It is clear that engineering of an improved RuBisCO, with either a faster overall rate of activity or a reduced relative oxygenation:carboxylation ratio, would have the potential for a large effect on the growth of plants expressing it. Many researchers have investigated different avenues for re-engineering RuBisCO, with limited success. Due to the difficulty in developing viable, re-engineered versions of RuBisCO with improved efficiency, some have suggested that in fact the enzyme may already be at, or close to, its evolutionary optimum, in spite of the enzyme's obvious shortcomings.
Limitations of protein modification
Attempts to modify RuBisCO highlight an important feature of protein modification, namely that it is often very difficult. There are several steps involved in producing an organism expressing a protein modified to improve a certain trait, all of which present potential problems:
- Identifying how to improve the protein - This means finding specific sites within the protein relevant to the function being modified and then altering them in the right way. It is often very difficult to know how to change a protein e.g. which amino acids to change and to what, in order to produce the desired effect.
- Ensuring normal function is not disrupted - Because different parts of the protein often interact with each other due to folding effects, change at one site can have long-ranging effects. It is important to ensure that changing a protein does not interfere with other important parts of its function, such as its interaction with other proteins or localisation to a specific part of the cell.
- Expressing the protein - Actually producing the protein in the organism of interest produces yet another roadblock. The genome of an organism has to be altered specifically and accurately, with appropriate expression levels. Often expressing a modified protein in an organism has unexpected effects on its overall phenotype which are hard to explain. For example, expression of an 'improved' RuBisCO in a tobacco line produced mutants with improved CO2 fixation rates but a slower growth rate.