ATP, PFK and CAM – an energetic look at an old story
Fluctuations in the acid content of the leaves of succulent plants – crassulacean acid metabolism (CAM) – were discovered a long time ago. The identification of the acid as malic acid and, in some species, its quantification over a day–night cycle were achieved early in the 20th century and since then it has been the subject of extensive research. However, there is still much that we do not know about CAM and it thus remains an interesting topic for study. This is exemplified by the work of Chen and Nose (Fuzhou, China and Saga, Japan, pp. 449–455). They point out that, in terms of the source of carbohydrate used for malate synthesis, CAM plants fall into two general groups. One group, exemplified by Kalanchoë species, uses starch whilst the other, exemplified by Ananas comosus (pineapple), uses hexose sugar. This difference implies a difference in the expenditure of ATP per malate molecule and is also associated with differences in enzyme activities. Kalanchoë has a much higher activity of ATP-dependent phosphofructokinase than PPi-dependent phosphofructokinase; the reverse is true in A. comosus. The authors predicted that these differences should be reflected in differences in the content of ‘energy-rich compounds’. Careful measurement of the relevant compounds during a day–night cycle confirmed this view. Both Kalanchoë and Ananas accumulated ATP in the dark and exhibited a higher adenylate energy charge (AEC) at night than during the day. However, the increases in ATP content and in AEC were greater in Kalanchoë than in Ananas, consistent with the view that, in ATP terms, starch is a less expensive source of carbon skeletons than hexose. However, a question remains. If indeed the use of hexose as a source of carbon skeletons for malate synthesis is more expensive than using starch, where does any selective advantage lie in using hexose? The ramifications of CAM are far from solved.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
