Biochemistry Chapter 11: Carbohydrates and Glycoproteins PDF

Chapter 11 Carbohydrates and Glycoproteins © 2023 W. H. Freeman and Company  CHAPTER 11 Carbohydrates and Glycoproteins  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. Descri...

Chapter 11 Carbohydrates and Glycoproteins © 2023 W. H. Freeman and Company  CHAPTER 11 Carbohydrates and Glycoproteins  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.  Ch.11 Outline 11.1 Monosaccharides Are the Simplest Carbohydrates 11.2 Monosaccharides Are Linked to Form Complex Carbohydrates 11.3 Carbohydrates Can Be Linked to Proteins to Form Glycoproteins 11.4 Lectins Are Specific Carbohydrate-Binding Proteins  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 Click on an asymmetric carbon atom. (1 of 2) © Macmillan Learning, 2023 Click on an asymmetric carbon atom. (2 of 2) © Macmillan Learning, 2023 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  Pyranose and Furanose Rings Can Assume Different Conformations Pyranose rings are not planar because of the tetrahedral geometry of its saturated carbon atoms. They can adopt two types of conformation: boat and chair. In chair form, substituents on the carbon ring atoms can be axial (nearly perpendicular) or equatorial (nearly parallel). Axial substituents sterically hinder each other if on the same side of the ring. 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.  Envelope Conformations of Furanose Rings Furanose rings are not planar and commonly adopt a conformation called the envelope form. In the ribose moiety of most biomolecules, there are two common confirmations: – C-2-endo (C-2 is out of the plane on the same side as C-5) – C-3-endo (C-3 is out of the plane on the same side as C-5)  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 Cu 2+ 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  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, 150 different oligosaccharides have been identified in human milk – not digested by breastfed infant – play a significant role in protecting them against bacterial infection (e.g., Streptococcus bacteria that may be transmitted during vaginal childbirth) These oligosaccharides may: – serve as a fuel source for beneficial bacteria. – prevent the attachment of microbial pathogens to the intestinal wall of the newborn.  Glycogen and Starch Are Storage Forms of Glucose Free glucose cannot be stored because high concentrations will disturb the cell's osmotic balance. polysaccharides (glycans) = large polymeric oligosaccharides formed by the linkage of multiple monosaccharides – plays roles in energy storage and structural integrity homopolymer = polymer in which all the monosaccharide units are the same  Glycogen glycogen = large, branched polymer of glucose residues – most common homopolymer in animal cells – storage form of glucose – most glucose units are linked by α-1,4-glycosidic linkages – branches are formed by α-1,6-glycosidic linkages – hydrolyzed by α-amylase Branching increases the surface area to allow better access for enzymes to rapidly breakdown glycogen.  Starch starch = homopolymer that serves as the nutritional reservoir in plants – two forms: amylose and amylopectin amylose = unbranched type of starch composed of glucose residues in α-1,4 linkage amylopectin = branched type of starch with ~1 α-1,6 linkage per 30 α-1,4 linkages – identical structure to glycogen but with a lower degree of branching Amylose and amylopectin are hydrolyzed by α-amylase.  Cellulose Is the Main Structural Polysaccharide of Plants cellulose = unbranched polymer of glucose residues joined by β-1,4 linkages – serves a structural role instead of a nutritional role The β configuration allows cellulose to form long, straight chains that interact with one other through hydrogen bonds – yields a rigid, supportive structure The α linkages of starch and glycogen form compact hollow cylinders suitable for accessible storage. Glycosidic Linkages Determine Polysaccharide Structure  Insoluble and Soluble Fiber Are an Important Part of the Diet Mammals cannot digest cellulose because they lack cellulases, but plant fibers are still important in the mammalian diet. Insoluble fibers increase the rate at which digestion products pass through the large intestine. – softens stools and makes them easier to pass Soluble fibers (e.g., pectin or polygalacturonic acid) slow the movement of food through the gastrointestinal tract. – facilitates absorption of nutrients from the diet  Chitin Is the Main Structural Polysaccharide of Fungi and Arthropods chitin = homopolymer of β-1,4 linked N-acetylglucosamine – found in fungal cell walls and exoskeletons and shells of arthropods – Fibers are often crosslinked and composited with minerals and proteins to increase rigidity and strength.  Chitin Can Be Processed to a Molecule with a Variety of Uses Cellulose is a major constituent of paper, bioadhesives, and clothes. Chitin could be recovered from the shellfishing industry by processing the shells into the more versatile chitosan through microbial/enzymatic processes. Chitosan can be used as: – a carrier to assist in drug delivery. – a component of cosmetic and food products. – a surgical dressing.  Section 11.3 Carbohydrates Can Be Linked to Proteins to Form Glycoproteins glycoprotein = a carbohydrate group covalently attached to a protein – makes up 50% of the human proteome glycosylation increases the complexity of the proteome – glycoforms = different glycosylated forms – may occur when a protein has several potential glycosylation sites  Three Classes of Glycoproteins glycoproteins = predominantly proteins – play a variety of roles, including cell adhesion proteoglycans = predominantly carbohydrates and the protein component is conjugated to a glycosaminoglycan – function as structural components and lubricants mucins (mucoproteins) = predominantly carbohydrates and the protein components is extensively glycosylated at Ser or Thr residues, usually by N-acetylgalactosamine – key component of mucus – function as lubricants  Carbohydrates Can Be Linked to Proteins Through N-Linked or O-Linked N-linkage = links the sugars in glycoproteins to the amide nitrogen atom in the side chain of Asn – Asn must be part of an Asn-X- Ser or Asn-X-Thr sequence, where X is any residue except proline O-linkage = links the sugars in glycoproteins to the oxygen atom in the side chain of Ser or Thr  N-Linked Oligosaccharides Have a Common Core N-linked polysaccharides have a common pentasaccharide core that consists of three mannoses and two N-acetylglucosamine residues. Click on the N-linkage. (1 of 2) © Macmillan Learning, 2023 Click on the N-linkage. (2 of 2) © Macmillan Learning, 2023 The Glycoprotein Erythropoietin Is a Vital Hormone erythropoietin (EPO) = a glycoprotein secreted by the kidneys into the blood serum to stimulate production of red blood cells – cloned recombinant form has improved treatment for anemia, but has been abused by some endurance athletes – glycosylation enhances the stability of the protein in the blood  Oligosaccharides Attached to Erythropoietin N-glycosylated at three Asn residues O-glycosylated a Ser residue 40% carbohydrate by weight  Glycosylation Functions in Nutrient Sensing GlcNAcylation = the post- translational, covalent attachment of a single N- acetylglucosamine (GlcNAc) to Ser or Thr residues of proteins – catalyzed by O-GlcNAc transferase – occurs when nutrients are abundant – reversible  O-GlcNAc Transferase GlcNAcylation sites are also potential phosphorylation sites. – O-GlcNAc transferase and protein kinases may be involved in cross talk. Improper regulation of O-GlcNAc transferase has been linked to: – insulin resistance. – diabetes. – cancer. – neurological pathologies.  Proteoglycans Have Important Structural Roles Proteoglycans are up to 95% glycosaminoglycan by weight) – resembles a polysaccharide more than a protein Proteoglycans: – function as lubricants and structural components in connective tissue. – mediate adhesion of cells to extracellular matrix. – bind factors that regular cell proliferation.  Glycosaminoglycans glycosaminoglycans = composed of repeating units of disaccharides containing a derivative of an amino sugar – amino sugar derivative is either glucosamine or galactosamine – at least one of the two sugars in the unit has a negatively charged carboxylate or sulfate group The inability to degrade glycosaminoglycans causes diseases marked by skeletal deformities and reduced life expectancies. Glycosaminoglycans Are Made of Repeating Units  Proteoglycans Are Important Components of Cartilage Cartilage contains the protein collagen protein and the proteoglycan aggrecan. aggrecan = large molecule with three globular domains – site of glycosaminoglycan (keratan sulfate and chondroitin sulfate) attachment is in the extended region between G2 and G3 – G1 noncovalently binds to a central polymer of hyaluronate The Proteoglycan From Cartilage Has an Enormous and Complex Structure  Aggrecan Cushions Compressive Forces Water is bound to the glycosaminoglycans to cushion compressive forces. – Water is squeezed from the glycosaminoglycan under pressure. – Water rebinds when pressure is released. osteoarthritis = form of arthritis that results when water is lost from proteoglycan with aging  Mucins Are Glycoprotein Components of Mucus tandem repeats (VNTR) region = region of the protein backbone of mucins that is rich in O-glycosylated Ser and Thr residues Core carbohydrate structures are conjugated to the protein component of mucin.  Functions of Mucins Mucins: – adhere to epithelial cells and act as a protective barrier. – hydrate the underlying cells. – play roles in fertilization, the immune response, and cell adhesion. Overexpression occurs in bronchitis, cystic fibrosis, and adenocarcinomas.  Cartilage contains: (1 of 2) a. aggrecan, a proteoglycan that provides structure and tensile strength. b. collagen, a triple helix of protein that serves as a shock absorber. c. proteoglycan monomers that emerge laterally from opposite sides of a central hyaluronate filament. d. multiple polymers of heparin and dermatan sulfate. e. covalent bonds between the G1 domain of aggrecan and the central polymer of hyaluronate. © Macmillan Learning, 2023  Cartilage contains: (2 of 2) a. aggrecan, a proteoglycan that provides structure and tensile strength. b. collagen, a triple helix of protein that serves as a shock absorber. *c. proteoglycan monomers that emerge laterally from opposite sides of a central hyaluronate filament. d. multiple polymers of heparin and dermatan sulfate. e. covalent bonds between the G1 domain of aggrecan and the central polymer of hyaluronate. © Macmillan Learning, 2023 Protein Glycosylation Takes Place in the Lumen of the Endoplasmic Reticulum and in the Golgi Complex Endoplasmic reticulum (ER) and Golgi complex are organelles that play central roles in protein trafficking. N-linked glycosylation begins in the ER and continues in the Golgi complex. O-linked glycosylation occurs only in the Golgi complex.  Dolichol Phosphate dolichol phosphate = specialized lipid molecule located in the ER membrane – contains about 20 isoprene (C5) units. – location where large oligosaccharides destined for attachment to the Asp residues are assembled – the terminal phosphate is the site of attachment  The Golgi Complex Is a Sorting Center Golgi complex = a stack of flattened membranous sacs proteins proceed to lysosomes, secretory granules, or the plasma membrane – based on signals encoded within their amino acid sequences and three-dimensional structures  Specific Enzymes Are Responsible for Oligosaccharide Assembly glycosyltransferases = catalyze the formation of glycosidic linkages Activated sugar nucleotides are the most common carbohydrate donor for glycosyltransferases. Blood Groups Are Based on Protein Glycosylation Patterns Blood groups are designated by the presence of one of the three different carbohydrates (A, B, or O) attached to glycoproteins and glycolipids on the surfaces of red blood cells. All blood groups have a core O antigen. Specific Glycosyltransferases Add the Extra Monosaccharide to the O Antigen A and B antigens have one extra monosaccharide through an α-1,3 linkage to a galactose moiety of the O antigen – added by specific glycosyltransferases type A transferase = adds N-acetylgalactosamin to form the A antigen type B transferase = adds galactose to form the B antigen The A, B, and O Oligosaccharide Antigens Share a Common Core Structure Blood Type Phenotypes Result from the Enzymes Present Individuals with the: – O blood type lack both enzymes. – AB blood type express both enzymes. – A blood type express only type A transferase. – B blood type express only type B transferase. have important implications for blood transfusions. – If an antigen not normally present is introduced, the immune system recognizes it as foreign.  The Cholera Toxin cholera = disease caused by a toxin from Vibrio cholerae Individuals with blood type O are ~8 times more likely to have severe disease. – The O antigen binds more tightly to the toxin than other blood type antigens.  Errors in Glycosylation Can Result in Pathological Conditions congenital disorders of glycosylation = pathological conditions resulting from improper modification of proteins by carbohydrates and their derivatives – examples: certain types of muscular dystrophy are linked to improper glycosylation and I-cell disease  I-Cell Disease (1 of 2) lysosomes = organelles that degrade and recycle damaged cellular components or endocytosed material I-cell disease = a lysosomal storage disease that causes severe psychomotor impairment and skeletal deformities – affected lysosomes contain undigested glycosaminoglycans and glycolipids – active enzymes responsible for degradation are synthesized – enzymes lack appropriate glycosylation and are exported instead of being sequestered in lysosomes  I-Cell Disease (2 of 2) A mannose 6-phosphate residue of the N-oligosaccharide directs the enzymes from the Golgi complex to lysosomes. In I-cell disease, the mannose lacks a phosphate because patients are deficient in the N-acetylglucosamine phosphotransferase. Biochemists Use Several Techniques to Analyze Oligosaccharide Components of Glycoproteins Oligosaccharides can be detached from the protein using enzymes that cleave oligosaccharides at specific linkages. Mass of an oligosaccharide can be determined using MALDI-TOF or other mass spectrometric techniques. – Many possible oligosaccharide structures have the same mass.  Determining Oligosaccharide Structure and Points of Attachment Structure can be determined by combining additional cleavage of the oligosaccharide with mass spectrometry. Points of attachment can be determined by applying proteases to glycoproteins and performing chromatography. – Fragments attached to oligosaccharides have chromatographic properties that with protease treatment. – followed by mass spectrometry or direct peptide sequencing Oligosaccharides can be sequenced by using enzymes that cleave specific glycosidic bonds. MALDI-TOF is used to identify the released sugars.  Oligosaccharides Can Be Characterized by Mass Spectrometry  Section 11.4 Lectins Are Specific Carbohydrate-Binding Proteins glycan-binding proteins = bind to specific carbohydrate structures on neighboring cell surfaces lectins = class of glycan-binding proteins – example: the mannose 6-phosphate receptor that binds and directs lysosomal enzymes to the lysosome  Lectins Promote Interactions Between Cells and Within Cells Lectins: – function to facilitate cell–cell contact. – usually contains 2+ carbohydrate-binding sites. – are linked to carbohydrates by a number of weak noncovalent interactions.  Lectins are Organized into Two Large Classes C-type (calcium-requiring) lectins = found in animals – function in receptor-mediated endocytosis and cell–cell recognition L-type lectins = rich in seeds of leguminous plants – serve as potential toxins to herbivorous insects – come act as chaperones in the eukaryotic ER  C-Type Lectins Use Calcium Ions to Bind Carbohydrates Ca2+ on lectin acts as a bridge between lectin and the sugar. Two Glu residues in lectin bind to Ca2+ and the sugar. Other hydrogen bonds form between lectin side chains and the carbohydrate.  Selectins selectins = members of C-type lectins – bind immune-system cells to sites of injury in the inflammatory response – play a role in recruiting leukocytes to inflammation sites different forms of selectins: – L form = bind to carbohydrates on lymph-node vessels – E form = bind to carbohydrates on endothelium – P form = bind to carbohydrates on activated blood platelets Click on the bridge between lectin and the sugar. (1 of 2) © Macmillan Learning, 2023 Click on the bridge between lectin and the sugar. (2 of 2) © Macmillan Learning, 2023  Influenza Virus Binds to Sialic Acid Residues hemagglutinin = influenza virus lectin protein that binds to carbohydrates sialic acid residues linked to galactose residues on cell-surface glycoproteins – the virus is engulfed after binding The virus replicates inside the cell and viral particles bud off from the cell.  Neuraminidase Cleaves Oligosaccharide Chains Assembled viral particles are attached to sialic acid residues of the cell membrane by hemagglutinin. neuraminidase (sialidase) = influenza virus protein that cleaves the glycosidic linkages between sialic acid and the rest of the glycoprotein – frees the virus to infect new cells – inhibitors of neuraminidase (Tamiflu and Relenza) are important anti-influenza agents Influenza Virus Uses a Lectin for Specific Cell Binding Remember to complete your exit poll tonight! © Macmillan Learning, 2023