BIOCHEMISTRY OF CARBOHYDRATES CARBOHY DRATES UST – FMS General Formula: Formula: C x(H2 O) y or (CH2 O)n Carbon compounds having Carbonyl Carbon (C=O) and hydroxyl (-OH) functional groups Carbonyl Functional Groups : st Aldehyde Aldehyde (Pol yhydroxyal dehydes): dehydes): 1 C (C=O) nd Ketone (Polyhydroxyketones): 2 C (C=O) Classification 1) Size of base Carbon chain Triose (3C), Tetrose (4C), Pentose (5C), Hexose (6C), Heptose (7C), Nanose (9C) 2) Number of sugar units Monosaccharide 1 CHO unit Disacchar ide 2 CHO units Oli gosaccharide 3-10 CHO units Polysacchari de >10 units 3) Location of Carbonyl carbon Aldose Ketose Nomenclature Aldotriose
Aldotetrose
Aldopentose
Aldohexose
*Aldohexose *Aldohexose s: ALL ALTruists ALTruists GLadly MAke MAke GUm IN GALlon TAnk. nd 2 C: alternate –OH rd 3 C: alternate –OH by 2 th st 4 C: 1 4 Right –OH, Last 4 Left –OH 5 th C: all –OH at right side
Ketoses
Ketotriose
Ketotetrose
Ketopentose
Ketohexose
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Fischer Projection Sugars drawn in straight chain Perspective structural formula: 3D Fisher Haworth Projection Cyclic forms which show the molecules as cyclic and planar with substituents above or below the ring Boat and Chair conformation: more plausible bent forms Fischer projection
Haworth projection
Optical Activity Ability to rotate plane of polarized light All CHO’s contain assymetrical (chiral) carbon and are, therefore, optically active. a. Dextrorotatory (+): D isomer o Rotates to ri ght o In Fischer, -OH is at the right side of penultimate Carbon o In Haworth, last Carbon is above the ring b. Levorotatory (-): L isomer o o o
Rotate to left In Fi scher, -OH is at the left side of penultimate Carbon In Haworth, last Carbon is below the ring
*Assymetri c or Chiral Carbon: carbon with 4 different substituents *Penultim ate Carbon: chiral carbon farthest from functional group
Stereochemistry Isomers: same molecular formula and bonds but differ in spatial arrangement A. Constitutional Isomers Different a tom connectiviti es B. Stereoisomers Same a tom connectivi ty, different spatia l arr angement 2 types: Configurational and Conformational 1. Configurational Isomers o Interconverted only by breaking coval ent bonds (separa ble) o 4 types: Enantiomer, Diastereomer, Epimer, Anomer a. Enantiomer Stereoisomers which are non-superimposable mirror images of each other (Eg. D-glucos e and L-glucos e) b. Diastereomer Stereoisomers which are non-superimposable non-mirror images of each other (Eg. D-gal actose and D-glucos e) c.
d.
Epimer Stereoisomers which differ in one stereocenter (different -OH position al ong 1 Carbon atom only) Example: D-glucose, D-mannos e and D-galac tose Anomer Stereoisomers which differ only i n the configurati on around the carbon (anomeric carbon, usually C1) which was involved in the intramolecular nucleophili c attack (Eg. ɑ and β anomers) Fischer Projection: ɑ anomer (Cis): –OH of anomeric Carbon and hemibridge on sa me side β anomer (Trans): –OH of anomeri c Carbon and hemibridge on opposite s ide Haworth Projection: ɑ
anomer (Trans ): C6 up, -OH of C1 (anomeric ca rbon) down if in D isomer C6 down, -OH of C1 (anomeric carbon) up if in L isomer
β anomer (Cis): C6 up, -OH of C1 (anomeric carbon) up
if in D isomer
C6 down, -OH of C1 (anomeric carbon) down if in L isomer *Mutarotation : ɑ and β are in equilibrium Marco Peri kar R. Di maano 1BMed Class 2015
2.
Conformational Isomers o Related by rotation around single bond (bending and twisting) Interchange without breaki ng covalent bonds o o Boat and Chair conformation a. Boat conformation: less stable due to steric hindrances b. Chair conformation: more stable *Axial Bond: perpendicular to plane *Equatorial Bond: parallel to plane
Monosaccharides Glucose
Central sugar in metabolism Can cycl ize through intermolecular nucleophil ic attack of one of the OH’s on the Carbonyl Carbon of the aldehyde Occurs if stable 5 or 6 member rings can form Furanose (5 member) or Pyranose (6 member) On nucleophilic attack to form the ring, carbonyl O becomes an OH
Fructose: 67% pyranos e, 33% furanose Ribose: 25%pyranose, 75% furanose * Glucose is exclusively pyranose. Fructose and Ribose a re exclusively furanose.
Monosaccharide Derivatives 1.
Sugar Acids Oxidized forms in which a ldehyde and/or al cohol functional groups are oxidized to carboxylic acid ( Oxidation) a. Aldonic Acid Aldehyde group is oxidi zed (Eg. Gluconic Acid) o b. Uronic Acid o Terminal a lcohol i s oxidized (Eg. Glucuronic Acid ) c.
Aldaric Acid o Both aldehyde and terminal alcohol are oxidized
2. Sugar Alcohol
Reduction of Carbonyl group to OH (-ol) (Eg. Dulcitol:
excess causes cataract in galactosemi a patients) 3.
4.
5.
6.
Phosphorylated Sugar Phosphate is added by ATP forming phosphoester derivatives Eg. Glucose-6-Phospha te Amino Sugars Amino group replaced hydroxyl group (-OH to -NH) Eg. Glucosamine, Galactosamine Acetylated Amine Derivative Sugars derived from amino sugars Eg. N-acetylglucosamine, N-acteylgalactosamine Lactone Forms
Glucose-6-Phosphate
Intramolecular esters Hydroxyl group attacks Car bonyl carbon that was pr eviousl y oxidi zed to Carboxylic acid (Eg. Gluconolactone) Deoxysugars One or more Carbon atoms have been reduced, losing hydroxyl group (-OH to -H) (Eg. Deoxyribose)
7.
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8.
Condensation Products of Sugar Derivatives with Lactate and Pyruvate Forms Muramic Acid (glucosamine + lactic acid) Forms Neuraminic Acid (mannosamine + pyruvic acid) N-acetylmuramic Aci d (MurNAc or NAM): GlcNac + la ctic a cid (ether li nk at C3) o found in bacterial cell walls Sialic Acids: o Found on surface of all cells o Involved in cell contact/communication o Involved i n recognition bacteria ( cholera) and vir uses (influenza) o N-acetyl-neuraminic Acid (NANA): N-acetylmannosamine (ManNac) + pyruvic acid found only in humans lack hydrolase gene (92 base pairs of gene missing) o N-glycoyl-neuraminic Acid: N-glycoylmannosamine + pyruvic acid Have hydroxylas e
Neuraminic Acid
Oligosaccharides
Polysaccharides Homopolysaccharides: polys accha rides with 1 type of repeating monosaccha ri de unit Starch: found in plants; composed of: Amylose (20%) o Linear chain of Glc in ɑ 1-4 links (or repeating maltoses) Amylopectin (80%) o Branched chain i n ɑ 1 -6 links o Major part: Glc chain of 24-30 units (amylose) then branches off (amylopectin) Glycogen o Main carbohydrate storage in animals o Composed of Glc residues in ɑ 1 -4 links and ɑ 1 -6 branches (greater than starch) o Synthesized on Glycogenin protein primer o Reason why glycogen i s s tored rather than glucos e: Has l ess os motic pr essur e than glucos e, therefore, does not easi ly reacts with water o Source: Muscles (greatest s ource in terms of total glycogen mass sourc e) and li ver (greatest source in terms of grams glycogen per gram tis sue) Marco Peri kar R. Di maano 1BMed Class 2015
Cellulose Linear chain of Glc residues in β1-4 li nks (or repeating cellobiose) o o Held together by intra- a nd inter-chain H-bonds o Most abundant biological molecule in nature; cannot be broken down by humans (l ack of cellula se) Chitin o Linear chain of GlcNAc in β1-4 links Heteropolysaccharides: polys accha rides with 2 different monosaccha ride units Complex Oligosaccharide Units Mucopolysaccharides/Glycosaminoglycans (MPS/GAG) o Amino sugar + negatively charged sulfate or carboxyl group (uronic acid: glucuronic or iduronic acid) o Form matrix to hold protein component of skin, connective tissue and extracellular matrix o Often covalently attached to proteins to form proteoglycans Hyaluronic Acid o Hyaluronic Acid Glucuronate(β1-3)GlcNAc Water s oluble; found in synovial fluid Backbone for attachment proteins Dermatan Sulfate o L-Iduronate(β1-3)GalNAc-4-Sulfate Chondroitin Sulfate o D-Glucoronate(β1-4)GalNAc-4or6-Sulfate Heparin o Chondroitin Sulfate D-Glucoronate-2-Sulfate(ɑ 1-4)GlcNSulfo-6-Sulfate Antithrombin, naturally-occurring anticoagulant o Keratan Sulfate D-Gal( β1-4)GlcNAc-6-Sulfate No uronic acid component o Syndecan Heparan Sulfate Binds through intracellular domain to the cytoskeleton Interacts with fibronectin Glypican Heparin o Attached to outer s urface of pla sma membrane via phosphatidyl i nositol lipid Peptidoglycans Bacterial Cell Walls o Offer protection from hypotonic condition and high i nternal osmotic pressure Long chain of GlcNAc( β1-4)MurNAc (NAG,NAM) Gram (+) Bacteria Multi-layered; cell wall can be Gram stained (violet) Chains are covalently connected by a Pentaglycine Bridge through the ε-Amino group of tetrapeptide Lysine on one strand and D-Alanine on another strand Teichoic Acid Alternating r esidue of D-Ala a nd NAG in C2 Gl ycerol or Ribitol Phosphate backbone Multiple glycerols are linked through Phosphodiester Bonds Often attached to C6 of NAM Make up 50% of cell wall dry weight Present a foreign a ntigenic sur face to infected host Serve as receptors for ba cteriophages
Dermata n Sulfate
Heparin
Keratan Sulfate
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Gram (-) Bacteria Cell wall cannot be Gram-stained (red) No pentaglycine bridge; chains are connected by direct amide bond between the ε-Amino group of tetrapeptide Lysi ne on one stra nd and D-Alani ne on another stra nd Hydrophobic protein covalently attaches (through Lys amide bond) to the last amino acid in the tetrapeptide unit of cell wall (actuall y diaminopimeli c aci d/DAP, which replaces 10% of D-Ala in cell wall) No teichoic aci d; Cell wall sandwiched between l ipi d bil ayer; Peripl as mic s pace – space between lipid bilayers Lipopolysaccharide (O anti gen) coats the outer membrane and determines a ntigenic ity of ba cteri a Proteoglycans GAG covalently O-linked to protein (usually to Ser residue o of Ser-Gly di peptides) o May contain N-li nked oli gosaccharide groups Carbohydrates > Proteins o o Soluble o CHO part provides an incredible variety of binding structures (acts linke glue) in connecting intra- and extracellula r cell functions Syndecan: protein + heparin sulfate + chondroitin sulfate; o binds through its intracellular domain to the internal cytoskeleton of the cell while interacting with fibronectin in the extracell ular matrix o Aggrecan: pr otein + Chondroitin sul fate + Keratan sulfate; binds hyal uronic ac id; important in hydration of cartila ges o Versican: protein + Chondroitin sul fate; binds hyaluronic aci d in extracell ular matrix Glycoproteins/Glycosylated Proteins o Proteins post-trans la tional ly modified by attachment of carbohydrates o Usual ly attached through either Asn or Ser side chai ns o Involved in recognition of binding molecules, prevention of aggregation during protein foldi ng, protection from preoteolys is , increase in protein half-life, blood clotting, immunologic protection and ABO blood groups. N-linked glycoproteins o Carbohydrate a ttached to either GlcNAc or GalNAc to an Asn in a X Asn-X-Thr sequence of protein Core oli gosaccharide: (Man)3 (GlcNAc) 2 attached to Asn 3 types: Mannose, Complex, Hybrid O-linked glycoproteins o Carbohydrate usually attached from a Gal( β1-3)GalNAc to a Ser or Thr of a protein Eg. Blood Group Antigens Storage Polysaccharides: Starch, Glycogen Structural Polysaccharides: Cellul ose, Chitin, GAGs, Peptidoglycans
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Hemiacetal and Hemiketal Formation
Aldehyde or ketone group of monosaccha ri des can cyclize through intramolecul ar nucleophil ic a ttack of a hydroxyl group ( -OH) at the Carbonyl car bon in a n additi on reaction forming Hemiacetal or Hemiketal, respectively. On addition of acid: anomeric –OH is protonated, forming water, a leaving group Another al cohol ca n be added forming Acetal or Ketal
Reducing Property of Sugars
Reducing Sugars: sugars which can form an aldehyde at C1 or have an ɑ -hydroxymethyl ketone group which ca n i somerize to an a ldehyde under bas ic conditions, such as fructose o Eg. All common monosaccharides, maltose o Eg. Lactose: Since Glc is attached through the OH on C4, its anomeric carbon could revert to noncyclic aldehyde form, which is susceptible to oxidation , thus, subsequently reduced. Non-Reducing Sugar: s ugars in whic h there are no al dehyde or ketone group to react; sugar ri ngs ar e locked or not capable of opening o Eg. Sucrose: Since the anomeric carbons of both Glc and Fru are linked, it cannot be reduced (neither of the rings can be opened). Tests for identifying Reducing Sugars: o Benedict’s: Copper Sulfate + Alkaline Citrate; deep blue brick red ppt o Fehling’s: Copper Sulfate + Alkali ne Tartra te; deep blue brick red ppt Tollen’s : Sil ver Nitrate + Aqueous Ammonia; col orles s silver mirror o
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