Carbohydrates 2 Flashcards
Two monosaccharides can be joined via ?
A glycosidic bond between two anomeric carbons
The product has ?
Two acetal groups and one hemiacetal
Some disaccharides retain?
A reducing end (e.g. lactose); others do not (e.g. sucrose)
A bond formed between the anomeric C atom and an O atom of an alcohol is called ?
An O-glycosidic bond, and the product is called a glycoside
Sucrose is obtained from?
Sugar cane/beets and is composed of a glucose linked to a fructose. The linkage is α for glucose and β for fructose, and is cleaved by sucrase
Lactose is the disaccharide of ?
Milk that consists of a galactose linked to a glucose by a β-1,4 linkage. Lactase cleaves lactose
Maltose, a degradation product of ?
Large oligosaccharides, is composed of two glucose molecules linked by an α-1,4 linkage. Maltose is hydrolysed by maltase
Oligosaccharides are ?
Short chains of sugars
More than 150 different oligosaccharides have been identified in ?
Human milk, with the composition and amount varying among women
These carbohydrates are not digested by the infant, but ?
Play an important protective role against bacterial infection
How do they function ?
Function pro-biotically and as “decoys” for pathogenic bacteria
Polysaccharides can be:
- Homopolysaccharides (one type of monomer unit)
- Heteropolysaccharides (multiple types of monomer units)
- Linear (one type of glycosidic bond)
- Branched (multiple types of glycosidic bonds)
Glycogen and starch are insoluble due to ?
To their high molecular weight and often form granules in cells
Granules contain ?
Enzymes that synthesise and degrade these polymers
Glycogen and amylopectin (starch) have?
One reducing end but many nonreducing ends: Enzymatic processing occurs simultaneously in many nonreducing ends
What kind of polymers are both of these ?
Both are homopolymers of glucose
Glycogen functions as ?
The main storage polysaccharide in animals, in skeletal muscle and liver
What is Starch a mixture of ?
Two homopolysaccharides of glucose (Amylose and Amylopectin)
Explain Amylose and Amylopectin ?
- Amylose is an unbranched polymer of ( α-1,4) linked residues.
- Amylopectin is branched like glycogen, but the branch points with (α-1,6) linkers occur every 24–30 residues.
- Molecular weight of amylopectin is up to 200 million
Explain in depth Cellulose?
- Cellulose is a linear homopolysaccharide of glucose. Glucose monomers form (β-1,4) linked chains
- Hydrogen bonds form between adjacent monomers (blue dashed lines)
- There are additional H-bonds between chains.
- Structure is now tough and water insoluble.
- It is the most abundant polysaccharide in nature: cotton is nearly pure fibrous cellulose
What does the glycosidic bond determine ?
Polysaccharide structure
Explain how bonds can determine this ?
- Cellulose: bond points up, in starch: bond points down
- α-amylase (in saliva) breaks down the α-1,4-glycosidic bonds found in glycogen and starch but humans can’t digest cellulose (β-1,4 linkages)
- The β-1,4 linkages favor straight chains, which are optimal for structural purposes
- The α-1,4 linkages favor bent structures, which are more suitable for storage
Agar is a ?
Branched heteropolysaccharide composed of agarose and agaropectin
What does Agar serve as?
A component of cell wall in some seaweeds
Agar solutions form ?
- Agar solutions form gels that are commonly used in the laboratory as a surface for growing bacteria
- Agarose solutions form gels that are commonly used in the laboratory for separation DNA by electrophoresis
Explain Glycoproteins ?
The protein is the largest component by weight. Glycoproteins play a variety of roles, including as membrane proteins
Explain Proteoglycans ?
The protein is attached to a particular type of polysaccharide called a glycosaminoglycan. By weight, proteoglycans are mainly carbohydrate. Proteoglycans play structural roles or act as lubricants
Explain Mucins or mucoproteins ?
Like proteoglycans, mucins are predominantly carbohydrate. The protein is characteristically attached to the carbohydrate by N-acetylgalactosamine. Mucins are often lubricants.
Explain Glycolipids ?
Lipids modified by the attachment of carbohydrates
In all classes of glycoproteins, carbohydrates are attached to the ?
The nitrogen atom in the side chain of asparagine (N-linkage) or to the oxygen atom of the side chain of serine or threonine (O-linkage).
The N-linked glycan groups are attached to asparagine residues in the protein using a ?
GlcNAc monosaccharide
O-linked glycan groups are attached to serine or threonine residues using a ?
GalNAc monosaccharide.O-linked glycan groups are GalNAc often followed by Gal or GlcNAc
What is common to all N-linked oligosaccharides and serves as the foundation for a wide variety of these compounds ?
A pentasaccharide core (shaded gray) of mannose (green circles) and N-acetylglucosamine (blue square) residues
The glycoprotein erythropoietin EPO is secreted by ?
The kidneys and stimulates the production of red blood cells
EPO is composed of ?
165 amino acids and isN-glycosylated at three asparagine residues andO-glycosylated on a serine residue
How much of carbphydrate by weight is the mature EPO ?
The mature EPO is 40% carbohydrate by weight, and glycosylation enhances the stability of the protein in the blood
The availability of recombinant human EPO has ?
Greatly aided the treatment of anaemias
However, some endurance athletes have used recombinant human EPO to increase ?
The red-blood-cell count and hence their oxygen-carrying capacity
Glycoproteins have an important role in ?
Cell recognition & signalling. Immune cells communicate with each other by cell–cell interactions between glycoproteins and glycan-binding proteins (e.g. lectin) on the cell surface: intrinsic recognition
Extrinsic reactions occur between ?
Host cells and pathogens
Many pathogens gain entry into cells by first binding to ?
Carbohydrates on the cell surface
Infection spreads when?
A viral protein, neuraminidase (sialidase), cleaves the glycosidic bonds between the sialic acid residues and the rest of the cellular glycoprotein, freeing the virus to infect new cells.
N-linked glycosylation begins and continues in the ?
N-linked glycosylation begins in the endoplasmic reticulum and continues in the Golgi complex
O-linked glycosylation occurs only in ?
The Golgi complex
Glycoproteins are sorted according to ?
Signals encoded in the amino acid sequence and structure
Proteoglycans are key components of ?
The ECM and serve as lubricants
What are Proteoglycans either ?
They are either membrane-bound through a GPI anchor or a transmembrane α helix on the protein or are secreted into the ECM
Mucopolysaccharidoses, such as Hurler disease, are ?
Pathological conditions that result from the inability to degrade proteoglycans
What are Glycosaminoglycans composed of ?
Glycosaminoglycans are composed of repeating units of a disaccharide, one of which is a derivative of an amino sugar and one of which carries a negative charge, either as a carboxylate or sulfate
Cartilage is composed, in part, of ?
The proteoglycans aggrecan, chondroitin sulphate and keratan sulphate, with the protein collagen
The glycosaminoglycan component of aggrecan cushions ?
Joints by releasing water on impact, and then rebinding water
What is the second most abundant biomolecule in nature ?
Chitin
Chitin is ?
A glycosaminoglycan: linear homopolysaccharide of N-acetylglucosamine
N-acetylglucosamine monomers form ?
β-1,4-linked chains
What do β-1,4-linked chains form ?
Extended fibres that are similar to those of cellulose: they are tough and flexible, insoluble and cannot be digested by vertebrates
Where is Chitin found ?
In cell walls in mushrooms and in exoskeletons of insects, spiders, crabs, and other arthropods
In mucins, the protein component is ?
Extensively glycosylated to serine and threonine residues, with the first carbohydrate being N-acetylgalactosamine
A region of the protein backbone rich in serine and threonine, called ?
Variable number of tandem repeats (VNTR), is the site of glycosylation
Mucins serve as ?
Lubricants and adhere to epithelial cells, acting as a protective barrier
In gram-negative bacteria, lipopolysaccharides (LPS) cover ?
The peptidoglycan layer
Different bacterial species have ? But have in common ?
Subtly different LPS structures, but they have in common a lipid region (lipid A), a core oligosaccharide also
The O-specific chain is ?
The principal determinant of the serotype (immunological reactivity) of the bacterium.
The outer membranes of the gram-negative bacteria S. typhimurium and E. coli contain ?
So many LPS molecules that the cell surface is virtually covered with O-specific chains
The human ABO blood antigens contain ?
75% glycoproteins and 25% glycolipids
The blood groups are determined in part by ?
The type of sugars located on the head groups in glycosphingolipids
The structure of sugar is determined by ?
An expression of specific glycosyltransferases
Who will have the O antigen ?
Individuals with no active glycosyltransferase will have the O antigen
Who will have the A blood group ?
Individuals with a glycosyltransferase that transfers an N-acetylgalactosamine group
Who will have the B blood group ?
Individuals with a glycosyltransferase that transfers a galactose group
Blood types must be carefully ?
Matched for donation to avoid an immune response
What are both Blood Groups O and Group AB?
Blood group O are “Universal donors” whereas group AB are “Universal receivers”
