Cysteine synthase inhibition

In bacteria, cysteine regulates its own synthesis by feedback inhibition of serine acetyltransferase and by repression of the synthesis of cysteine synthase (Umbarger, 1978). In plants, neither ofthese effects have been observed. To attain a 44% inhibition of serine acetyltransferase from kidney bean seedlings, 0.5 mM L-cysteine was required (Smith and Thompson, 1971), a concentration 500 times greater than that required to inhibit the bacterial enzyme by 85% (Kredich and Tomkins, 1966). The activities of serine acetyltransferase in the roots of kidney beans growing either under conditions of sulfur deficiency or with adequate sulfate nutrition were not significantly... 

Cysteine synthase is inhibited by high concentrations of cysteine and of a number of other amino acids in the sulfur assimilation pathway (Ng and Anderson, 1978 Ascaho and Nicholas, 1977), but the high concentrations required cast doubt on the physiological significance of these inhibitions. The amounts of cysteine synthase were comparable in kidney beans growing either under conditions of sulfur deficiency or adequate sulfate nutrition (Smith, 1972), and in L. minor growing either in the presence of sulfate as a sole source of sulfur, or supplemented with HjS (Brunold and Schmidt, 1976) or cysteine (Brunold and Schmidt, 1978). Such studies further indicate that the amounts of cysteine synthase may not be regulated appreciably by repression and derepression. 

The level of sulfotransferase in Lemna and in Phaseolus vulgaris is also subject to strong inhibition by gaseousH2S(Brunoldand Schmidt, 1976,1978 Wyss and Brunold, 1979). However, the extractable acti vity of cysteine synthase is not similarly affected. Removal of H2S firom the gas phase results in rapid restoration of activity which, based on a study of labeling of the enzyme (von Arb and Brunold, 1980), was attributed to synthesis ofthe enzyme de novo. HjS also inhibits the level of APS sulfotransferase in cell suspension cultures of Nicotiana sylvestris in this tissue neither the ATP-sulfiirylase or cysteine synthase activity was affected by H2S or cysteine (Brunold etal., 9Sl). Importantly, the inhibition of APS sulfotransferase by H2S was correlated with an enhanced level of cysteine, suggesting that the H2S inhibition could have been mediated via this reaction product. Uptake of exogenous sulfate was also inhibited by H2S in this system (Brunold et al., 1981). 

Cysteine synthase is subject to weak inhibition by free sulfide (BertagnoUi and Wedding, 1977), although the effect of organic sulfides as implicated in the sulfate assimilation pathway has not been studied. No attempt seems to have been made to resolve the sensitivity of the enzyme from Phaseolus to sulfide in vitro with the insensitivity of cysteine synthase to HjS reported in Lemna (Brunold and Schmidt, 1976 Wyss and Brunold, 1979). However, inhibition of the enzyme in vitro is observed only at concentrations greater than 1 mM (BertagnoUi and Wedding, 1977). 

The internal regulation of the sulfate assimilation pathway and its coordination with the nitrate assimilation pathway are summarized in Fig. 3. It shows that cysteine is a negative effector of serine transacetylase and that it also controls the level of APS sulfotransferase. The inhibitory effects of HjS on the level of APS sulfotransferase are probably mediated via cysteine, though HjS itself at high concentrations inhibits cysteine synthase. 

The first disclosed natural product was cerulenin (15), an irreversible inhibitor of FabB. This hydrophobic epoxide locates itself in the hydro-phobic groove of the acyl site and reacts covalently with the active site cysteine [26]. However, 15 was also found to inhibit eukaryotic fatty acid synthase. 

NO is recognized as a mediator of bone cell metabolism, where it regulates osteoblast and osteoclast activity [141-143]. Osteoporosis, which frequently occurs in postmenopausal women, is a systemic skeletal disease associated with abnormal bone resorption. Addition of NO or NO donors to osteoclasts in vitro results in a reduction in bone resorption, whereas NO synthase inhibitors increase bone resorption, both in vitro and in vivo. Further research has shown that NO reduces bone resorption, via inhibition of the cysteine protease cathepsin K, which is believed to be a key protease in bone resorption. Most of the NO donors, i.e., nitroglycerin, 3-... 

Table II shows that UDP-pyridoxal had a similar inhibitory effect on red beet glucan synthase. It inhibited activity at much lower concentrations than other covalent modification reagents, such as N-ethylmaleimide (cysteine), phenylglyoxal (arginine) and formaldehyde (lysine). UDP-pyridoxal had an I50 that is 62-fold lower than formaldehyde.

Nitric oxide generation from L-arginine and nitric oxide donors and the formation of cGMP. L-NMMA inhibits nitric oxide synthase. Some of the nitric oxide donors such as furoxans and organic nitrates and nitrites require a thiol cofactor such as cysteine or glutathione to form nitric oxide. 

Substrates are processed directly through cysteine thiols, and cerulenin and iodoacetamide are able to inhibit the activity of plant PKSs by modifying sulf-hydryl groups of these residues [ 134]. Although chalcone and stilbene synthases do not share a high level of overall sequence similarity with FASs and PKSs, the cysteine that is modified by cerulenin is conserved, suggesting that the active sites may be structurally related [51]. 

Chlorolaevulinic acid 28 is a potent competitive inhibitor of bovine PEG synthase, presumably due to binding in the active site in place of ALA, and it also inactivates the enzyme by alkylation of a specific cysteine residue [24] (Scheme 9). The concentration required for the inactivation is much greater than that required for competitive inhibition, however, which suggests that the processes occur at different sites on the enzyme. Electrospray mass spectrometry has shown that 28 can alkylate at multiple sites on the B. subtilis enzyme without causing more than about 50% loss of activity [18]. It is likely that there is no cysteine residue in the active site of this latter enzyme. 

As illustrated in Fig. 1, methionine synthase is positioned at the intersection between transsulfuration and methylation pathways. As a consequence, its level of activity exerts control over cellular redox status, since it determines the proportion of HCY that will be diverted toward cysteine and GSH synthesis. Methionine synthase activity is exceptionally sensitive to inhibition during oxidative stress, primarily because its cobalamin cofactor is easily oxidized (Liptak and Brunold, 2006). This allows methionine synthase to serve as a redox sensor, lowering its activity whenever the level of oxidation increases, until increased GSH synthesis brings the system back into balance. Electrophilic compounds, such as oxygen-containing xenobiotic metabolites, also react with cobalamin, inactivating the enzyme and increasing diversion of HCY toward GSH synthesis (Watson et al., 2004). Thus, methionine synthase is a sensor of both redox and xenobiotic status. 

The polyketide synthesis chemically and biochemically resembles that of fatty acids. The reaction of fatty acid synthesis is inhibited by the fungal product ceru-lenin [9]. It inhibits all known types of fatty acid synthases, both multifunctional enzyme complex and unassociated enzyme from different sources like that of some bacteria, yeast, plants, and mammalians [10]. Cerulenin also blocks synthesis of polyketides in a wide variety of organisms, including actinomycetes, fungi, and plants [11, 12]. The inhibition of fatty acid synthesis by cerulenin is based on binding to the cysteine residue in the condensation reaction domain [13]. Synthesis of both polyketide and fatty acids is initiated by a Claisen condensation reaction between a starter carboxylic acid and a dicarboxylic acid such as malonic or methylmalonic acid. An example of this type of synthesis is shown in Fig. 1. An acetate and malonate as enzyme-linked thioesters are used as starter and extender, respectively. The starter unit is linked through a thioester linkage to the cysteine residue in the active site of the enzymatic unit, p-ketoacyl ACP synthase (KS), which catalyzes the condensation reaction. On the other hand, the extender... 

Cysteine inhibits cystathionine 3-synthase and, therefore, regulates its own production to adjust for the dietary supply of cysteine. Because cysteine derives its sulfur from the essential amino acid methionine, cysteine becomes essential if the supply of methionine is inadequate for cysteine synthesis. Conversely, an adequate dietary source of cysteine spares methionine that is, it decreases the amount that must be degraded to produce cysteine. 

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