Biochemistry-Lehinger CH7
INCOMPLETE/UNSORTEDHome Intersection Biochemistry
Carbohydrates are the most abundant biomolecuels on earth Oxidation of carbohydrates is the central energy yielding pathway in most nonphotosyntehtic cells Insoluble carbohydrate polymers serve as structural and protective elements in the bact and plant cell walls Other carbohydrate polymers lubricate skeletal joints and participate in recognition and adhesion between cells Glyoconjugates: More complex carbs covalently attached to proteins or lipids act as singlas that determine the intracellular location or metabolic fate Carbs are polyhyroxy aldehydes or ketones that yield such compounds on hydrolysis Many have empirical formula (CH2O)n that may contain N, P or S Three Major Clases: consists of a single aldehyde or ketone Mono 5+ Cs tend to have cyclic Most abundant – diassharides Sucrose – d-Glucose+d-Fructose Colorless crystalline solids that are freely soluble in water but insoluble in nonpolar solvents Monosaccharide Families: 1) Aldose – aldehyde Simplest monosaccharides: glyceraldehydes Nomenclature: 8 d-aldohexoses have own name ALL 5+ C atoms are predominantly as cyclic strucutures – carbonyl group has formed a covalent bond with the O of a hydroxyl group along the chain Hemiacetals or hemiketals: Formation of these ring structures is the result of a general RXN between OH and OOH or =O to form derivatives Contain asymmetric carbon atom and thus can exist in two stereoisomeric forms Pyranose: 6 membered ring resemble pyran More stable than aldofuranose ring ONLY aldoses containing 5+ C atoms can form pyranose rings Furanose: 5 membered ring reselbe furan 2) Ketose – ketone Simplest monosaccharides: dihydroyacdtone Dihydroxyacetone – ONLY MONOSACHARIDE not containing 1+ chiral centers and occur optically active Nomenclature: 4-5C ketoses are designated by inserting “ul” into the name of the corresponding aldose Structural Characteristics: Chiral centers (general rule) 2^n where n is the number of chiral centers Stereoisomers divided into two groups that differ about the chiral center most distant from the carbonyl carbon Reference molecule – glyceraldehydes d glyceraldehyde – D isomers l glyceraldehydes – L isomers some occur naturally as L form Epimers: sugars that differ only in configuration around one C (EX d-glucose and d-mannose or d-glucose and d-galactose) Anomers: isomeric forms that differ only in their configuration about the hemiacetal/ketal carbon atom Anomeric carbon: hemiacetal carbon Mutarotation: in aq solutions alpha and beta interconversion Haworth perspective formulas – commonly used to show stereochemistry of ring forms of monosaccharides Pyranoses ARE NOT PLANAR – chair conformations Two conformations of a molecule are interconvertible without the breakage of covalent bonds Two configurations can be interconverted only by breaking a covalent bond Specific 3d conformations of the monosaccharide units are important in determining the monosaccharide units are important in determining biological properties and functions of some polysaccharides General Nomenclature: Tetrose – 4C Pentose – 5C Hexose – 6C d-glucose – aldo (most abundant – dextrose) d-fuctose – keto both most common in nature Heptose – 7C Organisms contain a variety of Hexose Derivatives Nubmer of sugar derivaties in which a OH group of a parent comound is replaced with another substituent or a carbon atom is oxidized to a carbonyl group Hydroxyl at C2 of the parent compound is replaced with an amino group Glucosamine, galatosamine, mannosamine the OH at C2 of the parent comound is replaced with an amino group Nearly always condensed with acetic acids as in N-acetylglucosamine – derivative is part of many structural polymers Oxidation of the carbon at the other end of the carbon chain forms the corresponding uronic acid – glucouronic, galactouronic, mannuronic Alsonic and uronic acids form stable intramollecular esters called lactones Salic acid – N-acetylneuramic acid (derivative of N-adetylmannosamine) component of many glycoproteins and glycolipids in animals Carboxylic acid – glucoronate, galactoruonate – carboxylic acid groups of acidic sugar derivatives ionized at pH7 Synthesis and metabolism of carbohydrates and the intermediates are very often not the sugars themselves but their physiological derivatives Condensation of phosphoric acid wit hone of the OH groups of a sugar forms a phosphate ester Sugar phosphates are relatively stable at neutral pH and bear a negative charge Sugar phosphorylation within cells is to trap the sugar inside the cell Most cells do not have the plasma membrane transporters for phosphorylated sugars Phosphorylation also activates sugars for subsequent chemical transformation Can be oxidized by relatively mild oxidizing agents such as ferric Fe+3 or cupric ions Cu+2 Reducing sugars: capable of reducing ferric or cupric Basis of Fehling’s reaction – test for reducing sugars, allowing the estimation of sugars, used as diabetic test
Disaccharides - contain Glycosidic Bond (gBOND) – concsit of two monosaccharides joined covalently by O-glycosidic bond EX of Disacc Malatose (glc and glc) a14 = reducing sugar – contains a free anomeric C (C1 of 2nd glucose) which can be alpha (trans) or beta (cis) Lactose (gal and glc) b14 = reducing sugar Sucrose (glc and fru) a1b2 = NONREDUCING sugar (glycoside) Trehalose (glc and glc) a1a1 = NONREDUCING sugar (glycoside) Hemolymph of insects serving as ENG storage O-glycosidic bond: formed when a OH of one sugar reacts with the anomeric carbon of the other RNX represents the formation of acetal from hemiacetal and OH Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base Disaccharides can behydrolyzed to yield their free monosaccharide components by bloiling in dilute acid N-glycosyl - bonds joint the anomeric carbon of a sugar to N in glycoproteins and nucleotides Oxidation of anomeric carbon by cupric or ferric ion occurs only with the linear form – exists in EQ with the cyclic forms When anomeric C is involved in glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a nonreducing sugar Reducing end: end of a chain with free anomeric C – not involved in gBOND Nomenclature: (three letter abbreviations are used) Configuration of the anomeric carbon joining the first monosaccharide (on the L) (use the Orgo-“yl”) (also include D or L isomerism) Name nonreducing residue Indicate in parentheses how two atoms are joined by gBOND Name second residue If there is third residue, describe the gBOND
Oligo Short chains of monosaccharides joined by glucosidic bonds Most consists of 3+ units do not occur as free entities
Poly (aka glycans) – DO NOT HAVE DEFINITE MWs 20+ monosaccharides Cellulose – linear chains Glycogen – branched Both are recurring units of dGlucose Glycans differ from each other in the identifty of their recurring monosaccharide units in the length of their chains types of bonds linking the units degree of branching Types of glycans: 1) Homopolysaccharides: contain ONE SINGLE TYPE of monomer
- Serve as STORAGE forms of monosaccharides that are used in fuels
-Amylose – contains long unbranched, d-glc residues connected by a14 (positive I(-3) test)
-Amylopectin – highly branched, glycosidic a14 and branches are a16 (every 24-30 residues) Glycogen – abundant in liver and skeletal muscle
-Long unbranched, a14
-Branched a16 (every 8-12 residues) As many nonreducing ends as its branches ONLY ONE REDUCING END Utilized as ENG source, glucose units are removed one at a time from the nonreducing ends Degrading E act only at nonreducing ends simultaneously at many branches Dextrans – bact and yeast
-Long branches a16
-Branched a13 with some a12 or a14 Dental plaque formed on surface of teeth Used in fractionation of proteins by size exclusion
- Serve as STRUCUTRAL elements in plant cell walls and animal exoskeletons
-Linear unbranched 10-15k b-d-glc units, b14 Chitin
-Linear unbranched N-acetylglucosamine, b14 2) Heteropolysaccharides: contain TWO+ DIFFERENT kinds Sereve as extracellular support for organisms Peptidoglycan and (saccharides for extracellular space) Lysozyme (tears, egg whites, bactphages, penicillin) breaks b14 Peptidoglycan: exact strucuture depends on bact species
-Linear Nag-Nam b14 Agar: argarose (double helix 3 residues – water trapped inside), agaropectin
-Argarose - linear d-gal and l-gal C3C6 Inert supports for electrophoresis – DNA sequencing, agar colonies, packaging pharmaceuticals
-Agaropectin – branched Glycosaminoglycans: Extracellular space is filled with extracellular matrix (ground substance) holds cells together in porous pathway for diffusion of nutrients and O to individual cells extMatrix composed of interlocking meshwork of heteropolysaccharides and fibrous proteins such as collagent, elastin, fibronectin and laminin Glycoaminoglycans: family of linear polymers composed of repeating disaccharide units One of the two are either NAG or NAM Other of the two are uronic acid (d-glucuronic or l-iduronic acids) Carboxylate groups are very high neg density Assume repulsive forces Comparison: Sugars and Proteins Sugars DO NOT HAVE DEFINITE MWs as opposed to proteins Proteins synthesized on a template of defined sequence and length Sugars – no template, based on enzymes that catalyze the polymeration with no specific stopping point (HOM)Glycan Stereochemistry: same principal as those governing polypeptide structure Subunits with more or less rigid structure is dictated by covalent bonds from 3d macromolecular strucutures that are stabilized by weak interactions within or between molecules Hbond, Hydropho, vdWall (Charged – electrostatic) Phi C1-O Psi O-C4 – free rotation around both linking residues Rotation is limited by steric hindrance of substituents Why Not Store Glucose In Monomeric Form? Hepatocytes store glycogen equivalently to a [glucose] of 0.4M Actual [glycogen] 0.01uM (microM) - insoluble and contributes little to the osmolarity of the cytosol If cytosol contained 0.4M of glucose, the osmolarity would be threateningly elevated rupture of cell and external [glucose] 5mM and the free energy change for glucose uptake into cells would be large
