Transduction (biophysics)
In biophysics, transduction is the conveyance of energy from one electron (a donor) to another (a receptor), at the same time that the class of energy changes.
Photosynthesis
Photonic energy, the kinetic energy of a photon, may follow the following paths:
- be released again as a photon of less energy;
- be transferred to a recipient with no change in class;
- be dissipated as heat; or
- be transduced
In photosynthesis, when the electrons of the "chlorophyll pair" receive the photon energy from the "collecting" associated pigments, the photonic energy is "destined" to link one molecule of phosphate to one of NAD. The resulting NADP in turn will use the stored energy in the generation of ATP, which is the end point of the light-induced photosynthetic process. This means that the photon's energy ends up its circuit by being transduced to an electron that takes part in the formation of a molecular link of energy-rich phosphate.
In the pathway of this end-point transduction, the energy is transferred along a number of molecules (cytochromes), in a downward way so that energy is partially dissipated at each step. The liberated heat energy serves the homeostasis of the plant, and at the end of the chain the remaining energy is perhaps exactly the one that is needed to build NADP.
This process is committed; i.e. there is no return path. Homeostasis, theoretically, might save the day only at the beginning: before the luminic energy transferred to the "chlorophyl pair" is conveyed to the first element of the cytochrome chain, there is a gap in the process when the energy is carried as a series of excitons. These are now called resonant-energy-transferring molecules of the chlorophyll class, which transfer what is considered electromagnetic energy, from one to its neighbor with no participation of electrons nor enzymes. At this stage, if the first pigment has received an excess of light, the "exciton" perhaps might dissipate the energy as heat.
Phototransduction
Visual phototransduction is a process by which light is converted into electrical signals in photoreceptors such as the rod cells, cone cells and photosensitive ganglion cells of the retina of the eye. The rods and cones contain a chromophore (11-cis-retinal, the aldehyde of Vitamin A1 and light-absorbing portion) bound to a cell membrane protein, opsin. Rods deal with low light level. Cones can code the colour of an image through comparison of the outputs of the three different types of cones. Each cone type responds best to certain wavelengths, or colours, of light because each type has a slightly different opsin. The three types of cones are L-cones, M-cones and S-cones that respond optimally to long wavelengths (reddish colour), medium wavelengths (greenish colour), and short wavelengths (bluish colour) respectively.
There is an ongoing outward potassium current through nongated K+-selective channels. This outward current tends to hyperpolarise the photoreceptor at around -70 mV (the equilibrium potential for K+). There is also an inward sodium current carried by cGMP-gated sodium channels. This so-called 'dark current' depolarises the cell to around -40 mV. Note that this is significantly more depolarised than most other neurons. A high density of Na+-K+ pumps enables the photoreceptor to maintain a steady intracellular concentration of Na+ and K+.
Photoreceptors are depolarized in the dark. Light hyperpolarizes and switches these cells off. Once switched off the next cell is activated and sends an excitatory signal down the neural pathway. In the dark, cGMP levels are high and keep cGMP-gated sodium channels open allowing a steady inward current, called the dark current. This dark current keeps the cell depolarised at about -40 mV.
The depolarisation of the cell membrane opens voltage-gated calcium channels. An increased intracellular concentration of Ca+ causes vesicles containing special chemicals, called neurotransmitters, to merge with the cell membrane, therefore releasing the neurotransmitter into the synaptic cleft, an area between the end of one cell and the beginning of another neuron. The neurotransmitter released is glutamate, an excitatory neurotransmitter.
In the cone pathway glutamate:
- Hyperpolarizes on-center bipolar cells. Glutamate that is released from the photoreceptors in the dark binds to metabotropic glutamate receptors (mGluR6), which, through a G-protein coupling mechanism, causes non-specific cation channels in the cells to close, thus hyperpolarizing the bipolar cell.
- Depolarizes off-center bipolar cells. Binding of glutamate to ionotropic glutamate receptors results in an inward cation current that depolarizes the bipolar cell.
In the light
- A light photon interacts with the retinal in a photoreceptor. The retinal undergoes isomerisation, changing from the 11-cis to all-trans configuration.
- Retinal no longer fits into the opsin binding site.
- Opsin therefore undergoes a conformational change to metarhodopsin II.
- Metarhodopsin II is unstable and splits, yielding opsin and all-trans retinal.
- The opsin activates the regulatory protein transducin. This causes transducin to dissociate from its bound GDP, and bind GTP, then the alpha subunit of transducin dissociates from the beta and gamma subunits, with the GTP still bound to the alpha subunit.
- The alpha subunit-GTP complex activates phosphodiesterase.
- Phosphodiesterase breaks down cGMP to 5'-GMP. This lowers the concentration of cGMP and therefore the sodium channels close.
- Closure of the sodium channels causes hyperpolarisation of the cell due to the ongoing potassium current.
- Hyperpolarisation of the photoreceptor results in a decrease in the amount of the neurotransmitter glutamate that is released by the cell.
- A decrease in the amount of glutamate released by the photoreceptors causes depolarization of On center bipolar cells (rod and cone On bipolar cells) and hyperpolarization of cone Off bipolar cells.
Deactivation of the phototransduction cascade
GTPase Activating Protein (GAP) interacts with the alpha subunit of transducin, and causes it to hydrolyse its bound GTP to GDP, and thus halts the action of phosphodiesterase, stopping the transformation of cGMP to GMP.
Guanylate Cyclase Activating Protein (GCAP) is a calcium binding protein, and as the calcium levels in the cell have decreased, GCAP dissociates from its bound calcium ions, and interacts with Guanylate Cyclase, activating it. Guanylate Cyclase then proceeds to transform GTP to cGMP, replenishing the cell's cGMP levels and thus reopening the sodium channels that were closed during phototransduction.
Finally, Metarhodopsin II is deactivated. Recoverin, another calcium binding protein, is normally bound to Rhodopsin Kinase when calcium is present. When the calcium levels fall during phototransduction, the calcium dissociates from recoverin, and rhodopsin kinase is released, when it proceeds to phosphorylate metarhodopsin II, which decreases its affinity for transducin. Finally, arrestin, another protein, binds the phosphorylated metarhodopsin II, completely deactivating it. Thus, finally, phototransduction is deactivated, and the dark current and glutamate release is restored. It is this pathway, where Metarhodopsin II is phosphorylated and bound to arrestin and thus deactivated, which is thought to be responsible for the S2 component of dark adaptation. The S2 component represents a linear section of the dark adaptation function present at the beginning of dark adaptation for all bleaching intensities.
All-trans retinal is transported to the pigment epithelial cells to be reduced to all-trans retinol, the precursor to 11-cis retinal. This is then transported back to the rods. All-trans retinol cannot be synthesised by humans and must be supplied by vitamin A in the diet. Deficiency of all-trans retinol can lead to night blindness. This is part of the bleach and recycle process of retinoids in the photoreceptors and retinal pigment epithelium.
Signal transduction
In biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another, most often involving ordered sequences of biochemical reactions inside the cell, that are carried out by enzymes and linked through second messengers resulting in what is thought of as a "second messenger pathway".
Many, if not all, of the signal conversions may convey or use energy from one electron (a donor) to another (a receptor), while the form of energy remains unchanged or usually does not change. Energy consumption occurs. But the pathway may not always be committed or leading.
A signal transduction is usually rapid, lasting on the order of milliseconds in the case of ion flux, to minutes for the activation of protein and lipid mediated kinase cascades. In many signal transduction processes, the number of proteins and other molecules participating in these events increases as the process eminates from the initial stimulus, resulting in a "signal cascade". Often a relatively small stimulus elicits a large response.
Signal transductions can occur in many directions. Some occur along the cytoplasmic surface of the cell membrane, like the epinephrin pathway.
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Others occur inwardly, such as the MAPK/ERK pathway, ultimately arriving inside the cell nucleus.
If a possible set of proteins for the MAPK/ERK pathway of signal transduction were linked together physically without the proteins being unfolded, their connected length as suggested in the table below would only be ~64 nm from the cytoplasmic side of the cell membrane to the cytoplasmic side of the nuclear envelope. For a typical cell of diameter 10 µm, with a nucleus of about 5 µm in diameter, about 2400 nm would remain for the signal transduction to occur across (the 6 gaps). The first 5 gaps involve activation and phosphorylation. The series of kinases from RAF to MKNK1 (MNK) is a protein kinase cascade lasting on the order of minutes.
| Protein | Mass (Da) | Likely diameter (nm) | Connected length (nm) |
|---|---|---|---|
| GRB2 | 25k | ~6 | ~6 |
| SOS1 | 152k | ~13 | ~19 |
| RASD1 | 32k | ~8 | ~27 |
| RAF1 | 73k | ~10 | ~37 |
| MAP3K1 | 164k | ~13 | ~50 |
| MKNK1 | 30k | ~8 | ~58 |
| CREB1 | 25k | ~6 | ~64 |
Physiological transduction
In physiology, transduction is the conversion of a stimulus from one form to another.
Transduction in the nervous system typically refers to synaptic events wherein an electrical signal, known as an action potential, is converted into a chemical one via the release of neurotransmitters. Conversely, in sensory transduction a chemical or physical stimulus is transduced by sensory receptors into an electrical signal. Each of these probably involves a loss of energy.
Genetic transduction
Genetic transduction is the process by which bacterial DNA is moved from one bacterium to another usually by a virus.
While the form of energy probably does not change, energy is lost and conveyed.
Acknowledgements
The content on this page was first contributed by: Henry A. Hoff.
Initial content for this page in some instances came from Wikipedia.