Lecture 4 – the Screening Hypothesis explains why the world goes round!

At the end of this lecture you should know: Why the chemical co-evolution model to explain NP diversity is wrong! How another evolutionary model, the Screening Hypothesis, provides a much more probable evolutionary scenario to explain not only NP abundance but also the properties of NPs.  That potent and specific biological activity is a rare property for a molecule to possess.  That because potent, specific biological activity is a rare property, organisms gaining fitness through making biologically active NPs must have evolved metabolic properties that enhance the chances of making and retaining chemical diversity

The chemical co-evolution model – widely accepted for 40 years Phytoalexins

The chemical co-evolution model – widely accepted for 40 years Allelopathy

The chemical co-evolution model – widely accepted for 40 years..“Volatile metabolites of Lycopersicon species are of interest because of their roles in tomato flavour and in host defense against arthropod herbivores..” (Colby et al., 1998).

Why is the chemical co-evolution model wrong? Because it cannot explain why most NPs seem to possess, at best, very weak biological activity. Thus only a very small % of the known NPs have ever had any role ascribed to them. The fragmentation of the subject hides this fact.

Why is the chemical co-evolution model wrong? The chemical co-evolution model is based on the idea that every NP, is, or has been at some stage in evolution, biologically active. PMBCDE

Why is the chemical co-evolution model to explain NP diversity wrong? Because potent biological activity is a rare property for any one molecule Evidence Study ,000 microbial cultures were screened over 10 a year period and only three utilisable compounds were found Study 2. 21,830 isolates screened in one year to give 2 possible compounds Study 3. 10,000 microorganisms were screened but only one clinically effective agent was found This low “hit rate” is not what the chemical co-evolution model predicts. The co-evolution model predicts that organisms should contain a few highly active compounds. The reality is that they contain many, largely inactive chemicals.

Why is the chemical co-evolution model to explain NP diversity wrong? Because biological activity is a rare property for any one molecule. Evidence In this respect NPs do not differ from synthetic chemicals. Tens of thousands of chemicals are “screened” every day by pharmaceutical companies in their search for new drugs. The “hit rate” is very low. < 1 in 1000.

Why is the chemical co-evolution model to explain NP diversity wrong? Because biological activity is a rare property for any one molecule Evidence Tens of thousands of chemicals are “screened” daily for agrochemical use – insecticides, herbicides, fungicides, etc.. The “hit rate” is very low. < 1 in Nearly different synthetic organophosphorus derivatives were made in a search for insecticides but <100 commercially useful products were found.

Why is potent specific biological activity rare? At low concentrations, a reversible, non-covalent interaction between a small molecule and a protein only occurs if the small molecule fits precisely into the protein structure

Why is potent specific biological activity rare? At low concentrations, a reversible, non-covalent interaction between a small molecule and a protein only occurs if the small molecule fits precisely into the protein structure

Why is the chemical co-evolution model wrong? Because it cannot explain why most NPs seem to possess, at best, very weak biological activity and that means that …….. PMBCDE The chance of every compound in a sequence possessing potent, specific biological activity is so low that is very hard explain why the pathways have evolved … The balance between opportunities for cost savings and enhanced fitness changes as the biosynthetic chains lengthens. Once D nolonger has a lasting benefit, there are more opportunites to gain immediate finess benefits by cost reduction than there are opportunities to gain fitness by the low probability event of making biologicall active E

Why is the chemical co-evolution model wrong? Because it cannot explain why most NPs seem to possess, at best, very weak biological activity and that means that …….. PMB Worse still, the organism that is selected against by B will evolve to cope with B Two basic stragtegies seem common. 1.The organism alters the receptor protein so that B fits less well in its binding site. 2.The organism reduces the effective concentration of B by not taking B up, by breaking down B, pumping it out of the cell or sequestering B in some way Hence a chemical similar to B, like the next compound C is facing a preadapted organism capable of handling molecules sharing chemical features with B C

How to improve the odds of making a rare biologically active product Old paradigm – one enzyme, one productNew paradigm – one enzyme, many products

How low enzyme substrate specificity could help improve fitness One mutation could generate many new products

The Screening Hypothesis – key points Based on overwhelming evidence that potent, specific biological activity is very rare Can explain why most NPs are not impressively biologicall active Can explain why a very few NPs are very highly biologically active Can explain why weird types of activity occur in unexpected places e.g. why the bark of Taxus contain an anticancer drug

The Screening Hypothesis Predictions about the metabolism making NPs Many enzymes involved in NP biosynthesis will have a broad substrate tolerance Metabolic grids will be found Some enzymes might participate at more than one step of a pathways Some enzymes may produce multiple products - chemical reactions or rearrangements will be used to generate chemical diversity