12.7
Membrane Carbohydrates
The plasma membrane is a dynamic barrier composed of lipids, proteins, and carbohydrates. It is the epicenter of many cellular processes required for…
After lipids and proteins, carbohydrates are the third major component of biological membranes.
Glycoproteins, proteins with bound carbohydrates, comprise nearly ninety percent of membrane carbohydrates. Glycolipids are the remaining ten percent.
Both glycoproteins and glycolipids are found on the outer side of the membrane and perform diverse functions, including acting as antigens and receptors.
Carbohydrates are attached to proteins through N-glycosidic and O-glycosidic bonds.
N-glycosidic bond formation occurs between the amide group of asparagine and a sugar such as N-acetylglucosamine, whereas O-glycosidic bonds form between the hydroxyl group of serine or threonine and a sugar, such as N-acetylgalactosamine.
Bound carbohydrates protect proteins from proteases, change their overall charge and increase their water-binding capacity.
Glycolipids are derived from ceramide– a sphingolipid. Glucocerebroside or galactocerebroside is synthesized when glucose or galactose binds to ceramide.
Gangliosides are formed when an oligosaccharide and sialic acid bind to ceramide. The negatively charged gangliosides regulate the concentration of positively charged calcium ions and thereby help in neurotransmission.
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Q1: What are the two main types of membrane carbohydrates?
Glycoproteins and glycolipids are the two main types of membrane carbohydrates. Glycoproteins, which are proteins with bound carbohydrates, comprise nearly 90 percent of membrane carbohydrates, while glycolipids make up the remaining 10 percent. Both are located exclusively on the outer surface of the membrane and perform diverse functions including acting as antigens and receptors.
Q2: How are carbohydrates attached to proteins in membranes?
Carbohydrates attach to proteins through two types of glycosidic bonds. N-glycosidic bonds form between the amide group of asparagine and sugars like N-acetylglucosamine. O-glycosidic bonds form between the hydroxyl group of serine or threonine and sugars such as N-acetylgalactosamine. These bonds create glycoproteins that protect proteins from proteases, alter their charge, and increase water-binding capacity.
Q3: Why are glycoproteins and glycolipids found only on the outer membrane surface?
Glycoproteins and glycolipids are synthesized in the endoplasmic reticulum and Golgi apparatus, where they initially orient with their carbohydrate portions facing the lumen. When vesicles bud from the Golgi and fuse with the plasma membrane, the inner layer flips outward, exposing carbohydrates to the cell's exterior. This membrane asymmetry ensures carbohydrates remain on the exoplasmic side.
Q4: What are gangliosides and what role do they play in cells?
Gangliosides are glycolipids formed when an oligosaccharide and sialic acid bind to ceramide. These negatively charged molecules regulate the concentration of positively charged calcium ions in the membrane, thereby helping facilitate neurotransmission. Gangliosides represent a specialized class of membrane carbohydrates derived from sphingolipids.
Q5: How does carbohydrate structural diversity contribute to cell identity?
Carbohydrates are structurally more diverse than proteins or DNA because each sugar molecule can form multiple types of covalent bonds, creating different oligosaccharide structures from identical sugar molecules. This diversity allows each cell type to have a distinct identity. For example, blood group classification depends on specific antigens present on red blood cell surfaces, with A, B, AB, and O types determined by different carbohydrate antigens.
Q6: What is the glycocalyx and how does it form?
The glycocalyx is a sugar coat formed when eukaryotic and bacterial cells secrete large amounts of carbohydrates outside the cell. This carbohydrate-rich layer surrounds the cell and contributes to cell recognition and protection. The glycocalyx represents an accumulation of membrane carbohydrates and secreted polysaccharides that extend from the cell surface.
Q7: How do blood group antigens relate to membrane carbohydrates?
Blood group classification depends on specific carbohydrate antigens present on red blood cell surfaces. Type A blood has A antigens; type B has B antigens; type AB has both; type O has neither. Each person carries antibodies against the antigens they lack. These membrane carbohydrates serve as molecular identifiers that determine blood compatibility and immune responses.