Lecture Notes

1 An Introduction to Photosynthesis Welcome to Life from Light! In this first class, we will introduce ourselves and tell you a little about our backgrounds and research interests. We want to hear about your own interests and goals as well as your expectations for the course. For the remainder of the class, we will provide an overview of photosynthesis and touch upon each of the topics discussed in this course.
2 The Emergence of Oxygenic Photosynthesis Plants carry out one of the most difficult chemical reactions in biology: the oxidation of water to protons, molecular oxygen, and, most importantly, high energy electrons for subsequent use in reductive biosynthesis. Our first paper is a theoretical treatment, from a geochemical perspective, of how the sophisticated machinery doing this reaction may have arisen, what precursor chemistries may have been used, and what evolutionary steps occurred to get to oxygenic photosynthesis. The second paper addresses the same question but utilizes gene phylogenies to shed light on the ancestry of plant photosynthesis.
3 Anoxygenic Photosynthesis Anoxygenic photosynthetic bacteria can use light energy to extract electrons from molecules other than water. This process does not evolve O2. In the paper of Post and Arieli, the ability of P. hollandica to carry out anoxygenic photosynthesis under stressed conditions is examined. In the paper by Griesbeck, the enzymatic mechanism of sulfide-quinone reductase (SQR), which is essential for sulfide-dependent photosynthesis, is investigated.
4 Light Harvesting The first step in the conversion of light to chemical energy is the harvesting and transmission of light energy through extremely short-lived excited chemical states. The first paper describes the structure of the light-harvesting complex and provides insights into how accessory antennae interact with the photosynthetic reaction center as well as provides clues to its regulation. Photosynthetic organisms must have mechanisms to adjust their light-harvesting capacity in response to changing light levels; otherwise photodamage can occur. The second paper uses a genetic and biophysical approach to examine one kind of protein involved in the regulation of the light-harvesting complex, something essential for plant survival.
5 Primary Photochemistry of Photosystems II Photosystem II (PSII) is the site of water oxidation. This reaction occurs through a highly choreographed cascade of oxidation/reduction reactions beginning with a specialized chlorophyll, P680, and ending with the oxidation of a special 4 manganese cluster that subsequently oxidizes water. The oxidation of water is not a single event but requires multiple rounds of photosystem turnover, of which there are five (S0-S4) for each molecule of oxygen produced. In this session, we will discuss the various protein intermediates involved in the steps leading up to the oxidation of water as well as intermediates leading to the formation of the oxidized P680.
6 Primary Photochemistry of Photosystems I In this section we will discuss how charge separation occurs within photosystem I (PSI) and the use of the technique of electron paramagnetic resonance in detecting unpaired electrons and spin-correlated radical pairs. The rate of electron transport within PSI is mainly controlled by the flux of electrons transferred from PSII through the cytochrome b6/f complex and by the activity of dark carbon metabolism that uses NADPH formed at the acceptor side of PSI. We will discuss nonphotochemical control of energy dissipation and photochemical activity in PSI using the paper by Rajagopal et al.
7 Structure of Photosystems II and I The crystal structures of photosystems II and I represent the capstone of over 50 years of photosynthesis research and will almost certainly enable a new era of discovery and technology.
8 Electron Transfer Electron transport from water to NADP+ requires three membrane-bound protein complexes operating in series- PSII, the cytochrome b6/f complex and PSI. We will discuss how the very early events of photosynthesis are captured. We will also look at the structure of cytochrome b6/f complex- the electronic connection between PSII and PSI.
9 Production of ATP The conversion of proton electrochemical energy into chemical free energy is accomplished by a single protein complex known as ATP synthase. We are going to use two structural papers to discuss the structure, function, and regulation of ATP synthase.
10 Carbon Fixation- Ribulose 1,5-Bisphosphate Carboxylase All plants and algae remove CO2 from the environment and reduce it to carbohydrate by the Calvin cycle. The first step is the addition of CO2 to a five-carbon compound: Ribulose 1,5-bisphosphate. This key reaction is catalyzed by Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is the most abundant protein on earth. The structure, function and regulation of Rubisco will be discussed in this section.
11 C-4 vs. C-3 Photosynthesis Pathways Some plants have developed a preliminary step to the Calvin Cycle (which is also referred to as a C-3 pathway). This preamble step is known as C-4. C-4 photosynthesis involves the separation of carbon fixation and carbohydrate synthesis in space and time. In doing so, the plant manages to raise the concentration of CO2 in certain cells to prevent photorespiration. Phosphoenolpyruvate (PEP) carboxylase is the enzyme that catalyzes the carbon fixation in the C4 pathway. We will discuss the effect of environment on the adoption of the C3 and C4 pathways as well as the allosteric regulation of PEP carboxylase.
12 Project Day: TEM of Phycobilisomes Transmission electron microscopy is an important tool in many disciplines and has played a particularly important role in photosynthesis research. We will spend this class period beginning first with an introduction to the principles of TEM, an overview of the King laboratory's microscope (a JEOL 1200EXII), and conclude by having you operate the microscope and view isolated phycobilisomes negatively stained with uranyl acetate.
13 Bacterial Proteorhodopsin and "Extreme" Photosynthesis Although true photosynthesis is ultimately about energizing low energy compounds to generate reducing capacity, there are other strategies to live on light. Proteorhodopsin is a simple light-driven proton pump used to establish a proton gradient that is consumed during the generation of ATP. Beja et al. show the occurrence of this phenomenon in two unexpected places: bacteria and the ocean. In the second paper, an obligately photosynthetic organism is found deep in the ocean, well beyond where any sunlight could possibly sustain photosynthesis. The mechanism of its survival is quite impressive.
14 Nanotechnology of Photosynthesis In an age of incredible dependence on greenhouse gas emitting fuels, the notion of imitating or using biological systems to capture light energy and convert it to electrical or chemical energy is appealing. Life has had 2.8 billion years to perfect this process, so why not tap into it? In the first paper, researchers (some from MIT!) utilize PSI purified from spinach to generate electricity. In the second paper, an artificial photosynthetic system is assembled in vitro to generate a proton gradient capable of sustaining ATP synthesis.
15 Proposal Presentations