Carb Conjugation (13)
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Which carbon of monosaccharides is chemically reactive?
The anomeric carbon
the product that results from the condensation of a sugar with a hydroxyl-containing compound
What is the difference between a glucoside and a galactoside?
Glucosides are glycosides that have a glucose, whereas galactosides have a galactose
What is a glycosidic bond?
A covalent bond that joins a sugar to the hydroxyl group of another molecule (what was an OH turns into an --O-- )
- composed of two molecules of glucose in an alpha (1-->4) - linkage
- composed of one galactose and one glucose in a beta (1-->4) - linkage
- composed of one glucose and one fructose in an alpha (1-->2) - linkage
- storage forms of carbohydrates in plants & a major dietary source of carbohydrate for animals
- there are two main kinds of and BOTH forms are homopolymers of glucose
- 1) amylose
- 2) amylopectin
starch composed of glucose molecules arranged in alpha (1-->4) - linkages; it is an UNbranched polymer that forms a helical coil
starch composed of glucose polymers arranged in alpha (1-->4) - linkages; it is branched every 12 residues via an alpha (1-->6) - linkages
- a carbohydrate storage form in animals composed of glucose polymers arranged in alpha (1-->4) -linkages
- it has branches every 8-10 residues via alpha (1-->6) - linkages
- it exists as spherical particles in cytoplasm
- a typically unbranched homopolymer of glucose arranged in BETA (1-->4) - linkages
- it CANNOT be digested by mammalian enzymes
- (can be digested by intestinal bacteria)
- glucose polymer arranged in alpha (1-->6) -linkages with some branching alpha(1-->3) branching
- made by bacteria
Which can we NOT digest, alpha or beta (1-->4) -linkages?
BETA (1 --> 4) - linkages
- long unbranched polysaccharides of repeating anionic disaccharides that may be sulfated
- they are an important component of connective tissues
- e.g. glucuronic acid, iduronic acid, galactose, galactosamine, glucosamine
- (used to be called mucopolysaccharides)
Where is chondroitin sulfate found?
GAG found in bone, cartilage, & cornea formation
Where is keratin sulfate found?
GAG found in the cornea & connective tissue
Where is dermatan sulfate found?
GAG that binds to LDL in the plasma walls
Where is heparin sulfate found?
GAG found in the aortic wall and basement membrane
What is heparin used for?
Anticoagulant; it's present in mast cell granules and complexes with protein and histamine
What is hyaluronic acid used for?
GAG important for cell migration & lubrication
- GAG present in connective tissue intercellular fluid, notably in synovial fluid in joints and vitreous fluid in the eye
- Its structure is a repeating disaccharide made up of D-glucuronic acid and N-acetylglucosamine
Which glycosaminoglycan (GAG) is not linked to a core protein?
glycosaminoglycans that are covalently linked to a protein
What must happen for sugar-containing polymers to be synthesized?
the anomeric C of a sugar that will take part in a glycosidic bond is activated by sugar nucleotide formation involving either uridine (U), guanosine (G), cytidine (C) or thymidine (T) nucleotides
Which nucleotide is NOT involved in the activation of sugars?
How do proteins react with carbohydrates?
- non-enzymatically through Schiff base formation between the ALDEHYDE of the sugar and the AMINO GROUP on the protein --> a ketoamine results
- this chemical reaction is called Glycation
True or False: glycation is a form of tissue damage?
TRUE, it's thought to contribute to disease states
What type of enzymes or cofactors does glycation require?
NONE TRICK YA glycation doesn't require enzymes, cofactors, and especially doesn't require energy
What is the clinical significance of HbA1c (hemoglobin A1c)?
- HbA1c provides a 3-month 'history' of blood glucose levels; it's an example of a non-enzymatic protein/sugar interaction
- a low value is good
- a high value can mean you're prediabetic
What type of proteins undergo enzymatic protein glycosylation?
- 1) secreted proteins
- 2) proteins with extracellularly facing domains
- 3) intracellular O-GlcNAc. It's an enzyme.
enzymatic process that attaches glycans (polysaccharides) to proteins meant to be secreted, transported to the extracellular matrix, or stored in the ER lumen
What are the two major types of carbohydrate linkages in glycoproteins?
- 1) N-linked (to asparagine)
- 2) O-linked (to serine or threonine)
Which amino acids are involved in N-linked and O-linked glycosylation?
- N-linked: asparagine (almost always)
- O-linked: through the oxygen atom of serene or threonine
Which glycoprotein is typically NOT secreted from cells?
O-linked N-acetylglucosamine (O-GlcNac)
- glycosylated in the Golgi apparatus
- eg. blood group substances, salivary mucins
Which class of enzymes adds sugars to a growing O-linked glycoprotein? glycosyltransferases
many different glycosyltransferases are required for synthesis of a sugar sequence because a specific one is needed for each sugar residue addition
carbohydrate attachment via _________ can be well predicted from the primary peptide sequence
N-linkage; it is difficult to predict where carbohydrate modifications will occur on O-linked glycoproteins
How are O-linked carbohydrates added to create a glycoprotein?
just added straight onto the protein one sugar at a time
How are N-linked carbohydrates added to create a glycoprotein?
- carbohydrate molecules are first added as a pre-assembled 14 sugar block on a lipid molecule
- then transferred to desired protein's asparagine in one fell swoop
- process usually occurs CO-translationally as protein is being made on RER
Where in the cell are sugars added to proteins to form N-linked glycoproteins?
the ER membrane
What lipid carrier molecule is used for the synthesis of the branched oligosaccharide of N-linked glycoproteins?
CDGs (Congenital Diseases of Glycosylation)
diseases that vary widely in presentation; mistakes made in protein glycosylation
an antibiotic that inhibits the addition of N-acetylglucosamine-phosphate to dolichol phosphate
Proteoglycans usually consist of:
a core protein O-LINKED to a glycosaminoglycan
macromolecules in which up to 95% of the mass consists of negatively-charged polysaccharide glycosaminoglycan chains which (except for hyaluronic acid) are attached to a core of protein
Where can proteoglycans be found?
- in the intercellular matrix of cartilage and skin, serving as support structures
- in lubricating fluids in joints, pleural and mesenteric spaces
- in the cornea and vitreous fluid in the eye
What's the most abundant glycosaminoglycan in the human body?
a proteoglycan (composed of chondrotoin sulfate attached to collagen type II) that acts as a shock absorber [in knee cartilage] by cycling water in and out of cartilage
What are examples of how glycoproteins are important for recognition?
- 1) protein folding in the ER
- 2) protein sorting and transport (M6P)
- 3) first line of innate immunity defense (collectins)
- 4) determine half-life of serum proteins
- 5) direct cellular trafficking in vascular system (selectins)
enzymes responsible for degradation of glycoproteins
Deficiency of acid glucosidase results in what storage disease?
What is the enzymatic defect in Hunter syndrome?
Where are heparin sulfate, collagen, & laminin found in the cell?
they're interwoven in the basement membrane
Where is keratan sulfate type I typically found in the human body?
Where is keratan sulfate type II typically found in the human body?
What's the difference between 2,3 and 2,6 silica acid linkages?
- (2-3) linkages correspond to avian flu, while (2-6) linkages correspond to the human influenza virus
- -a change in receptor specificity or a mutation can be highly dangerous
What is the difference between the terminal sites of Type A, B and O blood groups?
- terminal sugar residues at the end of a chain are what differentiate the different blood types; the remainder of the carbohydrate chains are the same for all
- Type A has a glycosyltransferase for UDP-N-acetylgalactosamine
- Type B has a glycosyltransferase for UDP-galactose
- Type O has NEITHER transferase
H-pylori preferentially binds to gastric epithelial cells of individuals with which blood type?
Which sugars are used for the synthesis of the initial branched chain oligosaccharide of N-linked glycoproteins?
N-acetylglucosamine, mannose and glucose
Selectins & Leukocyte Adhesion
- endothelial cells express carbohydrate [SUGAR] structures and receptors for binding P selectin ligand
- -specifically they express L-selectin ligands and P selectin receptors
- Leukocytes express L selectin and P selectin
- interaction between selectins & their ligands acts like Velcro to provide a weak attachment that stops the leukocyte and signals for it to invade the tissue
Which class of proteins bind carbohydrates?
Leukocyte Adhesion Deficiency II (LAD II)
- a defect in fucosylation due to endothelial cells displaying the incorrect carbohydrate structures; leukocytes are poorly recruited and people have chronic infections
- failure to properly target white cells to where they need to be
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