Constraints on the evolution of biochemical pathways

The considerable constraints that can be imposed on the evolution of primary metabolic pathways are apparent in the carbon pathway of photosynthesis in plants. Here, the unfortunate dual nature of the key enzyme, rubisco, to act as both a carboxylase and an oxygenase has led to a variety of adaptations in higher plants to maintain the efficiency of photosynthesis as a response to climatic stress.
Constraints on secondary metabolism are less apparent, although evolution can proceed by both loss of enzymic ability (as in the shikimate pathway) and by gain (as in the mevalonate pathway). One evolutionary trend in angiosperms is towards more highly toxic allelochemicals to provide protection from herbivory or microbial attack. One constraint on alkaloid evolution is the concomitant need to develop a metabolic pathway to recycle the nitrogen for other purposes. In the terpenoid series of compounds, there is a correlation between increasing oxygenation of a carbon skeleton and increasing toxicity. However, increasing oxygenation, especially peroxide formation, may produce a compound too unstable to act effectively against predators. In the evolution of blue flower colour, the synthesis of delphinidin 3-glucoside is only the first step in the production of a stable pigment complex capable of providing intense in vivo colouration. In the coevolution of plants and animals, some insects (e.g. the Monarch butterfly) have developed the ability to sequester and store plant toxins to avoid the cost of synthesis but here again there are clear constraints on the processes involved.