Shape and volume affect biochemical reactions

If you’ve ever watched a time-lapse movie of a cell undergoing division, you’ll appreciate that a cell’s shape and volume aren’t static. Owe Orwar and colleagues at the Chalmers University of Technology (Sweden) suggest that the dynamic changes in cellular shape and volume influence the rates of biochemical reactions going on inside the cells. They demonstrate the feasibility of the idea by analyzing enzymatic reactions in biomimetic networks of microscale vesicles and nanotubes as well as by carrying out theoretical calculations.

The investigators first showed that in a solitary vesicle, reactions can be turned on/off and tuned by volume changes. The rate of volume change has to overwhelm the overall rate of product formation; the volume resulting from the expansion has to be large enough to dilute the substrate concentration well below the Michaelis–Menten constant K_M. Orwar and colleagues extended their calculations to the enzymes involved in the mitochondria’s Krebs cycle, thereby demonstrating that organelles can regulate the rates of enzymatic reactions by shrinking or expanding their volume.

The Chalmers investigators next examined whether a reaction could be initiated or boosted in a network of vesicles and nanotubes—the Golgi–endoplasmic reticulum network would be an example—as the arrangement of the network is manipulated. Orwar and colleagues studied fluorescently labeled alkaline phosphatase as the enzyme and fluorescein diphosphate as the substrate. They showed that the reaction system was extremely sensitive to changes in network topology, with the chemical reactions being initiated or boosted in certain places as a function of vesicle–nanotube connectivity. (Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 4099–4104)