Biochemistry: Complex Carbohydrates PDF

BIOCHEMISTRY CARBOHYDRATES: Jandoc, M.D. Complex Carbohydrates TOPIC OUTLINE 1. Responsible for the viscous lubricating properties of mucous I. O...

BIOCHEMISTRY CARBOHYDRATES: Jandoc, M.D. Complex Carbohydrates TOPIC OUTLINE 1. Responsible for the viscous lubricating properties of mucous I. Overview of Glycosaminoglycans secretions (led to the original naming of these compounds as A. GAGs mucopolysaccharides) B. Funtion 2. Stabilize and support cellular and fibrous component of tissue II. Structure of Glycosaminoglycan while helping the water and salt balance of the body A. A.Relationship between glycosaminoglycan 3. Connective tissues (in skin tendons, cartilage, ligaments, bone structure and function matrix) consist of insoluble protein distributed in the ground B. B.Classification of the glycosaminoglycans substance III. Synthesis of GAGs  Character of Connective Tissue - dependent to a large A. Synthesis of Amino Sugars extent on the relative proportions of the ground substance B. Synthesis of Acidic Sugars and embedded fibrous proteins C. Core Protein Synthesis i. Cartilage D. Synthesis of the Carbohydrate Chain – rich in ground substance E. Addition of Sulfate Groups ii. Tendon IV. Degradation of GAGs – composed primarily of fibers A. Phagocytosis of Extracellular GAGs iii. Synovial Fluid B. Lysosomal Degradation of GAGs – example of ground substance V. Mucopolysaccharide – lubricant in joints, tendon sheaths and bursae A. Characteristics 4. Essential component of extracellular matrix (in eukaryotes and in B. Other Disorders resulting from a deficiency of bacterial biofilms) → mediation of cell-cell interactions Lysosomal Enzymes VI. Overview of Glycoproteins II. STRUCTURE OF GLYCOSAMINOGLYCAN VII. Structure of Glycoprotein Oligosaccharides GAGs VIII. Synthesis of Glycoprotein  long, unbranched (in most IX. Structure of Proteoglycans cases), heteropolysaccharide X. Bacterial Cell Wall chains XI. Lysosomal Glycoprotein Degradation  composed of repeating XII. Summary disaccharide units (acidic sugar - amino sugar) n I. OVERVIEW OF GLYCOSAMINOGLYCANS a. Amino Sugar GROUND SUBSTANCE - amino group is usually acetylated → positive charge – viscous, clear substance eliminated – occupies the space between the cells and fibres within - may be sulfated on carbon 4 or 6 or on a connective tissue nonacetylated nitrogen Three Main Types of Macromolecules 1. Proteoglycans i. D-glucosamine ii. D-galactosamine 2. Glycoproteins 3. Fibrous Proteins  fibrous proteins and glycoproteins are embedded in a proteoglycan gel, where they form an extensive extracellular matrix that serves both structural and adhesive functions A. GAGs  large complexes of negatively charged heteropolysaccharides chains generally associated with a small amount of protein b. Acidic Sugar  generally associated with a small amount of protein - contain carboxyl groups that are negatively charged proteoglycans (consist of over 95% carbohydrate) at physiologic pH + sulfate groups → strongly negative Glycoproteins – consist primarily of protein with a nature small amount of carbohydrate i. D-glucuronic acid ii. L-iduronic acid  special ability to bind large amounts of water → gel-like matrix → body’s ground substance (with fibrous components such as collagen → extracellular matrix)  rigid, linear polysaccharides B.Functions ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 1 of 9  BIOCHEMISTRY CARBOHYDRATES: Complex Carbohydrates Note: A single exception is keratan sulfate, in which galactose rather Glycosaminoglycans Repeating Unit Tissue Distribution than an acidic sugar is present. Glucoronic acid–N- Hyaluronic acid Joint fluid, eye fluid acetylglucosamine A. Relationship between glycosaminoglycan structure and function Glucoronic acid–N- Chondroitin sulfate Cartilage, bone  Because of their large number of acetylglucosamine negative charges, these Galactose acid–N- Keratan sulfate Cartilage heteropolysaccharide chains: acetylgalactosamine  tend to be extended in solution Glucoronic acid–  repel each other Heparan sulfate Lung, muscle, liver glucosamine  surrounded by a shell of water Iduronic acid–N- molecules Dermatan sulfate Skin, lung acetylgalactosamine  when brought together → slip past each other (like 2 magnets of same 2. Types of GAGs polarity) → slippery consistency of a. Keratan Sulfate mucous secretions and synovial fluid - galactose rather than acidic sugar is present  when solution of GAGs is compressed b. Heparin → water is squeezed out and GAGs - free glycosaminoglycan occupy smaller volume → release of - anticoagulant pressure → spring back to original - intracellular component of mast cells hydrated volume because of the - found - near the walls of blood vessels repulsion of their negative charges → - endothelial cell surface resiliency of synovial fluid and vitreous c. Hyaluronic Acid humor of the eye - composed of D-glucuronate linked β (1→3) to N- B. Classification of the glycosaminoglycans acetylglucosamine which in turn is 1. The six major classes of glycosaminoglycans are divided - linked β (1→4) to the next glucuronate residue according to: - polyanionic nature → hyaluronic acid forms a. Monomeric compositions viscoelastic solutions → effective biological shock b. Type of glycosidic linkages absorber and lubricant c. Degree and location of their sulfate units d. Other Types of GAGs - composed of sulfated disaccharide units i. Chondroitin sulfate ii. Dermatan sulfate iii. Keratan sulfate III. SYNTHESIS OF GAGs  polysaccharide chains are elongated by the sequential addition of alternating acidic and amino sugars donated by their UDP- derivatives  The reactions are catalyzed by a family of specific glycosyltransferases  occurs in the: - Endoplasmic reticulum - Golgi apparatus A. Synthesis of Amino Sugars ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 2 of 9  BIOCHEMISTRY CARBOHYDRATES: Complex Carbohydrates  very active in connective tissues (20% of glucose flows to this  acylated at a different site pathway)  usually found as terminal carbohydrate residue of a. Amino Sugars are essential components of: oligosaccharide side chains of - GAGs  glycoproteins - glycoproteins  glycolipids - glycolipids  GAGs - some oligosaccharides - some antibiotics b. F6P - precursor of: i. N-acetylglucosamine ii. N-acetylgalactosamine iii. Sialic acids including N-acetylneuraminic acid (NANA) b.Carbons and Nitrogens in NANA Come From c. Hydroxyl Group of the precursor  N-acetylmannosamine - replaced by amino group (amino groups are almost  PEP always acetylated) donated by glutamine c. N-Acetylneuraminate-CMP-Phosphorylase 1. N-Acetylglucosamine and N-Acetylgalactosamine  removes pyrophosphate from CTP then attaches o Glutamine remaining CMP to NANA - provides the amino group replacing the hydroxyl B. Synthesis of Acidic Sugars group of the precursor  D-Glucuronic acid and L-iduronic acid are essential components of GAGs 1. D-Glucuronic Acid - structure is that of glucose with an oxidized carbon 6 (- CH2OH → -COOH) - required in detoxification reactions of insoluble compounds · bilirubin · steroids · several drugs - precursor of ascorbic acid a. Uronic Acid Pathway - provide a mechanism by which dietary D-xylulose can enter the central metabolic pathways b. Obtained From - diet (small amount) - intracellular lysosomal degradation of GAGs - uronic acid pathway c. End Product of Metabolism - D-xylulose 5-phosphate 2. NANA - enters hexose monophosphate pathway → produce  acidic monosaccharide – G3P  member of the family of sialic acids – F6P  reacts with cytosine triphosphate → converted into its active form → added to growing oligosaccharide a. Sialic Acids ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 3 of 9  BIOCHEMISTRY CARBOHYDRATES: Complex Carbohydrates d. UDP-Glucuronic Acid Formation - active form of glucuronic acid - produced by oxidation of UDP-glucose - donates sugars in: · GAG synthesis · other glucuronylating reactions 2. L-Iduronic Acid - carbon 5 epimer of D- 1. 3’-Phosphoadenosyl-5’-Phosphosulfate (PAPS) glucuronic acid - molecule of AMP with a sulfate group attached to the 5’-phosphate - synthesis occurs after D- - source of sulfate glucuronic acid has been - sulfur donor in glycosphingolipid synthesis incorporated into the 2. Sulfotransferase carbohydrate chain - sulfation of carbohydrate chain at specific sites 3. Defect of Sulfation - one of several autosomal recessive disorders →  Uronosyl 5-Epimerase defect in the proper development and maintenance - epimerization of the D- to the L-sugar of the skeletal system C. Core Protein Synthesis  synthesized on and enter the ER  glycosylated by membrane-bound transferases as it moves through the ER IV. DEGRADATION OF GAGs 1. In Lysosomes - contain hydrolytic enzymes which are active at pH 5 → acid hydrolases - low optimum pH is a protective mechanism for the cell D. Synthesis of the Carbohydrate Chain should lysosomal enzymes leak to the cytosol of neutral  initiated by the transfer of a xylose from UDP-xylose to the pH hydroxyl group of a serine (or threonine) catalyzed by 2. GAGs xylosyltransferase → 2 galactose molecules added → - short half-lives (3 days for hyaluronic acid, 10 days for completion of trihexoside linkage region → sequential addition chondroitin and dermatan sulfate) - except keratan sulfate (120 days) of alternating acidic and amino sugars and conversion of some D-glucuronyl to L-iduronyl residues A. Phagocytosis of Extracellular GAGs E. Addition of Sulfate Groups  phagocytosed material in a vesicle → fuse with lysosome →  sulfation of the carbohydrate chain occurs after the phagolysosome → GAG degradation monosaccharide to be sulfated has been incorporated into the growing carbohydrate chain B. Lysosomal Degradation of GAGs ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 4 of 9  BIOCHEMISTRY CARBOHYDRATES: Complex Carbohydrates  require large number of hydrolytic enzymes  polysaccharide chain cleavage by endoglycosidases → B. Other Disorders resulting from a deficiency of Lysosomal Enzymes oligosaccharides → further degradation - some lysosomal enzymes required for GAG degradation also participate in glycolipids and glycoprotein degradation → CLINICAL CORRELATES individual suffering from mucopolysaccharidosis → also have Glycosaminoglycans are important components of the fluid in joints lipidosis and glycoprotein-oligosaccharidosis (synovial fluid) and the vitreous humor of the eye. These solutions are mucous and highly compressible because water can be VI. OVERVIEW OF GLYCOPROTEINS ‘‘squeezed out’’ from between the chains. Patients with osteoarthritis have a relative deficiency of these important A. Glycoproteins ‘‘cushioning’’ molecules, resulting in damage to the joint. - proteins to which oligosaccharides are covalently attached · polypeptide chains are encoded by nucleic acids V. MUCOPOLYSACCHARIDOSES · oligosaccharide chains are products of enzymatic reactions  hereditary disorders (1:25,000 births) - relatively short carbohydrate chain (2-10 sugar residues) but  caused by a deficiency of any one of the lysosomal hydrolases can be long (longer for GAGs but can be very long)  clinically progressive · often branched · may or may not be negatively charged A. Characteristics - contain variable amount of carbohydrate 1. Accumulation of GAGs in Various Tissues · do not have serial repeats (glycosaminoglycans have - skeletal and extracellular matrix deformities, mental diglucosyl repeats) retardation 1. IgG - tissues most affected are those that produce large - contain 20% carbohydrate - deficiency of one of the lysosomal hydrolases normally 3. Mucin (Human Gastric Glycoprotein) involved in the degradation of heparin sulfate and/or  contain >80% carbohydrate dermatan sulfate → incomplete lysosomal degradation of glycosaminoglycans → presence of oligosaccharides in B. Physiologic Functions urine (which may be used to identify the structure present  structural molecules on the nonreducing end of the oligosaccharide → (components of cell walls and identification of the specific mucopolysaccharidosis) membranes) 3. Confirmatory Diagnosis  certain hormones (hCG, - measuring the patient’s cellular level of the lysosomal thyrotropin) hydrolases  immunologic components 4. Children who are Homozygous to the Disease (immunoglobulins, - apparently normal at birth → gradually deteriorate complement, interferon) - severe cases → death in childhood  cell surface recognition (by 5. All of the Deficiencies other cells, hormones, viruses) - autosomal and recessively inherited  cell surface antigenicity (blood - except Hunter’s syndrome (x-linked) group antigens) 6. Treatment  components of extracellular - no effective therapy at present matrix 7. Prenatal Diagnosis  component of mucin of GIT - possible and GUT as protective biologic 8. Bone Marrow Transplants lubricants - used successfully to treat Hunter syndrome  almost all of the human plasma - transplanted macrophages produce the sulfatase needed globular proteins and secreted to degrade glycosaminoglycans in the extracellular space enzymes and proteins are glycoproteins VII. STRUCTURE OF GLYCOPROTEIN OLIGOSACCHARIDES  oligosaccharide components of glycoproteins are branched heteropolymers composed of: ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 5 of 9  BIOCHEMISTRY CARBOHYDRATES: Complex Carbohydrates 1. D-Hexoses 2. Neuraminic Acid - in some cases - 9 carbon acidic monosaccharide 3. L-Fucose (6-Deoxyhexose) A. Structure of the Linkage Between Carbohydrate and Protein 1. N-Glycosidic Link - sugar chain is attached to the amide group of an asparagine side chain 2. O-Glycosidic Link - sugar chain is attached to the hydroxyl group of either a serine or threonine R-group Collagen – O-glycosidic linkage between galactose or glucose and the b. Pentasaccharide Core hydroxyl group of hydroxylysine · common to all types · linked to asparagine via N-acetylglucosamine B. N- and O-Linked Oligosaccharides  a glycoprotein may contain only 1 type of glycosidic linkage or may have both O- and N-linked oligosaccharides within the same molecule 1. O-Linked Oligosaccharides - membrane glycoprotein components - extracellular glycoproteins - ex: O-linked oligosaccharides → ABO blood group determinants VIII. SYNTHESIS OF GLYCOPROTEINS  most proteins are: – destined for the cytoplasm – synthesized on free ribosomes in the cytosol  proteins destined for: - cellular membranes - lysosomes - or to be exported from the cell  synthesized on ribosomes attached to the rough endoplasmic reticulum  contain signal sequences at their N-terminal end - direct proteins to their proper destinations - allow the growing polypeptide to be extruded into the lumen of the rough endoplasmic reticulum → transported via secretory vesicles → Golgi complex - glycoproteins to become components of the cell 2. N-Linked Glycoproteins membrane → integrated into Golgi membrane  sugars are attached via the amide NH2 group of an (carbohydrate portion oriented toward the lumen) asparagine residue - glycoproteins to be secreted from the cell → remain free a. 3 Major Classes in the lumen → vesicles bud off from the Golgi → fuse i. High mannose with cell membrane → release of free glycoprotein add ii. Hybrid the membrane-bound proteins of the vesicle to the cell iii. Complex membrane → carbohydrate portion on the outside of the cell A. Carbohydrate Components of Glycoproteins 1. Sugar Nucleotides - precursors of the carbohydrate components of glycoproteins - includes: · UDP-glucose ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 6 of 9  BIOCHEMISTRY CARBOHYDRATES: Complex Carbohydrates · UDP-galactose · UDP-N-acetylglucosamine · UDP-N-acetylgalactosamine · Others – GDP-mannose – GDP-L-fucose (synthesized from GDP-mannose) – CMP-NANA · negative charge at physiologic pH · may donate sugars to the growing chain 2. Oligosaccharides are covalently attached to specific amino acid R-groups of the protein - the 3 dimensional structure of the protein determines whether or not a specific amino acid R-group is 3. Fates of N-Linked Glycoproteins glycosylated - released by the cell B. Synthesis of O-Linked Glycosides - become part of cell membrane  the protein to which oligosaccharides are to be attached is - translocated to lysosomes synthesized on the ER → lumen of ER → glycosylation (transfer 4. Enzymes Destined for Lysosomes of N-acetylgalactosamine from UDP-N-galactosamine onto a a. N-Linked Glycoproteins specific seryl or threonyl R-group - can be phosphorylated at 1 or more specific mannosyl 1. Glycosyltransferases residues → bind to mannose 6-phosphate receptors → - bound to the membrane of ER of golgi apparatus translocation into lysosomes - synthesize the oligosaccharides b. I-Cell Disease 2. Movement and Secretion of Newly Synthesized - rare Glycoproteins - acid hydrolytic enzymes normally found in the lysosomes - sugars are added to the growing oligosaccharide as are absent → accumulation of substrates the protein move from the ER to the Golgi apparatus - high amounts of lysosomal enzymes are found in the → remain in the lumen → secretory vesicles bud off plasma suggesting that targeting process in the → fuse with cell membrane → release of lysosomes is deficient glycoproteins - lack the enzymic ability to phosphorylate mannose C. Synthesis of the N-Linked Glycosides residues of potential lysosomal enzymes → incorrect  occur in the lumen of the ER and Golgi apparatus targeting to extracellular sites  undergo additional processing steps - characterized by:  requires: · skeletal abnormalities - dolichol (lipid) · restricted joint movement - dolichol pyrophosphate (phosphorylated derivative) · coarse facial features · severe psychomotor impairment · death usually at 8 years 1. Synthesis of Dolichol-Linked Oligosaccharide - proteins are synthesized on the ER → lumen of ER - lipid-linked oligosaccharide constructed (as separate process) consisting of dolichol attached through a pyrophosphate linkage to an oligosaccharide containing: · N-acetylglucosamine · mannose · glucose - oligosaccharide is transferred from dolichol to an asparagine side group of protein by proteinoligosaccharide transferase IX. STRUCTURE OF PROTEOGLYCANS 2. Final Processing of N-Linked Oligosaccharides  all of the GAGs (except hyaluronic acid) are covalently attached - removal of mannosyl and glycosyl residues → addition of to protein → proteoglycan monomers variety of sugars (Nacetylglucosamine, N- 1. Proteoglycans (Mucopolysaccharides) acetylgalactosamine, additional mannoses, fucose or NANA - distinguished from other glycoproteins by the nature of the as terminal groups) → complex glycoprotein attached polysaccharide ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 7 of 9  BIOCHEMISTRY CARBOHYDRATES: Complex Carbohydrates - occur mainly in the extracellular matrix - combinations of proteins and glycosaminoglycans that associate by both covalent and noncovalent bonds - highly hydrated, so that, in combination with collagen → high resilience of cartilage a. Examples of glycosaminoglycans that are attached to proteoglycans  chondroitin sulfate  dermatan sulfate  heparan sulfate  keratan sulfate  hyaluronic acid b. Examples of proteoglycans that are characterized and named based on their structure and functional location i. Syndecan B. Formation of Proteoglycan Aggregates  integral membrane proteoglycan  proteoglycan monomers + hyaluronic acid → proteoglycan ii. Versican, Aggrecan aggregates  predominant extracellular proteoglycans  ionic interaction between the core protein and hyaluronic acid iii. Neurocan, Cerebrocan  stabilized by additional small proteins (link proteins)  found primarily in the central nervous system X. BACTERIAL CELL WALLS 2. Cartilage  rigid a. GAG Species  responsible in part for their virulence  chondroitin sulfate A. Gram-Positive Bacterial Cell Wall  keratan sulfate  consists of polysaccharide and polypeptide chains that are b. Proteoglycan Monomer Consists of covalently attached to form peptidoglycan i. Core Protein Peptidoglycan  linear glycosaminoglycan chains are covalently - bag-like structure attached - completely envelops the cell ii. Linear Carbohydrate Chains  composed of > 100 monosaccharides  extend out of the core protein (separated from each other because of charge repulsion) → “bottle brush” structure B. Gram-Negative Bacterial Cell Wall  relatively thin peptidoglycan cell wall surrounded by a complex outer membrane A. Linkage Region Between Carbohydrate Chain and Protein  most commonly through 1. Trihexoside (Galactose-Galactose-Xylose) 2. Serine residue  O-glycosidic bond is formed between xylose and hydroxyl group of serine ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 8 of 9  BIOCHEMISTRY CARBOHYDRATES: Complex Carbohydrates C. Bacterial Cell Walls Polysaccharide SUMMARY  some consists of alternating residues of β (1→4)-linked N- acetylmuramic acid and N-acetylglucosamine N-Acetylmuramic Acid Residues - linked via an amide bond to a tetrapeptide containing D-amino acids → continuous meshlike framework is formed by cross- linking adjacent peptidoglycan chains through their tetrapeptide side chains D. Lysozyme  cleave the glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine → peptidoglycan degradation E. Penicillin  inhibit formation of cross-links in peptidoglycan → antibiotic action XI. LYSOSOMAL GLYCOPROTEIN DEGRADATION A. Lysosomal Hydrolytic Enzymes  generally specific for the removal of 1 component of the glycoprotein (removing respective groups in sequence in the reverse order of their incorporation)  mostly exoenzymes B. Glycoprotein Storage Diseases (Oligosaccharidoses)  deficiency of 1 of the degradative enzymes → accumulation of partially degraded structures in the lysosomes → cell death → oligosaccharide fragments appear in the urine  often directly associated with the same enzyme deficiencies involved in mucopolysaccharidoses and the inability to degrade glycolipids ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 9 of 9