Carbohydrates Flashcards
What are the functions of carbohydrates in plants?
Cellulose = structure Starch = energy reserve
What are the functions of carbohydrates in humans? (6)
- Carb oxidation provides energy
- Glycogen = short term energy reserve
- Synthesis of biochemical substances e.g. protein, lipids, nuclei comes acids as they provide carbon
- Form part of structural framework for DNA and RNA
- Carbs linked to lipids = structural components of cell membranes
- Carbs linked to proteins = cell-cell or cell-molecule recognition processes
What is the basic empirical formula for carbohydrates
CnH2nOn
Define a carbohydrate
A polyhydroxy aldehyde/ketone or compound that produces polyhydroxy aldehyde/ketone
What are the 4 classifications of carbohydrates
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides
Monosaccharide
- contains single polyhydroxy aldehyde or ketone unit e.g. glucose and fructose
- water soluble
- crystalline solids
- often open chain and ring structures
E.g. disaccharides (2)
Sucrose (sugar)
Lactose (milk sugar)
Oligosaccharides
- contains less than 20 monosaccharides joined together by o-glycosidic bonds
- usually associated with proteins or lipids
Polysaccharides
- 20+ monosaccharides bonded together
Heteropolymer
Chain of variable monosaccharides
Homopolymers
Chain of identical monosaccharides
How can you tell if a molecule can be chiral
- central carbon is bonded to 4 groups
- the 4 groups must all be different
- ring structures can be chiral if the substituents in the ring are different and the 2 halves of the ring are different
Chiral
Molecule that’s mirror images are not super impossible
Achiral
Molecule whose mirror images are superimposable
Do monosaccharide tend to right handed or left handed molecules
Right
(Plants only produce right hand)
BUT
Amino acids are always left handed
Enantiomers
Stereoisomers who are nonsuperimposable images of eachother
L and D forms determined by configuration at high numbered chiral centre
Diastereomers
Stereoisomers that are not nonsuperimposable images of eachother
Epimers
Diastereomers whose molecules differ only in the configuration at one chiral centre
Ho do enantiomers differ?
Their interactions with plane polarised light
(Single enantiomer light rotated counter/clockwise depending on enantiomer)
Their interactions with other chiral substances
Constitutional isomer
Isomers in which the atoms have different connectivity
Skeletal isome
Isomers with different carbon atom arrangements and different hydrogen atom arrangements
Positional isomers
Isomers that differ in the location of the functional group
Functional group isomers
Isomers that contain different functional groups
Stereoisomers
Isomers with atoms of the same connectivity but that differ in orientation in space
Cis trans isomers
Stereoisomers that result from rotation amount chemical bonds
(Sometimes possible when ring or double bond present)
Describe aldoses
- 6C sugar
- carbonyl group at the end of the chain
- reducing sugars
- 16 possible isomers
Describe ketoses
- 6C sugar
- carbonyl group at C2 position
- 8 possible isomers
- reducing sugars
What is an anomeric carbon
Carbon bonded to the OH group and oxygen atom in a ring structure
Describe monosaccharide ring structure
Monosaccharides >5 carbons tend to be cyclic structures
Covalent bond takes place between carbonyl group and the oxygen of the hydroxyl group
Produces a chiral centre
There are 2 possible stereoisomers depending on whether OH group formed on top (beta) or bottom (alpha)
What are anomers
Cyclic monosaccharides that only differ in the positions of the substituents on the anomeric carbon
Describe pyranose rings
- ring with 6 carbons and an oxygen
not planar but either boat or chair form - due to saturation of carbons and its tetrahedral geometry
- axial substituents sterically hinder eachother
Describe furanose rings
Mono ring structure with 5 carbons and an oxygen atom
- not planar = envelope form
Monosaccharide derivatisation
- hydroxyl groups replaced with substituents
- addition of phosphate group
- addition of sulphate group
- addition of amine groups
- addition of hydrogen or alkyl groups
Monosaccharide bonding
via glycosidic bonds
- o glycosidic bond formed when hydroxyl group reacts with anomeric carbon
(N glycosidic bonds formed when anomeric carbon reacts with nitrogen )
What are the 3 main roles of sugars (with e.g.s )
Can fill so many roles as so diverse
Energy source (e.g. ATP, starch, glycogen)
Structure ( cells walls, exoskeletons, connective tissue)
Information: recognition (glycoproteins and glycolipids) and nucleic acids
Why are polysaccharides better than monosaccharides for energy storage
- compact granule
- binding means less osmotic pull so little water associated with it
What’s the key similarity between starch and glycogen
Both homopolymers of a-d-galactose (a 1-4 linkages)
Describe starch
Unbranched = amylose Branched = amylopectin (a 1-6 linkage every 30 residues)
Describe glycogen
Branched a 1-6 linkage every 10 residues
Describe cellulose
Used for cell wall structure in plants
Unbranched polymer of glucose residues joined by b1-4 linkages.
The b configuration allows formation of long straight chains.
Parallel chains can interact by hydrogen bonding forming Fibrils.
These fibres have high tensile strength.
Describe chitin
Used for cell wall structure of insects (exoskeleton)
Linear homopolymer composed of N-acetylglucosamine linked by b1-4 glycosidic bonds
Differs form cellulose by replacement of the C2 hydroxyl group with an acetylated amino group
Parallel chains interact by hydrogen bonding leading to high tensile strength
Describe bacteria cell walls
Sugar rich coating outside Plasma membrane.
Gram-positive bacteria retain a dye-iodine complex, washed out of Gram-negative bacteria.
Gram-positive bacteria- cross-linked multilayered proteoglycans, whereas gram negative- only a single layer.
Describe proteoglycans
Polysaccharides with protein associated to it
Proteoglycans chains are called glycosaminocans (GAG). Are anionic chains made from repeating disaccharide units:
Monosaccharide + amino sugar = N-acetylglucosamine or N-acetylgalactosamine.
Proteoglycans in connective tissue
Glycosaminoglycans attached to Core Protein
These units can attach to long filament of Hyaluronan to form enormous assemblies
These can interact with fibrous matrix protein e.g collagen
Forms cross-linked meshwork giving strength and resilience
Describe a sugar fingerprint
Sugar residues of our cell surfaces unique to us- recognise self from non- self.
Sugars allow more variety of structure than proteins
Very important for tissue typing.
Define blood groupings.
Mount immune response.
Describe oligosaccharides and blood typing
Oligosaccharides are synthesised by Glycosyltransferases and are specific to sugars being linked
In Blood ABO groups – carbohydrates attached to proteins and lipids on cell surface
A and B antigens differ by 1 monosaccharide – N-acetylgalactose (A) or Galactose (B) linked a1,3 to O antigen
Type A and B glycosyltransferase add specific sugars they differ by 4aa out of 354aa
O phenotype is result of mutation resulting in no active glycosyltransferases
Describe lectins
proteins that recognise and bind specific carbohydrate structures.
Help with cell-cell recognition and adhesion processes.
Recognise old erythrocytes and remove them. Eryth surface changes and lectins recognise and and take away when too old.
Microbial/viral pathogens utilise lectins for adhesion/toxin entry
Role played by lectins in influenza virus
Hemagglutinin (lectin) recognises sialic acid residues on host surface glycoproteins.
After penetrating the membrane neuramidase (sialidase) breaks glycosidic bond to release the virus
Without neuramidase, virus non invasive. Tamiflu and Relenza inhibit the enzyme.
