PDHc inhibitor

Although biorational design has enjoyed limited success to date, this approach is advancing fast and promising in terms of finding potential to create effective crop protection compounds with novel modes of action. An excellent example was designing PDHc inhibitors as herbicides [2]. Details can be found in the following sections. 

We tried to find a plant PDHc inhibitor by biorational design. 

There are a number of reports on the inhibition of PDHc, especially on its first enzyme, PDHc-El. Different chemical structures have been designed and synthesized in several laboratories for PDHc inhibitors research. Although PDHc-El can be extracted from different biological origins such as yeast, bacteria, plant, animal G eef heart or pig heart), and human, most of the PDHc-El inhibitor studies were carried out with PDHc El from Escherichia coli (E. coli). 

Reviewing literatures on OP PDHc inhibitors, we noticed that most OP compounds acted as inhibitors of E. coli PDHc El. Those OP inhibitors against E. coli PDHc El could be chemically classified into three types (1) The analogs of substrate... 

This work on the inhibition against E. coli PDHc by sodium O-methyl acetylphosphonate 1-1 exemplified a more modem approach to the design of specific enzyme (PDHc El) inhibitors. Kluger et al. had established a new standard to promote the design and smdy on PDHc inhibitors using a biochemical theory. 

Since binding interaction between an enzyme and a substrate is more favorable (>10 ) at the transition state than at the ground state, a transition state analog was expected to be a much better inhibitor than a substrate analog. Therefore, design of analogs of transition-state in the enzyme catalytic reaction was another popular field of research on PDHc inhibitors [54]. The normal enzyme-catalyzed pyruvate metabolic process is described in Scheme 1.13 [42]. 

In order to get effective PDHc inhibitors, one approach was to replace those of transition-state in the reaction of TPP and pyruvate by a corresponding analog that resembles the key transition state. Unfortunately, the reported analog was incapable of catalysis [27]. keeper et al. [50] followed up this idea and made compounds 12 and 13 (Scheme 1.14) as analogs of transition-state hydroxyethyl-TPP. It was observed that 12 and 13 did bind very tightly to E. coli PDHc El. Their inhibitory activity was similar to deaza TPP 9 in terms of rate and affinity for binding with the active site of PDHc. 

Another approach to get a PDHc inhibitor was to find a compound that could react with the transition-state of TPP to form an intermediate which could combine with a protein side chain and stop any further downstream reactions. 

A series of sodium salts of acetylphosphonic acid as potent inhibitors of E. coli PDHc was reported by Kluger and Pike in 1977 [40]. Ten years later, on the basis of work of Kluger and Pike, further research on PDHc inhibitor was conducted by Baillie et al. in 1988 [2]. Plant PDHc El (EC 1.2.4.1) was used as the target to design a herbicide based on the understanding of key metabolic processes. In these metabolic processes, oxidative decarboxylation of pyruvate into acetyl coenzyme was catalyzed by PDHc El. More than 50 alkyl acylphosphonates, acylphosphinates, and their salts as mechanism-based inhibitors of PDHc were synthesized. Then-inhibition against pea PDHc was evaluated. Some of them were found to be very powerful inhibitors (Table 1.5) [2]. 

The above results suggested that substituents R and R" on the skeleton of the monosodium of acetylphosphinic acid or acetylphosphonic acid 1 would gready affect the enzyme inhibitory ability. It is worth noting that when Me as R" stayed the same, a smaller, weaker electronegativity R on the phosphorus, there are noticeable increasing inhibition activity. Baillie et al. s work showed that both 1-2 and 1-3, which resemble pyruvate in strucmre, were good PDHc inhibitors. Compared with 1-2 (R = R" = Me), 1-3 (R = H, R" = Me) is a much better time-dependent inhibitor [2]. 

Several OP compounds have been demonstrated to be one of the most potent PDHc inhibitors. Arming with this newly found favorable selectivity in acely-Iphosphonate 1-1 we believe that further studies in this biorational design approach for finding powerful and selective inhibitors is possible. 

It has been demonstrated that susceptible plants died as a direct result of inhibition on plant PDHc El [2, 65], which was concerned only with the first step of a series of the reaction from pyruvate to acetyl CoA [2, 66], So far several PDHc inhibitors have showed promising activities in the greenhouse, but none of them was successfiiUy developed as a commercial herbicide. As PDHc is an exciting new target for weeds control, further optimization studies on the PDHc inhibitors are needed. This situation provides us with an opportunity to explore plant PDHc as a novel target of commercial herbicide. 

OP compounds have been reported to be the most effective inhibitors against plant PDHc El among oAict PDHc inhibitors. Optimal OP compounds therefore could be an effective, environmental friendly, and selective herbicides targeting on a novel mode of action site (plant PDHc El), which is different from the target of commercial herbicide. 

Compound, (9,(9-dimethyl l-(2,4-dichlorophenoxyacetoxy)-l-(fur-2-yl) methyl-phosphonate IG-21 (HWS) is another plant PDHc inhibitor. Its low toxicity to rat, bees, birds, fish, and silkworm warranted further evaluation as well. IG-21 was found to be an effective compound against broadleaf weeds at 50-300 g ai/ha rate and it was safe to maize and rice even at the rate as high as 0.9-1.2 kg ai/ha in the greenhouse. In maize field trials at different regions in China showed that IG-21 could control a broad spectrum of broadleaf and sedge weeds at 135-270 g ai/ha for post-emergence applications. IG-21 displayed good potential as a selective and environment-friendly herbicide. Detail discussion for IC-22 and IG-21 can be found in Chap. 8. 

Dialkyl (-(substituted phenoxyacetoxy)alkylphosphonates lA-IF including 103 compounds as potential PDHc inhibitors were conveniently synthesized by the condensation of 1-hydroxyalkylphosphonates M2 and substituted phenoxyacetyl chlorides M5 in the presence of base under mild reaction conditions. 

IC-22 showed much higher herbicidal activity than that of those repoted plant PDHc inhibitors, acylphosphinates and acylphosphonates[l, 27]. Those compounds exhibited 80-100 % inhibition against weeds at 2.8 kg/ha but at this rate they had shown unacceptable phytotoxicity to the crops. Compared with acylphosphinates and acylphosphonates, IC-22 exhibited promising herbicidal activity and selectivity for development as a selective post-emergence herbicide which may be used for broadleaf weed control in monocot crop fields. 

A detailed study of acylphosphinates and acylphosphonates showed that they were mechanism-based inhibitors of pyruvate dehydrogenase complex (PDHc) as analogues of pyruvate. However, these phosphinates and phosphonates were not active enough to be considered as herbicides [1-3]. As stated in Chap. 2, some 1-substituted alkylphosphonates IC and IG showed notable herbicidal activity. Furthermore, the substitution of R R, R, R" and Y in phosphonate lo could be directly relevant to their herbicidal activity. Among the 1-substituted alkylphosphonates lA-IC, IC-22 (clacyfos) was found to be most eflFective against broadleaf weeds as a competitive inhibitor of PDHc (Scheme 3.1) [4]. This result prompted us to study continually on the design of novel PDHc inhibitors as potential herbicides. 

IIB-1 (R =Na, R =H), IIB-2 (R =Na, R =Me), IIB-3 (R =Na, R =Et), IIB-4 (R =Na, R =n-Pr) and IIB-20 (R =Na, R =fur-2-yl) with 2,4-Cl2 on the phenoxy-benzene displayed 50-75 % pre-emergence herbicidal activity against tested dicotyledons. However their corresponding (9,(9-dimethyl phosphonates had no pre-emergence activity at 450 g ai/ha. These IIB had >90 % post-emergence herbicidal activity comparable to their corresponding (9,(9-dimethyl phosphonates at the same dose. Some sodium salts of (9-methyl l-(2,4-dichlorophenoxyacet-oxy)alkylphosphonic acids were further found to be potent PDHc inhibitors (see Chap. 7). 

In order to recognize the type of mechanism of the PDHc inhibitor, IC-22 was chosen to smdy its inhibition against plant PDHc by kinetic experiment. The maximum velocity (Pmax) and Michaelis constant (Km) were determined by measuring the enzyme catalytic effect at different concentrations of substrate (sodium pymvate) in the presence or absence of the IC-22 (Scheme 7.3). 

Clacyfos exhibited an obvious advantage over those acylphosphinates or acy-Iphosphonates in enzyme-selective inhibition, crop safely, and effectiveness. It seems to be the first compound which shows practical herbicidal activity as a plant PDHc inhibitor. These results proved the rationality and effectiveness of our study on the biorational design of plant PDHc El inhibitor. 

Pyruvate dehydrogenase complex (PDHc) is one of the most important oxido-reductases in living organisms. It catalyzes the oxidative decarboxylation of pyruvate to form acetyl CoA, which is a pivotal process in cellular metabolism. PDHc has been reported as a potential target enzyme affected by some herbicidally active compounds. Regrettably, the PDHc inhibitors reported so far were not as active as other commercial herbicides. Therefore PDHc as a potential herbicidal target needs further investigation. 

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