Plant myrosinase

Ingested glucosinolates Breakdown by plant myrosinase 0 Breakdown by bacterial myrosinase 0 Faeces... 

Western blots have shown that the antibody raised to aphid myrosinase (Wye Q) was highly specific to a single band in crude extracts of B. brassicae by SDS PAGE analysis. Wye Q did not cross react with proteins (also using Western blotting techniques) from Sinapis alba and did not show a reaction to proteins from other Brassica pests tested (data not shown). Anti-plant myrosinase antibodies did not cross-react with B. brassicae proteins, and anti-aphid myrosinase does not cross react with plant myrosinase. The results of the Western blots are summarized in Table 6.1. 

Table 6.1 Summary of the results of Western blots with anti-plant-myrosinase antibodies and the anti-aphid myrosinase antibody. Wye Q was raised against aphid myrosinase. Wye E, D, and DCJ were all raised against plant myrosinases. + indicates a positive reaction, - indicates a negative reaction. indicates that this combination was not tested.

We have previously developed a protein homology model based on the sequence of aphid myrosinase and templates from plant myrosinase and cyanogenic P-glucosidase.12 These enzymes all adopt the (p/a)8-barrel structure typical of this... 

The residues acting as proton donor and nucleophile in the hydrolysis of glucosinolates by aphid myrosinase have been identified as Glu 167 and Glu 374, respectively (Fig. 6.2). The equivalent residues in plant myrosinase are Gin 187 and Glu 409 (Fig. 6.2). A generalized mechanism for the aphid myrosinase is shown in Fig. 6.3 and for the plant myrosinase in Fig. 6.4. 

Recognition of the sulphate group of glucosinolates is probably mediated by residues Arg 194 and Arg 259 within a positive pocket in plant myrosinase. In aphid myrosinase, Lys 173 and Val 228 are similarly positioned, and it is possible that Lys 173, but not Val 228, may play a similar role (Fig. 6.2). In addition, the basic residue Arg 312 is located in the active site and may contribute to recognition of the sulphate group. Amino acid residues that play a part within the active site of plant myrosinase are shown in Fig. 6.5, while potential residues involved in aphid myrosinase are shown in Fig. 6.6. 

Figure 6.4 A generalized mechanism for plant myrosinase hydrolysis of glucosinolates based on Burmeister et al (transition states are not shown).16...

Figure 6.5 Amino acid residues associated with the active site of plant myrosinase.

JONES, A.M.E., BRIDGES, M., BONES, A.M., COLE R., ROSSITER, J.T., Purification and characterisation of a non-plant myrosinase from the cabbage aphid Brevicoryne brassicae (L.), Insect Biochem. andMolec. Biol., 2001,31, 1-5. 

Andreasson, E. and Jorgenson, L.B. (2003) Localization of plant myrosinases and glucosinolates, in Recent Advances in Phytochemistry Integrative Phytochemistry From Ethnobotany to Molecular Ecology (ed. J.T. Romeo). Pergamon, Oxford, pp. 79-99. 

GLSs are broken down enzymatically either by plant myrosinases in the small intestine or, if the plant myrosinases have been denatured by cooking as would frequently be the case, by bacterial myrosinases... 

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