one-electron reduction to form a peroxy intermediate. Breakdown of this intermediate, perhaps by HCO(3)(-)-mediated proton transfer, is proposed to yield a high-valent iron species, which is the true oxidizing reagent for the bound ACC.

Spectroscopic studies of 1-aminocyclopropane-1-carboxylic acid oxidase: molecular mechanism and CO(2) activation in the biosynthesis of ethylene

1-Aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO) catalyzes the last step in the biosynthesis of the gaseous plant hormone ethylene, which is involved in development, including germination, fruit ripening, and senescence. ACCO is a mononuclear non-heme ferrous enzyme that couples the oxidation of the cosubstrate ascorbate to the oxidation of substrate ACC by dioxygen. In addition to substrate and cosubstrate, ACCO requires the activator CO(2) for continuous turnover. NIR circular dichroism and magnetic circular dichroism spectroscopies have been used to probe the geometric and electronic structure of the ferrous active site in ACCO to obtain molecular-level insight into its catalytic mechanism. Resting ACCO/Fe(II) is coordinatively saturated (six-coordinate). In the presence of CO(2), one ferrous ligand is displaced to yield a five-coordinate site only when both the substrate ACC and cosubstrate ascorbate are bound to the enzyme. The open coordination position allows rapid O(2) activation for the oxidation of both substrates. In the absence of CO(2), ACC binding alone converts the site to five-coordinate, which would react with O(2) in the absence of ascorbate and quickly deactivate the enzyme. These studies show that ACCO employs a general strategy similar to other non-heme iron enzymes in terms of opening iron coordination sites at the appropriate time in the reaction cycle and define the role of CO(2) as stabilizing the six-coordinate ACCO/Fe(II)/ACC complex, thus preventing the uncoupled reaction that inactivates the enzyme.

The catalytic mechanism of 1-aminocyclopropane-1-carboxylic acid oxidase

It has been proposed that 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase catalyzes the oxidation of ACC to ethylene via N-hydroxyl-ACC as an intermediate. However, due to its chemical instability the putative intermediate has never been isolated. Here, we have shown that a purified recombinant ACC oxidase can utilize alpha-aminoisobutyric acid (AIB), an analog of ACC, as an alternative substrate, converting AIB into CO2, acetone, and ammonia. We chemically synthesized the putative intermediate compound, N-hydroxyl-AIB (HAIB), and tested whether it serves as an intermediate in the oxidation of AIB. When [1-(14)C]AIB was incubated with ACC oxidase in the presence of excess unlabeled HAIB as a trap, no labeled HAIB was detected. By comparing the acetone production rates employing HAIB and AIB as substrates, the conversion of HAIB to acetone was found to be much slower than that of using AIB as substrate. Based on these observations, we conclude that ACC oxidase does not catalyze via the N-hydroxylation of its amino acid substrate. ACC oxidase also catalyzes the oxidation of other amino acids, with preference for the D-enantiomers, indicating a stereoselectivity of the enzyme.

Increased ability of transgenic plants expressing the bacterial enzyme ACC deaminase to accumulate Cd, Co, Cu, Ni, Pb, and Zn

Transgenic tomato plants Lycopersicon esculentum (Solanaceae) cv. Heinz 902 expressing the bacterial gene 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, under the transcriptional control of either two tandem 35S cauliflower mosaic virus promoters (constitutive expression), the rolD promoter from Agrobacterium rhizogenes (root specific expression) or the pathogenesis related PRB-1b promoter from tobacco, were compared to non-transgenic tomato plants in their ability to grow in the presence of Cd, Co, Cu, Mg, Ni, Pb, or Zn and to accumulate these metals. Parameters that were examined include metal concentration and ACC deaminase activity in both plant shoots and roots; root and shoot development; and leaf chlorophyll content. In general, transgenic tomato plants expressing ACC deaminase, especially those controlled by the PRB-1b promoter, acquired a greater amount of metal within the plant tissues, and were less subject to the deleterious effects of the metals on plant growth than were non-transgenic plants.

Role of the nonheme Fe(II) center in the biosynthesis of the plant hormone ethylene

The final step of ethylene biosynthesis in plants is catalyzed by the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO). In addition to ACC, Fe(II), O2, CO2, and ascorbate are required for in vitro enzyme activity. Direct evidence for the role of the Fe(II) center in the recombinant avocado ACCO has now been obtained through formation of enzyme.(substrate or cofactor).NO complexes. These NO adducts convert the normally EPR-silent ACCO complexes into EPR-active species with structural properties similar to those of the corresponding O2 complexes. It is shown here that the ternary Fe(II)ACCO.ACC.NO complex is readily formed, but no Fe(II)ACCO.ascorbate.NO complex could be observed, suggesting that ascorbate and NO are mutually exclusive in the active site. The binding modes of ACC and the structural analog alanine specifically labeled with 15N or 17O were examined by using Q-band electron nuclear double resonance (ENDOR). The data indicate that these molecules bind directly to the iron through both the alpha-amino and alpha-carboxylate groups. These observations are inconsistent with the currently favored mechanism for ACCO, in which it is proposed that both ascorbate and O2 bind to the iron as a step in O2 activation. We propose a different mechanism in which the iron serves instead to simultaneously bind ACC and O2, thereby fixing their relative orientations and promoting electron transfer between them to initiate catalysis.