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Metabolism

Photosynthesis takes place at reaction centers on the cell membrane, which is folded into the cell to form sacs, tubes, or sheets, increasing the available surface area.

Like most other photosynthetic bacteria, purple bacteria do not produce oxygen (anoxygenic), because the reducing agent (electron donor) involved in photosynthesis is not water. In some, called purple sulfur bacteria, it is either sulfide or elemental sulfur. The others, called purple non-sulfur bacteria (aka PNSB), typically use hydrogen although some may use other compounds in small amounts. At one point these were considered families, but RNA trees show the purple bacteria make up a variety of separate groups, each closer relatives of non-photosynthetic proteobacteria than one another.

The reaction centers create a charge separation through a series of favorable redox reactions, after the excitation of the special pigment pair P870. The reduction of quinones leads to the take up of 2 protons from the cytoplasm. When the quinones are eventually oxidized, they release the protons in the periplasmic side. This builds up a proton motive force that is used by ATP synthase to produce ATP from ADP and phosphate.The ATP is finally used in biosynthesis.

Metabolism

Location

Photosynthesis occurs at reaction centers on the cell membrane, which is folded to form vesicle sacs, tubules, or single-paired or stacked lamellae sheets. This is called the intracytoplasmic membrane which has increased surface area to maximize light absorption.

Mechanism

Purple bacteria use cyclic electron transport driven by a series of redox reactions. Light-harvesting complexes harvest photons in the form of resonance energy, exciting chlorophyll pigment P870 located in reaction centres. Excited electrons are cycled from P870 to quinones, then passed to cytochrome bc₁, cytochrome c₂, and back to P870. The reduction of quinones involves taking up protons from the cytoplasm. Reduced quinones are oxidized by the cytochrome bc₁, fuelling proton pumping into the periplasm. The resulting charge separation between the cytoplasm and periplasm generates a proton motive force used by ATP synthase to produce ATP energy.

Electron Donors for Anabolism

Purple bacteria also transfer electrons from external electron donors directly to cytochrome bc₁ to generate NADH or NADPH used for anabolism. They are anoxygenic because they do not use water as an electron donor to produce oxygen. One type of purple bacteria, called purple sulfur bacteria (PSB), use sulfide or sulfur as electron donors. Another type, called purple non-sulfur bacteria, typically use hydrogen as an electron donor but can also use sulfide or organic compounds at lower concentrations compared to PSB.

Purple bacteria lack external electron carriers to spontaneously reduce NAD(P)⁺ to NAD(P)H, so they must use their reduced quinones to endergonically reduce NAD(P)+. This process is driven by the proton motive force and is called reverse electron flow.

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