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Lecture 9

Biochemistry 2280A Lecture 9: 9 - Carbohydrates

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Biochemistry 2280A
Eric Ball

Thursday, October 1, 2015 Carbohydrates McLachlin Topic 9 — Lecture 11 & 12 Learning Objectives - apply appropriate terminology to describe carbohydrates, without memorizing individual sugars - convert a cyclized monosaccharide between Fischer and Haworth projections - describe how monosaccharides are joined to make polysaccharides - compare the structures of polysaccharides used for energy storage - describe the difference between dietary fibre and carbohydrates used for energy metabolism - give examples of the major roles played by carbohydrates in biological systems Four Main Classes of Biomolecule - sugars (monosaccharides) and polysaccharides are carbohydrates - fibre and sugar are different - what makes up the rest of ‘carbohydrates’? Monosaccharides - general formula: (CH O) 2 n - ‘hydrated carbon’ - name them in terms of how many carbon they have - ‘-ose’ is the common ending - glyceraldehyde: • each carbon has bonds to OH and a double bond to O • the middle carbon is different because its bonded to 4 different atoms • this is a chiral carbon • not the same molecule if they don’t have the same atoms in the same places • so which version of glyceraldehyde are we talking about? • this is when we use projections - represented by Fischer projections - horizontal are coming out at you - vertical lines are going back - have to just assume this - D means right (the OH group is there) - L means left (the OH group is there) Some Hexoses - most biological monosaccharides are in the D orientation at the highest chiral carbon - numbers start where the O is double bonded - you’ll only have one carbon that is double bonded - the highest number of chiral carbons is what you look at for D or L - all of these are D sugars because of carbon #5 - safe to assume all carbohydrates are D sugars 
 1 Cyclization - most monosaccharides form ring structures - 5 or more carbons are likely to form ring structures - the cyclic form is predominant in solution - they can go in and out of cyclic structure Haworth Projections - cyclized monosaccharides are represented by Haworth projections - 3D structures - the 6 carbons are all in one plane in this picture - everything else is above and below - in reality, they’re either in boat configuration (when both sides are up) - or chair (when one side is up, one side is down) - when transferring fischer to haworth: rotate 90* to the right • • low carbons are usually on right hand side • carbons 2, 3 and 4 are ‘in front’ (aka slightly below) • for these, you just look at the rotated fischer • at 5, the CH2OH is always pointing up • the right top corner is just an O Anomeric Carbon - a monosaccharide can form one of two structures when it cyclizes - the right side H vs. OH position depends on how that carbon-oxygen bond forms - these are anomers of one another - they’re in equilibrium - this isn’t permanent because every time it cyclizes, is has a chance of being either position - ‘alpha’ is when OH is below (a with b) - ‘beta’ is when OH is above (b with a) - * it’s really when the highest number carbon is on the same or opposite side Cyclization of Fructose - the hydroxyl on C5 attacks the C2 ketone - still ‘a with b’ and ‘b with a’ - C1 in this is not part of the ring Pyranoses vs. Furanoses -
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