A research team led by the University of Bonn, and also involves researchers from Argentina, Canada and the University of Düsseldorf provides new insights into an enzyme that plays a key role in C4 photosynthesis.
Around three percent of all plants have developed a trick that enables them to use even the smallest amounts of CO2 in C4 photosynthesis. In this process, they first pre-fix CO2 by linking it to a transport molecule, producing an organic compound containing four carbons - hence the name C4 photosynthesis. This is transported into the bundle sheath cells, which are specially sealed. Here, the carbon dioxide is released again and is then available for further reactions of photosynthesis. This release step is catalyzed by the NAD-malate enzyme (C4-NAD-ME).
For a long time, it was unclear exactly how C4-NAD-ME functions. The research team investigated this using an ornamental plant of the genus Cleome. The team found that NAD-ME consists of two large building blocks, the alpha and the beta subunit. While the alpha unit is responsible for CO2 release, the beta subunit serves primarily to regulate the activity of the enzyme.
This regulation is extremely important because CO2 release takes place in the mitochondria, where important metabolic processes are constantly taking place. The beta subunit apparently prevents the two enzymes from getting in each other's way, regulating the reaction rate of C4-NAD-ME. To do this, it binds an intermediate product of the C4 photosynthesis cycle called aspartate. Aspartate ensures that the "photosynthetic variant" of NAD-ME becomes particularly active. The CO2 that is pre-fixed and intended for photosynthesis is thus mainly processed by the enzyme variant that "matches" it (and works much faster).
(Source: Crop Biotech Update, International Service for Acquisition of Agri-Biotech Applications. www.isaaa.org)

The plant in which the structure of the C4-NAD malate enzyme was elucidated is familiar to some people in Germany as an ornamental plant. Photo Credit: Meike Hüdig/University of Bonn