Carrier protein

Carrier proteins, (together with channel proteins) are Membrane transport proteins actively facilitating transport of molecules across cell membrane on which they are placed. Carrier proteins facilitate the diffusion of different molecules, to which they have specificity, one molecule at the time is allowed to bind to carrier protein and is released on the other side of the membrane. Carrier proteins are integral/intrinsic ^[1] membrane proteins; that is they exist within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport. These mechanisms of movement are known as carrier-mediated transport.^[2] Each carrier protein is designed to recognize only one substance or one group of very similar substances. Research has correlated defects in specific carrier proteins with specific diseases.^[3]

Active transportation

Active transport

The action of the sodium-potassium pump is an example of primary active transport. The two carrier proteins on the left are using ATP to move sodium out of the cell against the concentration gradient. The proteins on the right are using secondary active transport to move potassium into the cell.

Active transport is the movement of a substance across a membrane against its concentration gradient. This is usually to accumulate high concentrations of molecules that a cell needs, such as glucose or amino acids. When the lipid bilayer is impermeable to the molecule needing transport, active transport is also necessary. If the process uses chemical energy, such as adenosine triphosphate (ATP), it is called primary active transport. Secondary active transport involves the use of an electrochemical gradient, and does not use energy produced in the cell.^[4] Unlike channel proteins which only transport substances through membranes passively, carrier proteins can transport ions and molecules either passively through facilitated diffusion, or via secondary active transport.^[5] A carrier protein is required to move particles from areas of low concentration to areas of high concentration. These carrier proteins have receptors that bind to a specific molecule (substrate) needing transport. The molecule or ion to be transported (the substrate) must first bind at a binding site at the carrier molecule, with a certain binding affinity. Following binding, and while the binding site is facing the same way, the carrier will capture or occlude (take in and retain) the substrate within its molecular structure and cause an internal translocation so that the opening in the protein now faces the other side of the plasma membrane.^[6] The carrier protein substrate is released at that site, according to its binding affinity there.

Facilitated diffusion

Facilitated diffusion in cell membrane, showing ion channels (left) and carrier proteins (three on the right).

Facilitated diffusion is the passage of molecules or ions across a biological membrane through specific transport proteins and requires no energy input. Facilitated diffusion is used especially in the case of large polar molecules and charged ions; once such ions are dissolved in water they cannot diffuse freely across cell membranes due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the bilayers. The type of carrier proteins used in facilitated diffusion is slightly different from those used in active transport. They are still transmembrane carrier proteins, but these are gated transmembrane channels, meaning they do not internally translocate, nor require ATP to function. The substrate is taken in one side of the gated carrier, and without using ATP the substrate is released into the cell. They may be used as potential biomarkers

Examples

Each carrier protein, even within the same cell membrane, is specific to one type or family of molecules. For example, GLUT1 is a named carrier protein found in almost all animal cell membranes that transports glucose across the bilayer. Other specific carrier proteins also help the body function in important ways. Cytochromes operate in the electron transport chain as carrier proteins for electrons.^[4]

Pathology

A number of inherited diseases involve defects in carrier proteins in a particular substance or group of cells. Cysteinuria (cysteine in the urine and the bladder) is such a disease involving defective cysteine carrier proteins in the kidney cell membranes. This transport system normally removes cysteine from the fluid destined to become urine and returns this essential amino acid to the blood. When this carrier malfunctions, large quantities of cysteine remain in the urine, where it is relatively insoluble and tends to precipitate. This is one cause of urinary stones.^[7] Some vitamin carrier proteins have been shown to be overexpressed in patients with malignant disease. For example, levels of riboflavin carrier protein (RCP) have been shown to be significantly elevated in people with breast cancer.^[8]

References

1. Cooper, Geoffrey (2009). The Cell: A Molecular Approach. Washington, DC: ASM Press. p. 62. ISBN 9780878933006.
2. Thompson, Liz A. Passing the North Carolina End of Course Test for Biology. American Book Company, Inc. 2007. ISBN 1-59807-139-4. p. 97.
3. Sadava, David, Et al. Life, the Science of Biology, 9th Edition. Macmillan Publishers, 2009. ISBN 1-4292-1962-9. p. 119.
4. Ashley, Ruth. Hann, Gary. Han, Seong S. Cell Biology. New Age International Publishers. ISBN 8122413978. p. 113.
5. Taiz, Lincoln. Zeigler, Eduardo. Plant Physiology and Development. Sinauer Associates, 2015. ISBN 978-1-60535-255-8. pp. 151.
6. Kent, Michael. Advanced Biology. Oxford University Press US, 2000. ISBN 0-19-914195-9. pp. 157–158.
7. Sherwood, Lauralee. 7th Edition. Human Physiology. From Cells to Systems. Cengage Learning, 2008. p. 67
8. Rao, PN, Levine, E et al. Elevation of Serum Riboflavin Carrier Protein in Breast Cancer. Cancer Epidemiol Biomarkers Prev. Volume 8 No 11. pp. 985–990