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What are disaccharides and how can the linkage occur?
- Two monosaccharides linked by a glycosidic bond
- Linkages can occur between the anomeric carbon of one sugar and a hydroxyl on another sugar
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What are the name of linkages in disaccharides based on and in what form are sugars abosorbed in the body?
- Linkages are named based on the number of the carbon atoms involved
- In the body, sugars are absorbed as monosaccharides and digestion requires specific enzymes (e.g., lactase)
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What are the key disaccharides?
- Lactose
- Maltose
- Cellabiose
- Sucrose
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What is lactose, how is it formed, and what is its linkage?
- A disaccharide found in milk.
- A combination of galactose and glucose.
- Beta(1,4) linkage.
- Requires lactase to digest (expression decreases following childhood).
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What is maltose, how is it formed, and what is its linkage?
- A disaccharide that is the intermediate product of starch hydrolysis
- Alpha (1,4) linkage between two D-glucose molecules
- Does not appear freely in nature
- In solution the free anomeric carbon undergoes mutarotation
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What is cellobiose, how is it formed, and what is its linkage?
- A disaccharide that is the degradation product of cellulose
- Contains two D-glucose molecules with a beta (1,4) linkage
- Does not occur freely in nature.
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What is sucrose, how is it formed, and what is its linkage?
- A disaccharide: table sugar (cane or beet sugar)
- Formed from alpha-glucose and beta-fructose
- alpha, beta (1,2) linkage
- Produced in the leaves and stems of plants
- Non-reducing sugar
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What are polysaccharides?
- Also called glycans
- Composed of a large number of monosaccharides connected by glycosidic linkages
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What are oligosaccharides?
- Smaller glycans with up to 10-15 monomers
- Most often attached to polypeptides in glycoproteins and some glycolipids.
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What are the two classes of polysaccharides?
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What are homoglycans?
Polysaccharides composed of a single type of monosaccharide
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What are heteroglycans?
Polysaccharides composed of two or more different types of monosaccharides.
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What are the most abundant homoglycans found in nature and what are they made of?
- Starch, glycogen, and cellulose are all made from D-glucose
- Chitin is made of N-acetylglucosamine
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What is the molecular weight polysaccharides?
- Polysaccharides like starch and glycogen have no fixed molecular weight, unlike proteins and nucleic acids. This means there is not necessarily a limit as to how many monomers can be combined
- In a “well fed” state, the liver synthesizes glycogen until the blood sugar levels reach proper levels
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What is starch?
- Energy reservoir in plant cells and a significant energy (carbohydrate) source in the human diet (e.g., potatoes, rice, corn, wheat)
- A homoglycan
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What are the two different polysaccharides that form starch?
Amylose and Amylopectin
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What is amylopectin, how is it formed, and digested?
- One of the polysaccharides that form starch
- Made of branched chains of D-glucose with alpha(1,4) and alpha(1,6) linkages
- Digestion involves amylase in the mouth and small intestine, followed by other enzymes designed to release the glucose monomers
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What is amylose, how is it formed, and what is its useful function?
- One of the polysaccharides that form starch
- Made of unbranched chains of D-glucose with alpha (1,4) linkages
- Ideal for storage
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What are glycogens and where are they most abundant?
- A homoglycan
- Carbohydrate storage form used by vertebrates
- Most abundant in liver and muscle cells
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Describe the sturcture of glycogen
- Involves a compact branched chains of D-glucose similar to the structure of amylopectin
- The numerous non-reducing ends allows the cell to rapidly break glycogen down and release glucose
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Describe the similarities of linkages in gylcogen and amylopectin
These polysaccharides have both alpha(1,4) [for the chain] and alpha(1,6) [for thebranches] linkages
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What is cellulose, where is it most abundant?
- A homoglycan and structural polysaccharide in plants
- Probably the most abundant organic substance on earth
- Considered a good dietary fiber for animals
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Describe the formation, structure, and linkage of cellulose
- Made of D-glucopyranose
- Beta (1,4) linkages
- Unbranched chains pair together via H-bonds (microfibril)
- 40 pairs combine to form a bundle, which is the structural form found in plant cells
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What are 5 types of heteroglycans?
- N-glycans
- O-glycans
- Glycosaminoglycans (GAGs)
- Glycan components of glycolipids
- GPI anchors (glycosylphosphatidylinositol)
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What are N-glycans, how are they formed, and what is its linkage?
- A heteroglycan
- Sugar groups linked to the nitrogen of the side chain amide of an asparagine residue
- Involves a beta-glycosidic bond between nitrogen and the anomeric carbon of N-acetylglucosamine
- Involves several different monosaccharides
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What are O-glycans, how are they formed, and what is its linkage?
- Core of galactosyl-b-(1,3)-N-acetylgalactosamine linked to the protein with an a-glycosidic bond to
- the oxygen of the side chain of serine or threonine residues
- Collagens have a similar linkage with hydroxylysine using a different core
- Several other kinds sugars are attached to the core
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What are glycosaminoglycans (GAGs) and what does it generally contain?
- A heteroglycan
- Linear polymers of repeating dissacharides
- Generally contain a 6-carbon uronic acid (CH2OH to carboxylic acid)
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What does the repeating groups of Glycosaminoglycans (GAGs) contain and what are its features?
- Repeating groups contain both carboxyl and sulfate groups (negative charges!)
- Charge repulsion keeps chains separated and the hydrophilicity attracts and maintains large volumes of water – vastly increasing the volume of space occupied
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What are the five classes of glycosaminoglycans (GAGs) and how are they connected to proteins?
- hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin and heparan sulfate, and keratan sulfate
- All are connected to protein via serine or threonine except hyaluronic acid which connects via asparagine
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What is chondroitan sulfate?
- A heteroglycan (GAG)
- Component of cartilage
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What is dermatan sulfate?
- A heteroglycan (GAG)
- Found primarily in skin
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What is Heparin?
- A heteroglycan (GAG)
- Anticoagulant, found in mast cells
- Heparan very similar but has fewer sulfate and more acetyl groups
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What is keratan sulfate?
- A heteroglycan (GAG)
- Found in cornea, cartilage, and intervertebral disks
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What is hyaluronic acid?
- A heteroglycan (GAG)
- Found in the vitreous humor of the eye and synovial fluid of joints
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What are glycoconjugates?
- Compounds that covalently bind carbohydrates with either lipids or proteins
- Serve structural and functional roles generally on the surface of cells or in the extracellular matrix
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What are the classes of glycoconjugates?
- Glycolipids
- Proteoglycans
- Glycoproteins
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What is the difference between proteoglycans and glycoproteins
- Proteoglycans have a higher carbohydrate content
- Glycoproteins do not usually have uronic acids, sulfate groups, and disulfide repeating groups (GAGs)
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Where are proteoglycans present?
Present on the cell surface or secreted into the extracellular matrix
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How are proteoglycans formed and what is its structure?
- Made of GAGs linked to proteins (called core proteins) using N- and O-glycosidic linkages
- Proteoglycan aggregates are a series of proteoglycans linked to a common central GAG backbone (hyaluronic acid)
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What are 3 examples of proteoglycans?
- Syndecans
- Glypicans
- Aggrecan
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What are syndecans?
A class of heparan sulfate and chondrotin sulfate containing proteoglycans which include a transmembrane core protein
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What are Glypicans?
Are proteoglycans that contain heparan sulfate and are linked to membranes via GPI anchors
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What are aggrecans?
- Proteoglycans found in cartilage
- Combination of chondroitin sulfate and keratan sulfate attached to a core
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What are the functions of proteoglycans?
Signaling and Structural (compressive stiffness)
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What is the backbone of a proteoglycan aggregate?
Hyaluronic acid (GAG) backbone
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What are glycoproteins and how are they linked?
- Glycoconjugates
- Proteins that are linked through N- or O-linkages to carbohydrates (i.e., N- or O-glycans)
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What is the range of carbohydrate content in glycoproteins?
- Carbohydrate content ranges from ~1-85%
- Carbohydrates involved include monosaccharides, disaccharides, and several types of oligosaccharides
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How are the N and O linkages formed in gylcoproteins?
- N-linked carbohydrate chains are formed initially and then added to the protein during synthesis (carbohydrate added on protein)
- O-linked chains are built directly on the protein in the Golgi (carbohydrate built on protein)
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What are 5 examples of glycoproteins?
- Immunoglobulin (antibodies)
- Hormones: chorionic gonadotropin (HCG), follicle-stimulating hormone (FSH)
- Metal transport proteins: transferrin and ceruloplasmin
- Membrane protein such as Na+-K+-ATPase (Na+-K+pump)
- ABO blood group system- most common example
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What is the glycocalyx?
- Layer outside of most eukaryotic cells containing a large number of carbohydrates
- These serve structural, signaling, and recognition roles for cells and tissues
- Glycoproteins are components of the glycocalyx
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What are lectins?
- Considered to be the translators of the sugar code.
- Carbohydrate-binding proteins that are not abs and have no enzymatic activity.
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How does the cell recognize the vast array of different carbohydrates?
- Proteins known as lectins are able to bind to specific carbohydrate groups
- These are used in numerous types of interactions for recognition – especially cell-cell (e.g., selectins)
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What is glycolysis?
Break down of glucose to produce energy
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What is gluconeogenesis?
Production of glucose from specific precursors
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What is pentose phosphate?
Involved in the production of ribose for nucleotides
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What is considered glycogen metabolism?
The building (glycogenesis) or breakdown (glycogenolysis) of glucose
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What daily value percentage is considered low and high of total carbohydrates?
- 5% or less is Low
- 20% or more is High
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What are the dietary sources of carbohydrates?
- Sugar
- Starch- complex carbohydrate
- Dietary fiber- complex carbohydrate
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Where is sugar found?
- In a range of food sources such as milk (lactose), fruit (fructose), refined sugar for baking (sucrose), honey (fructose, glucose, maltose, sucrose)
- These are digested by the body
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Where is starch found?
- In bread, pasta, rice, potatoes, sweet potatoes,etc.
- Are digested by the body
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Where is dietary fiber found?
- In many plant foods (fruits and vegetables) and in whole grain foods
- Cannot be broken down by the body (includes cellulose), but are important components of our diet
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