GPI anchors are cell membrane glycoproteins

Glycosylphosphatidylinositol (GPI)-anchored proteins are long and firmly embed within the membrane and leave an extension out over the surface of the membrane. One end of the protein stays embedded firmly within the cell membrane and the other end can attach to a variety of important molecules such as enzymes and antigens. The enzyme or antigen is held above the cell membrane in a position that makes it available to be activated on the cell surface.

The phosphatidylinositol end is lipid (oil or fat) based and dissolves well in the fatty acid rich environment found within the membrane. The glyco- or sugar part of the molecule is able to dissolve in water or form bonds with other proteins or carbohydrates and is found on the end of the molecule that sticks out over the surface of the membrane.

GPI anchor proteins are essential for life. Mice that were experimentally made to lack the gene thought to encode for GPI anchor proteins did not survive. Experimental “knockout” mice are usually observed to see what types of function the knocked out gene might have performed. The experiment showed that GPI anchors were necessary for basic survival of baby mice. (Ref. 1, Brooks, Dwek, Schumacher, 2002, p 225) When a protein is found to be so essential that a “knockout” mouse doesn’t survive than more minor differences are attempted to be made in order to try to find out what types of functions are changed or are missing from the more slightly modified “knockout” mice.

Background information: GPI anchors are found in some types of G-protein couple receptors and may have importance within the cannabinoid receptor system which has been found to play early and essential roles in implantation of the newly fertilized egg within the mother’s uterus.

/Disclosure: This information is provided for educational purposes within the guidelines of fair use. While I am a Registered Dietitian this information is not intended to provide individual health guidance. Please see a health professional for individual health care purposes./

  1. Brooks SA, Dwek MV, Schumacher U., Functional and Molecular Glycobiology, (Bios, 2002, Oxford, UK)
  2. Landry Y, Niederhoffer N, Sick E, Gies JP., Heptahelical and other G-protein-coupled receptors (GPCRs) signaling., Curr Med Chem. 2006;13(1):51-63. [ncbi.nlm.nih.gov/pubmed/16457639]
  3. Maccarrone M, Bernardi G, […], and Centonze D., Cannabinoid receptor signalling in neurodegenerative diseases: a potential role for membrane fluidity disturbance., Br J Pharmacol. 2011 August; 163(7): 1379-1390 [ncbi.nlm.nih.gov/pmc/articles/PMC3165948/]

Additional note on GPI anchors:

  1. Fujita M, Kinoshita T. “GPI-anchor remodeling: potential functions of GPI-anchors in intracellular trafficking and membrane dynamics.” Biochim Biophys Acta. 2012 Aug;1821(8):1050-8. doi: 10.1016/j.bbalip.2012.01.004. Epub 2012 Jan 11.  Abstract: [http://www.ncbi.nlm.nih.gov/pubmed/22265715] “and discuss how GPI-anchors regulate protein sorting, trafficking, and dynamics.”

/Disclosure: This information is provided for educational purposes within the guidelines of fair use. While I am a Registered Dietitian this information is not intended to provide individual health guidance. Please see a health professional for individual health care purposes./

Neuraminic acid was known first as sialic acid

Neuraminic acid, or sialic acid as it was first called, is a monosaccharide with nine carbons. It has a negative electric charge which gives compounds containing it a negative charge. This is useful for keeping molecules like red blood cells from getting too near to each other. The negative charge on the surface glycoproteins repels the red blood cells from each other or from the walls of blood vessels which also have compounds containing sialic acid.

Mature red blood cells have an active life for about seven days.  White blood cells remove older red blood cells and de-sialylation of the surface proteins is one way the older cells are identified. Cancer cells with the ability to produce excess surface sialyation may have an increased chance to metastasize and turn up somewhere else in the body. [13]

Our bodies need to be healthy and well enough nourished overall to keep the whole system working. The neuraminic acid is produced within our cells from other chemicals in a series of membranous channels called the endoplasmic reticulum and the golgi apparatus. The channels have embedded enzymes along the way somewhat like an assembly line in a factory. We can not just eat more sialic acid in our diet and have it show up on our cell surfaces – we have to be healthy enough and well enough nourished over all in order to be able to manufacture our own supply of sialic acid. All of the different enzymes within the assembly line like system of the endoplasmic reticulum and Golgi apparatus would have to be present and working which would mean trace minerals such as zinc might be essential for producing neuraminic acid/sialic acid.

Therapeutic glycoproteins are being developed and the problem of just the right amount of sialylation is one of the hurdles being studied. [2] In addition to the negative charge sialic acid tends to stabilize and stiffen the protein portion of the glyco-compound.  The proteins that line vessels were described to be somewhat like bottle-brushes; the protein being somewhat like the sturdy wire handle of the brush and with the negatively charged sialic acid acting as bristles that electrically repel other molecules of sialic acid. [1]

/This article was originally posted on 8/21/2013./ /Disclaimer: Information presented on this site is not intended as a substitute for medical care and should not be considered as a substitute for medical advice, diagnosis or treatment by your physician./

More recent research from the scientists at the University of Zurich, regarding sialic acid, found an association between the presence of autoimmune disease and reduced levels of sialic acid on the individual’s antibodies, which are important for the body’s immune cells to be able to recognize and remove infected or foreign or decaying cells: “Specific Sugar in Antibodies Structure Determines the Risk of Autoimmune Diseases,” Oct. 7, 2015, [molecularbiologynews.org]

References:

  1. S.A. Brooks, M. V. Dwek, U. Schumacher, Functional and Molecular Glycobiology, (BIOS Scientific Publishers, Ltd., 2002), Amazon.
  2. Bork K, Horstkorte R, Weidemann W., “Increasing the sialylation of therapeutic glycoproteins: the potential of the sialic acid biosynthetic pathway.” J Pharm Sci. 2009 Oct;98(10):3499-508. doi: 10.1002/jps.21684.  [ncbi.nlm.nih.gov]
  3. R. T. Almaraz, et. al., “Metabolic Flux Increases Glycoprotein Sialylation: Implications for Cell Adhesion and Cancer Metastasis.” Mol Cell Proteomics. 2012 July; 11(7): M112.017558. Published online 2012 March 28. doi:  10.1074/mcp.M112.017558 [ncbi.nlm.nih.gov]