BCH3033 Stryer Biochemistry 10e Chapter 11 Lecture Slides PDF

Chapter 11 Carbohydrates and Glycoproteins © 2023 W. H. Freeman and Company  Ch.11 Learning Goals By the end of this chapter, you should be able to: 1. Describe the structure and main roles of carbohydrates in nature. 2. Describe how simple carbohydrates are linked to form com...

Chapter 11 Carbohydrates and Glycoproteins © 2023 W. H. Freeman and Company  Ch.11 Learning Goals By the end of this chapter, you should be able to: 1. Describe the structure and main roles of carbohydrates in nature. 2. Describe how simple carbohydrates are linked to form complex carbohydrates. 3. Explain how carbohydrates are linked to proteins and what functions the linked carbohydrates play. 4. Describe the three main classes of glycoproteins and explain their biochemical roles. 5. Define what lectins are and outline their biochemical functions.  Section 11.1 Monosaccharides Are the Simplest Carbohydrates carbohydrates = carbon-based molecules high in hydroxyl groups – empirical formula: (CH2O)n – can have additional groups or modifications – better described as polyhydroxy aldehydes and ketones (and their derivatives) Monosaccharides are aldehydes or ketones that contain two or more hydroxyl groups. The smallest monosaccharides are composed of three carbons. Monosaccharides exist in many isomeric forms.  Monosaccharides monosaccharides = carbohydrates that are three to seven carbons in length – also called simple sugars  Monosaccharide Nomenclature Nomenclature is based on carbon-chain length: – three carbons: trioses – four carbons: tetroses – five carbons: pentoses – six carbons: hexoses – seven carbons: heptoses Nomenclature is also based on the identity of the most oxidized group: – keto group: ketose – aldehyde group: aldose  Isomers constitutional isomers = molecules with identical molecular formulas that differ in how the atoms are ordered stereoisomers = molecules that differ in spatial arrangement but not bonding order – have either D or L configuration – can be enantiomers (mirror images of each other) or diastereoisomers (not mirror images of each other) – number possible = 2n where n is the number of asymmetric carbon atoms Isomeric Forms of Carbohydrates  Common Monosaccharides epimers = sugars that are diastereoisomers differing in configuration only at a single asymmetric center  Most Monosaccharides Exist as Interchanging Cyclic Forms An aldehyde can react with with an alcohol to form a hemiacetal A ketone can react with an alcohol to form a hemiketal  Pyranose Formation called pyranose because of similarity to pyran  Furanose Formation called furanose because of similarity to furan  Anomers of Glucose anomer = a diastereoisomeric form of sugars that forms when a cyclic hemiacetal is formed and an additional asymmetric center is created In glucose, C-1 (the anomeric carbon atom) becomes an asymmetric center, forming two ring structures: – α-D-glucopyranose (hydroxyl group attached to C-1 is on the opposite side of the ring as C-6) – β-D-glucopyranose hydroxyl group attached to C-1 is on the same side of the ring as C-6)  D-Fructose Rapidly Interchanges Between Four Distinct Ring Structures C-2 is the anomeric carbon atom. The pyranose form predominates in solution due to reduced steric hindrances. The furanose form predominates in fructose derivatives. The Most Common Monosaccharides Exist Primarily in Their Ring Forms Chair and Boat Forms of β-D-Glucose The chair form predominates because all axial positions are occupied by hydrogens. The boat form is disfavored because it is sterically hindered.  D-Glucose Is an Important Fuel for Most Organisms blood sugar = D-glucose circulating in the blood – only fuel used by the brain in non-starvation conditions – only fuel used by red blood cells potential reasons why D-glucose an important fuel: – glucose is formed from formaldehyde under prebiotic conditions and may have been available as a fuel source for primitive biochemical systems – glucose is relatively inert – the most stable ring structure is β-D-glucopyranose  Solutions of Glucose The two anomeric forms (α and β) are in an equilibrium that passes through the open-chain form. There is a roughly 2:1 ratio of β-to-α anomer conformations for D-glucose in an equilibrium solution.  D-Glucose Is a Reducing Sugar and Reacts Nonenzymatically with Hemoglobin In its linear form, glucose can react with oxidizing agents. example: linear glucose reacts with Cu2+ yielding Cu+ and gluconic acid  Reducing Sugars Fehling's solution = solutions of Cu2+ that test for the presence of sugars that adopt an open structure reducing sugars = sugars that react with oxidizing agents – all monosaccharides that can adopt linear structures in solution non-reducing sugars = sugars that do not react with oxidizing agents  https://i.pinimg.com/originals/39/92/ e7/3992e73dbb2702f3de3bca98c5ebd3b4.jpg A solution of glucose contains one-third α anomer, two-thirds β anomer, and about 1% open chain. Mutarotation – change in the specific rotation of a cyclic sugar as it reaches an equilibrium between its alpha and beta anomeric forms. The two anomeric forms are in equilibrium that passes through an open-chain form, and the free open-chain form reacts with oxidizing agents. Sugars that react with oxidizing agents are called reducing sugars, whereas those that do not are nonreducing sugars.  Glucose is a Reducing Sugar Glucose can react with hemoglobin, forming glycated hemoglobin (hemoglobin A1c), which is fully functional https://www.medicographia.com/wp-content/uploads/ https://biologicmodels.com/wp-content/uploads/2016/05/ 2009/12/46.jpg HbA1c_fcs_shapeways_vB1.png  Glycation of Sugars glycation = nonenzymatic addition of a carbohydrate to another molecule – can be benign or detrimental example: Reducing sugars nonspecifically react with free amino groups on proteins (often Lys or Arg) to form a stable covalent bond. D-glucose has a low tendency to glycate proteins unless concentrations of sugar and protein are very high for long periods of time.  Advanced Glycation End Products (AGEs) advanced glycation end products (AGEs) = products resulting from cross-linking following the primary modification – implicated in aging, arteriosclerosis, diabetes, and other pathological conditions  Assessing Treatments for Diabetes Mellitus by Monitoring A1C Levels D-glucose reacts with hemoglobin to form glycated hemoglobin (hemoglobin A1c, A1C). – has no effect on O2 binding In nondiabetic individuals,