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6.7: Simple Phototrophs

Phototrophs are organisms that capture particles of light (photons) and transform their electromagnetic energy into energy stored in unstable molecules, such as ATP and carbohydrates. Phototrophs eat light. Light can be considered as both a wave and a particle (that is quantum physics for you) and the wavelength of a photon determines its color and the amount of energy it contains. Again, because of quantum mechanical considerations, a particular molecule can only absorb photons of specific wavelengths (energies). Because of this property, we can identify molecules at great distances based on the photons they absorb or emit, this is the basis of spectroscopy. Our atmosphere allows mainly visible light from the sun to reach the earth's surface, but most biological molecules do not absorb visible light very effectively if at all. To capture this energy, organisms have evolved the ability to synthesize molecules, known as pigments to capture, and therefore allow organisms to use (absorb) visible light. The color we see for a typical pigment is the color of the light that is it does not absorb but rather that it reflects. For example chlorophyl appears green because light in the red and blue regions of the spectrum is absorbed and green light is reflected. The question we need to answer is, how does the organism use the electromagnetic energy that is absorbed?

One of the simplest examples of a phototrophic system, that is, a system that directly captures the energy of light and transforms it into the energy stored in a chemical system, is provided by the archaea Halobacterium halobium174. Halobacteria are extreme halophiles (salt-loving) organisms. They live in waters that contain up to 5M NaCl. H. halobium uses the membrane protein bacteriorhodopsin to capture light. Bacteriorhodopsin consists of two components, a polypeptide, known generically as an opsin, and a non-polypeptide prosthetic group, the pigment retinal, a molecule derived from vitamin A175. Together the two, opsin + retinal, form the functional bacteriorhodopsin protein.

Because its electrons are located in extended molecular orbitals with energy gaps between them that are of the same order as the energy of visible light, absorbing of a photon of visible light moves an electron from a lower to a higher energy molecular orbital. Such extended molecular orbitals are associated with molecular regions that are often drawn as containing alternating single and double bonds between carbons; these are known as conjugated π orbital systems. Conjugated π systems are responsible for the absorption of light by pigments such as chlorophyll and heme (the pigment that makes blood red). When a photon of light is absorbed by the retinal group, it undergoes a reaction that leads to a change in the pigment molecule’s shape and composition, which in turn leads to a change in the structure of the polypeptide to which the retinal group is attached. This is called a photoisomerization reaction.

The bacteriorhodopsin protein is embedded within the plasma membrane, where it associates with other bacteriorhodopsin proteins to form patches of proteins. These patches of membrane protein give the organisms their purple color and are known as purple membrane. When one of these bacteriorhodopsin proteins absorbs light, the change in the associated retinal group produces a light-induced change in protein structure that results in the movement of a H+ ion from the inside to the outside of the cell. The protein (and its associate pigment) then return to its original low energy state, that is, its state before it absorbed the photon of light. Because all of the bacteriorhodopsin molecules are oriented in the same way in the membrane, as light is absorbed all of the H+ ions move in the same direction, leading to the formation of a H+ concentration gradient across the plasma membrane with [H+]outside > [H+]inside. This H+ gradient is based on two sources. First there is the gradient of H+ ions. As light is absorbed the concentration of H+outside the cell increases and the concentration of H+ inside the cell decreases. The question is – where is this H+ coming from? As you (perhaps) learned in chemistry water undergoes the reaction (although this reaction is quite unfavorable):

\[H_2O \rightleftharpoons H^+ + OH^–\]

\(H^+\) is always present in water from the autoionization (\([H^+] = 1 \times 10^{-7}\) for neutral water at room temperature) and it is these H+s that move.

In addition to the chemical gradient that forms when \(H^+\) ions are pumped out of the cell by the bacteriorhodopsin + light + water reaction, an electrical field is also established. There are excess positive charges outside of the cell (from H+ being moved there) and excess negative charges inside the cell (from –OH being left behind). As you know from your physics, positive and negative charges attract, but the membrane stops them from reuniting. The result is the accumulation of positive charges on the outer surface of the membrane and negative charges on the inner surface. This charge separation produces an electric field across the membrane. Now, an \(H^+\) ion outside of the cell will experience two distinct forces, those associated with the electric field and those arising from the concentration gradient. If there is a way across the membrane, the \([H^+]\) gradient will lead to the movement of H+ ions back into the cell. Similarly the electrical field will also drive the positively charged \(H^+\) back into the cell. The formation of the [H+] gradient basically generates a battery, a source of energy, into which we can plug in our pump.

So how does the pump tap into this battery? The answer is through a second membrane protein, an enzyme known as the \(H^+\) -driven ATP synthase. \(H^+\) ions move through the ATP synthase molecule in what is a thermodynamically favorable (\(\Delta G < 0\)) reaction. The ATP synthase couples this favorable movement to an unfavorable chemical reaction, a condensation reaction:

\[\text{ATP synthase} \longrightarrow\]

\[H^+_{outside} + ADP + \text{inorganic phosphate} (P_i) \rightleftharpoons ATP + H_2O + H^+_{inside}\]

\[ \longleftarrow \text{ATP hydrolase (ATP synthase running backward)}\]

This reaction will continue as long as light is absorbed. Bacteriorhodopsin acts to generate a H+ gradient and the H+ gradient persists. That means that even after the light goes off (that is, night time) the H+ gradient persists until H+ ions have moved through the ATP synthase. ATP synthesis continues until the \(H^+\) gradient no longer has the energy required to drive the ATP synthesis reaction. The net result is that the cell uses light to generate ATP, which is stored for later use. ATP acts as a type of chemical battery, in contrast to the electrochemical battery of the \(H^+\) gradient.

An interesting feature of the ATP synthase molecule is that as H+ ions move through it (driven by the electrochemical power of the H+ gradient), a region of molecule rotates. It rotates in one direction when it drives the synthesis of ATP and in the opposite direction to couple ATP hydrolysis to the pumping of H+ ions against their concentration gradient. In this form it is better called an ATP hydrolase:

\[ \text{ATP hydrolyse} \longrightarrow\]

\[ATP + H_2O + H^+_{inside} \rightleftharpoons H^+_{outside} + ADP + \text{inorganic phosphate} (P_i)\]

\[ \longleftarrow \text{ATP synthase (ATP hydrolase running backward)}\]

Because the enzyme rotates when it hydrolyzes ATP, it is rather easy to imagine how the energy released through this reaction could be coupled, through the use of an attached propeller or paddle-like extension, to cellular or fluid movement.

Questions to answer & to ponder

  • In a phototroph, why does the H+ gradient across the membrane dissipate when the light goes off? What happens to the rate of ATP production? When does ATP production stop and why?
  • What would limit the “size” of the H+ gradient that bacteriorhodopsin could produce?
  • What would happen if bacteriorhodopsin molecules were oriented randomly within the membrane?
  • What is photoisomerization? Is this a reversible or an irreversible reaction?
  • Indicate how ATP hydrolysis or tapping into the H+ gradient could lead to cell movement.

References

174 http://youtu.be/4OkN1QC4hyY

175 As we will return to later, proteins are functional entities, composed of polypeptides and prosthetic group. The prosthetic group is essential for normal protein function. The protein without the prosthetic group is known as the apoprotein.

Contributors

  • Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.