Cyanoalanine synthase

Cysteine itself is the substrate for / -cyanoalanine synthase (EC 4.4.1.9) which catalyzes the jS-replacement reaction yielding -cyanoalanine 144... 

Although tracer studies using extracts of several plant species indicated that both L-serine and L-cysteine were substrates for the formation of j -cyano-L-alanine, the use of purified fractions (such as from Lupinus spp) revealed that cysteine was the authentic precursor serine was converted into this by the appropriate enzyme in the crude preparations. The cyanoalanine synthase from Lupinus was purified to homogeneity and its mechanism of action utilizing pyridoxal phosphate as a coenzyme was elucidatedj -Cyanoalanine hydrolyase which catalyses the hydration of 102- 103 has also been purified ... 

Toxicity Causes neurolathyrism and cystathionuria by inhibition of cystathionine y-lyase (EC 4.4.1.1). Biosynthesis metabolism From serine or cysteine and cyanide catalyzed by 3-cyanoalanine synthase (EC 4.4.1.9). 3-Cyanoalanine hydratase (EC 4.2.1.65) hydrates C. to asparagine. ... 

In conclusion there is little doubt that cyanide can be converted to asparagine. It must be assumed that an active defense mechanism has evolved to prevent the accumulation of cyanide even at very low levels. If the localization of -cyanoalanine synthase in the mitochondria is correct then the protection of the sensitive electron transport chain is enhanced. The suggestion that the cyanide pathway contributes a major source of carbon and nitrogen to the synthesis of asparagine is doubtful. 

Manning, K., Ethylene production and (3-cyanoalanine synthase activity in carnation flowers, Planta, 168, 61-66 (1986). 

Because 1 mole of cyanide is produced for each mole of ethylene generated from ACC (1-amino-cyclopropane-1-carboxylic acid) see Chapter 13), all plants must contain small amounts of cyanide (Peiser et al., 1984). However, due to a concomitant increase in the amount of (3-cyanoalanine synthase, the ability to detoxify cyanide in the tissues is much higher (Manning, 1988). 

Plants convert HCN into asparagine (75) and p-cyano-alanine (76) (Fig. 16.15). When H CN is introduced in plants, the label is incorporated into the amide carbon of asparagine (Davis, 1991). This suggests that the cyanogenic compounds are being synthesized and catabolized, and the HCN produced in the intact plant is subsequently incorporated into asparagine by the action of 3-cyanoalanine synthase and (3-cyanoalanine hydrolase. (3-Cyanoalanine synthase has been shown to occur in many plants (Conn, 1981 Harbome, 1989 Nahrstedt, 1992). 

Fig. 161. Synthesis and transformation of L-j -cyanoalanine in higher plants 1 Cyanoalanine synthase (coenzyme pyridoxalphosphate) 2 j5-cyanoalanine hydrolase...

Table 1. Changes in ethylene production rate (v,), extractable )8-cyanoalanine synthase activity (V), and tissue cyanide content in ripening apple fruit. Data are from Yip and Yang [35]...

Wen J.-Q., Huang F., Liang W.-S., Liang H.-G. Increase of HCN and fi-cyanoalanine synthase activity during ageing of potato tuber slices. Plant Science, 125 147-149 (1997). 

However, the situation is apparently, different for one subgroup among the PLP-dependent lyases extensively studied in recent.years in our laboratory on comparative lines,cf. surveys When highly purified, lyases belonging to this subtype, namely, cysteine lyase, serine sulfhydrase, cystathionine E-synthase, and E-cyanoalanine synthase (enzy-mes 1 - it Table 1 2 and obviously are species-specific... 

L-Cy stein/ Cyanide -Cyanoalanine-synthase -Cyanoalanine, HS" Tris-Acetate pH 8.5 AgjS [202]... 

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