Glycoconjugates and Membrane Carbohydrates

 

Cells recognize one another because of the saccharides attached to cell surfaces. 

They  are present usually as oligosaccharides associated through covalent links to lipids and/or proteins forming Glycoconjugates. The lipid or protein part is integrated into the cell membrane structure, with the saccharide part towards the external membrane surface.

Membrane carbohydrates  (2-10% of the membranes) are on the extracellular surface bounded to lipids or proteins of the membrane, forming glycoconjugates that serve as docking sites in cell recognition, adhesion and receptor action. These sugars include mainly glucose, galactose, mannose, fucose, N-acetyl galactosamine and  N-acetyl glucosamine.

The different kinds of Glycoconjugates include:

Proteoglycans: In the Proteoglycans, the Glucosaminoglycan moety forms the greater fraction of the molecule (tipically a proteoglycan consists of 95 % of carbohydrates) and is the main site of biological activity, providing multiple binding sites. They are found mainly in the extracellular matrix. They are major components of connective tissue.

Glycoproteins: Membrane bound glycoproteins participate in a wide range of cellular phenomena, including cell recognition, cell surface antigenicity, etc. In the glycoproteins, the majority of the molecule consist of proteins; they have one or more oligosaccharides attached to a protein, and they usually are branched and do not have serial repeats, so they are rich in information, forming highly specific sites for recognition and high affinity binding by other proteins

Glycolipids: are membrane lipids in which the hydrophilic head groups are oligosaccharides.

 As in glycoproteins, glycolipids  act as specific sites for recognition by carbohydrate binding proteins.  The four types of human RBC have different oligosaccharides (antigens) in their cell membranes. Blood groups depends on the gangliosides (a kind of sphingolipid) in the surface of the RBC .

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About Dietary Reference Intake (DRI)

 

 

The Dietary Reference Intake can be defined  a system of nutrition recommendations that estimates the amounts of nutrients required to prevent deficiencies and maintain optimal Health.

 

The DRI includes some concepts that sometimes are used as equivalents, when they are not. In other cases, the same concept receive different names, depending of the source of the article you are reading.

DRIs consists of four dietary reference standards:

 

1.- Estimated Average Requirement (EAR): Average daily nutrient intake level estimated to meet the requirement of one half of the healthy individuals in a specific age and gender group.

 

 2.- Recommended Dietary Allowance (RDA): Average daily Dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97-98%) individuals (EAR+2DS) on each life state and gender group. It is approximately 20 % higher than the EAR. RDA are printed in food labels in the United States and Canada.  (The definition of RDA is similar to the definition of  Recommended Nutrient Intake (RNI)  used by the World Health Organization and other international organizations).

 

 3.- Adequate Intake (AI): Estimation of nutrient intake in healthy people (used when EAR and RDA are not known)

 

 4.- Tolerable Upper Intake Level (UL): Highest average daily intake level from food, water and supplements that is likely to pose no risk of adverse health effect from excess in almost all apparently healthy individuals in an age and gender specific population group. (This concept is important mainly in relation to nutritional supplements, since the ingestion of natural foods is regulated in the body through mechanisms of absorption and excretion), In absence of observations of known adverse effects, it is used a default value of 10 times de Recommended Dietary Allowance.

 

More information about these reference standards can be found at:

 

Dietary Reference Intake

Reference dialy Intake 

 Interactive DRI for health care professionals

 Yates, A.A.: Dietary Reference Intakes –What is new and how to use them (Power point presentation)

Who Experts Committee:

Vitamin and Mineral requirements in human nutrition

World Health Organization, 2004