1. Title: Lesson 10 B.10 Stereochemistry in biomolecules Learning Objectives: – Describe the stereochemistry in proteins, lipids, carbohydrates and vitamins

2. Main Menu Stereochemistry in proteins  Stereoisomers represent different spatial arrangements of the atoms in a molecule, and their study involves a 3D approach  Many biopolymers can exist as stereoisomers, and metabolic reactions are usually stereospecific, meaning only one form of the isomer has the required activity  In amino acids, the amino group, the carboxylic acid group, a hydrogen atom, and a variable R group are all attached to the same carbon atom, known as the alpha carbon or carbon-2 in the numbering of the chain  Four different groups, this carbon atom is chiral  Amino acids are optically active and can exist as two different stereoisomers (enantiomers) The stereochemistry of alanine An exception to the rule is glycine, because R=H so the carbon atom is not chiral L and D forms of amino acids have identical physical properties and chemical reactivities apart from: the direction in which they rotate plane-polarized light Their reactions with reagents that are chiral Enzymes themselves are proteins and chiral, so can distinguish completely the L and D forms of amino acids. Biological systems have evolved to use only L forms of amino acids…

3. Main Menu Stereochemistry in lipids  Unsaturated fatty acids in fats and oils contain carbon-carbon double bonds  These exist in two forms, known as cis-trans isomers, which arise due to the restriction on the rotation around the double bond  The cis form occurs when the same group, e.g. hydrogen, has the same orientation relative to the double bond  The trans form occurs when the same group has opposite orientation across the double bond Most naturally occurring unsaturated fats are in the cis form… Molecules of the cis isomer cannot easily arrange themselves side by side to solidify, so they tend to have lower m.p than the corresponding trans isomer… Free rotation is not possible around a double bond because of the sigma and pi bond (pi bond over laps two p orbitals. Free rotation would push the p orbitals out of position and the pi bond would break…

4. Main Menu Hydrogenation of fats  Takes place in the food industry when hydrogen is added across the carbon-carbon double bond using a metal catalyst such as nickel  The product is a fat that, being more saturated, has a higher m.p, and therefor is a more convenient form for packing/storage as a solid or semi-solid  Hydrogenated fats usually have a longer shelf life than liquid oils  Problem with this process is partial hydrogenation… Where only some of the carbon-carbon double bonds in a fat are broken, and those that remain often get chemically modified from the cis position to the trans position…  Resulting fatty acids are therefore known as trans fats and are common in processed foods  Consuming trans fats raises LDL Cholesterol levels  risk of heart disease  Trans fats also reduce the blood levels of HDL cholesterol  less protection against heart disease…

5. Main Menu Stereochemistry in carbohydrates  All simple sugars are chiral molecules as they contain at least one chiral carbon atom  These stereoisomers are described as D and L  For sugars that have two or more chiral carbon atoms, the prefixes D and L refer to the configuration of the chiral atom furthest away from the carbonyl carbon  D sugars are the most abundant form in nature This glucose has more than one chiral carbon… Furthest chiral carbon from the carbonyl group at the top…

6. Main Menu Alpha and beta forms of glucose  Conversion of straight chain to the ring form, creates an additional type of isomers, known as alpha and beta forms  These are distinguished by the relative position of the groups attached to the carbon atoms that close the ring by forming an ether link with oxygen  In glucose, alpha and beta forms are determined by the positions of the –OH group at C 1  Alpha form has the –OH at C 1 on the opposite side to the ring to C 6  Beta form has the –OH on the same side of the ring as C 6 R-O-R’ is the ether link NOTE: Count all the carbons, not just the ones in the ring!

7. Main Menu Fructose alpha and beta forms  Focus is on the C 2 instead of the C 1 The structural differences between alpha and beta glucose have a large effect on the properties of their polymers:  Starch and glycogen are polymers of alpha-glucose. Starch forms a relatively compact spiral structure and is stored as starch grains in plant cells  Cellulose is a polymer of beta-glucose. Linear polymer with 1-4 links known as beta-glycosidic links…  These position the sugars at a different angle from the alpha- glycosidic links so the cellulose chain forms an uncoiled linear structure with alternate glucose monomers ‘upside down’ with respect to each other.  Hydroxyl groups can form hydrogen bonds with the hydroxyls of other cellulose molecules lying parallel. NOTE: Count all the carbons, not just the ones in the ring!

8. Main Menu Properties of cellulose  Due to the hydrogen bonding between parallel hydroxyl groups, cellulose forms cables (microfibrils) of parallel chains that give it a rigid structure…  Cellulose is found in cell walls and the main support material in plant cells  Wood is rich in cellulose and a great building material!  Starch and glycogen can be relatively readily hydrolysed by action of digestive enzymes  Humans do not produce the enzyme to hydrolyse the beta-glycosidic links in cellulose  Cellulose will pass through the gut intact, contributing to increase dietary fibre  The cellulose microfibrils abrade the digestive tract wall and stimulate the production of mucus which helps in the passage of undigested food through the gut Scanning electron microscope of cellulose microfibrils in a plant cell wall… microfibrils measure between 5 nm and 15 nm in diameter

9. Main Menu Stereochemistry in vitamins  Vitamin A, also known as retinal, is involved in the visual cycle, the photochemical changes associated with our ability to detect light  The retina of the eye contains two types of light sensitive cells (rods and cones)  Rods are stimulated by low intensity light (non colour vision)  The main photoreceptor pigment in rods is a large conjugated protein molecule called rhodopsin  This consists of a protein, opsin, tightly bound to 11-cis-retinal (derived from vitamin A)  When rhodopsin is exposed to light a transformation of 11-cis-retinal occurs, changing it to all- trans retinal

10. Main Menu  This causes the all-trans isomer to dissociate from the opsin (protein), which triggers a nerve impulse  Rhodopsin is regenerated from opsin and 11-cis-retinal after the all-trans form isomerizes back to the 11-cis form in a series of steps catalyzed by enzymes Structures are given in section 35 of the IB data booklet

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