David M. Kramer
- DOE - Project A Faculty, DOE - Project B Faculty, Algae Focus, Plant Focus
Hannah Distinguished Professor in Photosynthesis and Bioenergetics
Department of Biochemistry & Molecular Biology
Energetics and control of photosynthesis electron and proton transfer reactions reactive oxygen generation the conversion of light energy by plants into forms usable for life understanding the processes involved at both molecular and physiological levels, how they are regulated and controlled how these processes define the energy budget of plants and the ecosystem how plants have evolved to support life in extreme environments
Among the tools used in Dr. Kramer’s lab are spectroscopic approaches including absorption, fluorescence, circular dichroism and electron spin resonance (EPR) applied to isolated membranes, organelles and intact plants. Students in the laboratory gain wide exposure to biophysical techniques and the important area of bioenergetics.
My laboratory seeks to understand how plants convert light energy into forms usable for life, how these processes function at both molecular and physiological levels, how they are regulated and controlled, how they define the energy budget of plants and the ecosystem and how they have adapted through evolution to support life in extreme environments. Some of the specific projects are outlined below.
The multiple roles of the thylakoid proton motive force
Energy conversion by the chloroplast involves capture of light energy and its funneling through a series of pigment-excited states, electron and proton transfer reactions, ultimately into the NADP+/NADPH and ATP/ADP + Pi couples. In this way, photosynthesis drives essentially all biochemistry in our ecosystem. Many of the intermediates in energy conversion are also quite reactive, and at sufficiently high concentrations they can destroy the photosynthetic apparatus (photoinhibition or photodamage) or even kill the plant. To prevent photoinhibition, the efficiency of the light harvesting complexes is down-regulated via the dumping of excitation energy harmlessly as heat. This, of course, lowers the efficiency of photosynthesis, but prevents photodamage.
There are strong indications that the balancing of photoprotection and photochemical efficiency is important for acclimation to environmental challenges. We are focusing on the dual role of the transthylakoid proton gradient, or proton motive force (pmf), which serves a pivotal role in this balancing act, both as a key intermediate in energy conversion, driving the synthesis of ATP, as well as the trigger for initiation of NPQ. New research on the structure and function of the ATP synthase and the cytochrome b6f complex, as well as on the nature of the proton motive force, has begun to reveal how pmf balances these two key roles.
The mechanisms of the electron transfer-coupled proton pumps
To make ATP, the chloroplast couples exergonic electron transfer reactions to translocation of protons across the thylakoid membrane. The resulting pmf is used to drive the endergonic synthesis of ATP. We have focused on the reactions of the cytochrome b6f complex and related cytochrome bc1 complexes (of bacteria and mitochondria). These enzymes oxidize hydroquinones (quinols) and reduce soluble electron carriers. The energy released in this process is used to pump protons, probably via a mechanism called a Q-cycle, which is responsible for producing about one-third of all of the ATP in plants. Under adverse conditions, the cytochrome b6f/bc1 complexes can also produce, as a byproduct, superoxide, which in turn can lead to diseases, including some associated with aging in humans. To understand (and potentially control) these processes, we are investigating the molecular structure and mechanism of these complexes using a range of biophysical and molecular techniques.
Proton-coupled electron transfer reactions in energy conversion
It has been shown that key reactions in these complexes are a special type of electron transfer reaction, called a proton-coupled electron transfer reaction (PCET). Although applicable theory exists for straightforward electron transfer (ET), PCET is much more complex. We have been investigating PCET using a synthetic photoactive mimic of the Qo site of the cytochrome bc1 and b6f complexes.
How photosynthesis works in vivo
Ultimately, we aim to understand how the specific molecular mechanisms of the photosynthetic system determine how the plant survives and grows. This approach is enabled by our development of novel spectroscopic techniques that allow us to observe specific photosynthetic reactions both in isolated systems (which are easily manipulated), and in living plants. With these tools, we are now able to monitor many of the photosynthesis reactions in living plants under natural conditions. This allows us to test whether models developed on isolated systems truly operate in vivo. In some cases, such studies have led us to new conclusions about how enzymes operate at the molecular level. Finally, the technology developed through these efforts has potential applications in plant breeding and precision farming, by giving growers the ability to rapidly assess the physiological status of plants.