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To what extent has genetic manipulation of the Calvin cycle forced the reappraisal of our understanding of the control of metabolic pathways in plants. What do studies of transgenic plants reveal about the integration of metabolism?

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Introduction

To what extent has genetic manipulation of the Calvin cycle forced the reappraisal of our understanding of the control of metabolic pathways in plants. What do studies of transgenic plants reveal about the integration of metabolism? Within the past decade advances in genetics and molecular biology has facilitated brand new ways of looking at metabolic processes. Far from the traditional reductionalist approaches of the previous years, we are able undertake a more holistic approach towards understanding metabolic pathways and networks. The most important advance has been a move away from inferred models based on in vitro characteristics of enzymes to real-time studies in vivo of enzymes at work. One of the best understood metabolic networks (and also among the best funded in plants) is the primary pathway for Carbon fixation, the Calvin cycle. In this essay I shall describe how the regulation of metabolic pathways was originally approached, how the use of genetics has changed this approach and describe in detail some experiments on enzymes in the Calvin cycle and how the results from these has caused a reappraisal of our understanding of how metabolism is controlled in plants. Traditional Methods Metabolic pathways consist of a series of chemical modifications to a compound which results in substrates being turned into products. At each step of the way enzymes are used in order to allow the reaction to occur at physiological temperatures and at a speed conducive to homeostasis. The regulation of the flux of a pathway has been one of the key questions in understanding metabolism; is flux regulated by a series of steps that act as a bottleneck to the system or by co-limitation by several enzymes. Generally speaking enzyme activity can be modified via two different mechanisms. For short-term, "fine" changes, due to a change in the environment, enzymes can be modified by altering the existing enzymes kinetics, changing levels of substrate, inhibitors or activators and by post-translational modification (e.g. ...read more.

Middle

Photosynthetic carbon metabolism in higher plants is thought to be one determining factor in plant growth and yield. The cycle combines a five carbon compound (ribulose-1,5-bisphosphate) with a molecule of CO2 and through a process of reduction creates two triose phosphates which can be siphoned off into other metabolic pathways or used to regenerate the ribulose 1,5-bisphosphate. Overall there is a net gain in carbon, hence the term carbon fixation cycle. This is a diagram of the Calvin cycle showing the pathway of carbon as it is fixated. Of the 13 reactions involved in the cycle 11 of them are catalysed by enzymes. Irreversible reactions are catalysed by the following 4 enzymes. 1. ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) 2. sedoheptulose-1,7-bisphosphatase (SBPase) 3. fructose-1,6-bisphosphatase (FBPase) 4. ribulose-5-phosphate kinase (PRK) Although there are many flaws with the traditional approach these enzymes had already been well characterised and their encoding sequences were well known, making them perfect candidates for investigating the contribution they make to flux in the Calvin cycle. One question to be asked is why regulate the Calvin cycle at all? The surrounding environment is not static and the system must be able to make changes in different light intensities and CO2 concentrations. Since the Calvin cycle relies upon the products of the light dependant reactions in photosynthesis ATP/NADPH, it must be able to switch off during the dark otherwise it uses carbohydrates in order to create the ATP/NADPH which in turn is used to make new carbohydrates. Such changes in the relative concentrations of intermediates will effect other pathways which share them, such as glycolysis and oxidative PPP. I shall now outline some experiments performed on these enzymes and in turn describe what effects the results have had on the traditional ideas regarding regulation of the Calvin cycle. Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) Rubisco is the most abundant enzyme on the planet. It is responsible for the fixation of CO2 in the Calvin cycle and was generally thought to regulate (limit) ...read more.

Conclusion

Here we see the flux of one pathway directly affecting the flux of another, showing a true network of metabolism. The simplest explanation for this observation is that erythose-4-phosphate has been decreased which limits the flux into the shikimate pathway. Discussion Throughout this essay I have broached different observations that have required plant physiologists to reassess their initial preconceptions regarding the Calvin cycle. Here is a brief summary of the point observed. 1. The enzymes involved in metabolic pathways are substitute to much regulation by many different factors, but this does not necessarily mean they have a large "regulatory capacity". 2. The overall control of flux in normal conditions is controlled by a number of enzymes each playing a small part. Control is not solely reserved for enzymes that catalyse irreversible reactions but evidence is mounting to show that reversible enzymes are involved as well. 3. The contribution of a given enzyme to the control of flux is dependent on both the conditions flux is measured in and the conditions in which the plant has accumulated to. 4. In general adjustments in "fine" regulation are usually enough to compensate for changes in gene expression. However redundancy mechanisms are in place in case fine regulation is unable to cope. 5. Contrary to traditional thinking "non regulated" enzymes are not present in excess, usually only 2 to 3 fold over the minimum required to avoid severe flux limitations. 6. "Regulated" enzymes are often present in excess, yet only reach a small percentage of their maximum activity (how did they get so good in the first place if not selected for?). This would provide a redundancy zone to allow decreased gene expression to be accounted for. The use of transgenic plants provides us with a powerful tool but does not provide us with all the answers. There are many limitations in using transgenic plants which I do not have time to talk about here, but sufficed to say that we are not all the way home when it comes to understanding metabolic systems. ...read more.

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