Multi-site inhibitors

Forty-six different specific fungicide and bactericide modes of action are actually classified in the FRAC lists [3], including the unknown modes of action (Table 12.1). In addition, numerous multi-site inhibitors and plant defense inducers are available for the control of plant diseases worldwide. Therefore, a sufficient diversity of modes of action seems to be available for the control of plant diseases and for an effective resistance management. However, many fungicides are only available in a restricted number of regions and crops because they may not be registered everywhere or because the market size may not be big enough. 

Specific modes of action [unknown (U) included except host plant defense inducers (P) and multi-site inhibitors (M)]. 

Heavy atom-labelled inhibitors have the advantage that the specificity of the active site is exploited to generate a single site derivative. However, such reagents are likely to perturb the region of the enzyme of most interest. They have been useful in providing an approximate phase set that helps interpret a multi-site derivative and have then been discarded when the other derivatives are refined. 5-Iodouridine 2, 3 -phosphate was used in this way with ribonuclease-S [81]. 

Figure 2. Postulated denaturation pathway for a complex between two enzyme molecules (E, E ) and one multi headed inhibitor molecule (I). This hypothetical inhibitor consists of two structural domains, each with one reactive site (binding site for enzyme).

Zahniey, J. C. (1979). Tn7ependent heat stabilization of proteases associated with multi headed Inhibitors. Trypsins bound at nonequivalent sites on chicken ovoinhibitor. J. Biol. Chem. 254, 9721-9727. 

A plot of VQ against [S] for an allosteric enzyme gives a sigmoidal-shaped curve. Allosteric enzymes often have more than one active site which co-operatively bind substrate molecules, such that the binding of substrate at one active site induces a conformational change in the enzyme that alters the affinity of the other active sites for substrate. Allosteric enzymes are often multi-subunit proteins, with an active site on each subunit. In addition, allosteric enzymes may be controlled by effector molecules (activators or inhibitors) that bind to a site other than the active site and alter the rate of enzyme activity. Aspartate transcarbamoylase is an allosteric enzyme that catalyzes the committed step in pyrimidine biosynthesis. This enzyme consists of six catalytic subunits each with an active site and six regulatory subunits to which the allosteric effectors cytosine triphosphate (CTP) and ATP bind. Aspartate transcarbamoylase is feedback-inhibited by the end-product of the pathway, CTP, which acts as an allosteric inhibitor. In contrast, ATP an intermediate earlier in the pathway, acts as an allosteric activator. 

Target-site resistance of biotypes to ACCase inhibitors has up to now been confirmed for quite a few grass weed species of economic importance. The earliest cases of target-site based resistance were reported for biotypes of LoUum multi-Jlorum from Oregon, USA [30] and of LoUum rigidum from Australia [31]. 

Another Raf kinase inhibitor, sunitinib (Sutent), was approved in 2006 based on a multi-center, international randomized trial enrolling 750 patients with treatment-naive metastatic renal cell carcinoma (Motzer et al. 2007). In that study, patients were randomized to receive either Sutent or interferon-a (IFN-a). Common metastatic sites included lung, lymph nodes, bone, and liver. There were 96 events (25.6%) of progression/death on Sutent compared with 154 events (41.1%) on IFN-a. Median progression-free survival was 47.3 weeks for Sutent-treated patients and 22.0 weeks for patients treated with IFN. Objective response rate on the Sutent arm was 27.5 vs 5.3% on IFN-a arm. 

Target site studies can identify new processes for inhibition, ideally unique to the pest, and sometimes give information about the molecular geometry of active sites. However, investigations of fundamentally new targets is still limited. The tendency is to re-fkie understanding of well known inhibitors, or to look at untapped enzymes in proven multi-step pathways such as amino acid or sterol biosynthesis. 

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