Carbohydrates Flashcards
(46 cards)
Which class of compounds is most biologically abundant?
Carbohydrates, over 100 billion tonnes produced by organisms per year.
What are the 3 key functions of carbohydrates?
1) Source of energy by successive breakdown and oxidation steps to yeild NADH, FADH2 and ATP (either from substrate level phosphorylation or chemiosmotic coupling).
2) Energy storage: glycogen.
3) Structural: peptidoglycan cell walls in bacteria, chitin in arthropods, cellulose in plants, found in connective tissues of animals; part of extracellular matrix as proteoglycans.
4) Signalling as glycoproteins and glycolipids.
What are the differences between monosaccharides, oligosaccharides and polysaccharides?
Monosaccharides:
- single sugar unit (monomer), which consists of usually 3-6 carbons
- hexoses (6): glucose, fructose, galactose (most common because most stable)
- pentoses (5): ribose, deoxyribose, xylose, ribulose (second most common because second most stable)
- tetroses (4): erythrose, erythrulose
- trioses (3): glyceraldegyde, dihydroxyacetone (the simplest sugars)
Oligosaccharides:
- short polymers (2-10 units long)
- disaccharides (2 units): sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose)
Polysaccharides:
- long polymers (over 10 units long; normally thousands of sugar units)
- can be linear (cellulose, amylose) or branched (amylopectin, glycogen)
What are the physical and chemical properties of monosaccharides?
- colourless
- crystaline
- soluble in polar (water) but insoluble in non-polar solvents (hexane)
- unbranched carbon chain
- carbonyl group (C==O) at one of carbons
- if carbonyl is on the very end - aldose (glucose, glyceraldehyde, ribose, erythrose)
- if carbonyl on the middle of carbon chain - ketose (fructose, dihydroxyacetone, erythrulose, ribulose)
- the rest of carbons are attached to hydroxyl group (OH)
- empirical formula: CnH2nOn
Are monosaccharides chiral?
Most of them contain multiple chiral carbons (where OH are attached), except dihydroxyacetone which is not chiral at all.
Glucose, galactose and mannose are stereoisomers based on the differences in their spatial configuration of their multiple OHs.
- Glucose has OH on C2 on the opposite side towards the rest of OHs;
- Galactose has OHs on C2 and C3 on the opposite side towards the rest of OH.
“L” and “D” are used to distinguish enantiomers (for example, L-glucose and D-glucose). “D” configuration is much more common in organisms.
How to distiguish D and L forms on Fisher/perspective projection?
Put molecule in the way that:
- carbonyl is as high as possible and on the right
- the rest is below
If OH on the reference carbon (the carbon further away from carbonyl) is on the left than L; if OH on the reference carbon is on the right than D.
What is the difference between Fisher projections and perspective formulas?
Fisher projections: just thin solid lines; horisonal means above the plane, vertical means below the plane.
Perspective: solid wedges for above the plane, dashed wedges for below the plane.
What are epimers?
The pair of monosaccharides that differ in the configuration of only one of their OH bound carbons.
Examples:
- glucose and galactose only differ in configuration at C4 (C4 epimers)
- glucose and mannose are C2 epimers
When monosaccharides form cyclic (ring) structures?
Pentoses (result in furanoses) and hexoses (result in pyranoses) in aqueous solutions (dissolved in water) - dominant form.
Form by carbonyl group reacting with the carbon just above the reference carbon (in hexoses, C1 with C5; in pentoses, C1 with C4). The reference carbon itself makes a “tail” outside the ring.
This new bond forms a new chiral centre which is hemiacetal in aldoses and hemiketal in ketoses. This happens becase carbonyl is planar and can be attacked by OH either from “above” or “below”, resulting in two different (alpha and beta) anomeres.
What is the difference between alpha and beta anomeres?
Alpha when the reference carbon (“tail”) and C1 OH (the closest to O) are on the opposite sides of the ring plane.
Beta when the reference carbon (“tail”) and C1 OH are on the same sides of the ring plane.
How do Fisher projections correlate to ring Haworth projections?
Haworth down = Fisher right
Haworth top = Fisher left
How to distinguish alpha-D-glucopyranose, alpha-L, beta-D and beta-L on Haworth projection?
Alpha-D: C6 and C1 OH are on the opposite sides; C6 on the top (above the ring).
Alpha-L: C6 and C1 OH are on the opposite sides; C6 on the bottom (below the ring).
Beta-D: C6 and C1 OH are on the same side; C6 on the top (above the ring).
Beta-L: C6 and C1 OH are on the same side; C6 on the bottom (below the ring).
Are ring pyranoses and furanoses flat?
No, they have 2 chair conformations; one which has the most OHs and reference carbon (“tail”) on equatorial (horisontal) positions in relation to the ring plane is more preferred because it provides more space for bulky substituents.
What is mutarotation?
C1 OH can change its chiral position, interconverting the monosaccharide molecule from alpha to beta anomer and other way round.
Pathogen Haemophilus influenzae uses NanM mutarotase to convert alpha-Neu5Ac into beta-Neu5Ac for immune evasion.
Which anomer is more common, alpha or beta?
Beta is more common (64%), alpha is less common (36%) and linear form is extremely rare (much below 1%).
What are reducing sugars?
All aldose (and tautomeric ketoses like fructose) monosaccharides (only in their open linear form) and some oligosaccharides if their anomeric carbon is exposed (not involved in glycosidic bond) can reduce mild oxidising agents such as Cu2+ and Fe3+.
Othervise, when mutarotation of anomeric carbon is not possible due to modification or being part of glycosidic bond, the sugar is non-reducing
Reduction is done by carbonyl group and involves anomeric (C1) carbon.
Those sugars end up oxidised to carboxylic acids (glucose to gluconate).
What is Fehling reaction?
In the presence of reducing sugars such as glucose, the blue solution of Cu2+ is reduced to brown-reddish solution of Cu+. The former method to measure glucose levels in blood.
Was displaced with glucose oxidase method: glucose + O2 –> glucono-delta-lactone + H2O2
Which bond connects monomers into oligo- and polysaccharides?
O-glycosidic bond which is an acetal/ketal.
Condensation with release of water: anomeric carbon (hemiacetal group) of one monomer reacts with any hydroxyl group of another monomer.
If hemiacetal in the second monomer is still exposed, it can react further to make even longer chain.
What are the key features of maltose?
Formula: Glc(alpha1–>4)Glc
1) alpha means both glucose monomers are alpha anomers;
2) –> means reducing end is on the right glucose
3) 1 and 4 means O-glycosidic bond is formed between C1 of one glucose molecule with C4 of another
4) on the right glucose, anomeric carbon is still esposed, so maltose is reducing sugar.
Appears as starch is broken down by amylase.
Component of malt (specially soaked and dryed cereals), which is used to make alcohol (beer, whisky) and some sweets.
What are the key features of lactose?
Formula: Gal(beta1–>4)Glc
- both gal and glc are beta anomers
- glc has exposed anomeric carbon, so lactose is a reducing sugar
Found in milk (up to 7%), broken down to glucose and galactose by lactase (beta-galactosidase); this enzyme is present in all infants but can be abscent in some adults leading to lactose intolerance.
What are the key features of sucrose?
Formula: Glc(α1<–>2β)Fru
- glucose is an alpha anomer
- fructose is a beta anomer
- both formed O-glycosidic bond with their anomeric carbons so sucrose is non-reducing!
Sucrose can either be a more acessible storage sugar or a form to transport glycose from leaves (just after photosynthesis) to starch (bigger storage) or cellulose (structure) to the rest of the plant via phloem.
What are the key features of trehalose?
Formula: Glc(α1<–>α1)Glc
- both glucose molecules are alpha anomers
- O-glycosidic bond is between two anomeric carbons, which makes trehalose non-reducing sugar
Major component of insect blood - very acessible glucose storage (more beneficial than glycogen) during flying.
Has cyoprotective properties (prevents freezing).
What is another name for polysaccharides?
Glycans
