Glycoproteins Flashcards
What are glycosaminoglycans?
Unbranched polysaccharides consisting of alternating uronic acid and hexosamine residues
They form highly hydrated gels
The gel like matrix containing collagen and other proteins is largely comprised of glycosaminoglycans
What is an example of an important glycosaminoglycan?
Hyaluronate or Hyaluronic acid
It acts as a shock absorber and lubricant
It plays a vital role in connective tissue, synovial fluid and the vitreous humor of the eye
Consists of D-glucuronic acid and N-acetly-D-glucosamine
What are some other common glycosaminoglycans?
They are sulphated:
Keratan sulphate - variable sulphate content
Heparin - highly charged
What is heparin used for?
Occurs in intracellular granules of mast cells of arterial walls
It inhibits the clotting of blood - thought to prevent runaway clot formation after injury
It is used to inhibit clood clotting in postsurgical patients
What do plants have as an alternative to glycosaminoglycans?
Pectins
They are major components of cell walls
Can have a similar function as shock absorbers
They are heterogeneous polysaccharides with a core of α(1→4)-linked galacturonate residues interspersed with the hexose rhamnose
What is a biofilm?
Bacterial biofilms are a type of extracellular matrix, where bacteria can grow
It consists of highly hydrated polysaccharides
What are glycoproteins?
Proteins with carbohydrate content, varying from < 1% to >90% by weight
Microheterogeneity - glycoproteins with the same amino-acid sequence will vary in their carbohydrate composition as under enzymatic not genetic control
What are some roles of glycoproteins?
Stabilise or define protein structure
Mediate recognition event
They could be enzymes, transport proteins, receptors, hormones and structural proteins
What are proteoglycans?
Proteins and glycosaminoglycans in the extracellular matrix aggregate covalently and noncovalently to form a diverse group of macromolecules
What bonds are involved in eukaryotic proteins being glycosylated?
N-glycosidic bonds or O-glycosidic bonds
N-Glycosylation occurs co-translationally i.e. while being synthesised
Oligosaccharides are attached to asparagine
Even proteins identical in sequence can vary due to incomplete glycosylation and lack of specificity of glycosidases and glycosyltransferases
O-linked glycosylation occurs in the Golgi apparatus i.e. to a completed polypeptide chain
Oligosaccharides are attached to serine or threonine
No amino-acid sequence dependence – specified by secondary or tertiary structure
What is a bacterial cell wall made up of?
Peptidoglycan
linear chains of alternating β(1→4)-linked GlcNAc and N-acetylmuramic acid
Rigid cell wall, for hypotonic environments = prevent osmotic swelling
Gram-positive bacteria - thick cell wall
Gram-negative bacteria - thin cell wall (covered by an outer membrane)
What is peptidoglycan not resistant to?
Lysozyme
Antibiotics - that inhibit biosynthesis
What is a glycoform?
The varient species of a glycoprotein
Oligosaccharides may determine glycoprotein structure, function and recognition
How do oligosaccharides help define protein structure?
They tend to attach at surface loops
They shield proteins from proteolysis as sugars are hydrophilic and project away from the surface - occupying large volumes
They can limit the conformational freedom of the protein
They may stabilise the folded conformation by reducing backbone flexibility
How do oligosaccharides mediate recognition events?
The diversity of polysaccharides allows sugars to carrying more biological information
Lectins are proteins that bind carbohydrates, they are very specific and can recognise monosaccharides with a particular linkage to other sugars within an oligosaccharide
Cell-cell recognition (mediated by selectins) is important in leukocyte action as well as virus/bacterial/parasite invasion
