Ch 30 biochem
Synthesis of Glycosides, Lactose, Glycoproteins, and Glycolipids
 Many pathways for interconversion of sugars or formation of sugar derivatives use activated sugars attached to
nucleotides
o Both uridine diphosphate glucose (UDP-glucose) and UDP-galactose used for glycosyltransferase
reactions in many systems
o Lactose synthesized from UDP-galactose and glucose in mammary gland
o UDP-glucose can be oxidized to form UDP-glucuronate, which is used to form glucuronide derivatives of
bilirubin and xenobiotic compounds
 Glucuronide derivatives generally more readily excreted in urine or bile than parent compound
 Carbs often found in glycoproteins (carb chains attached to proteins) and glycolipids (carb chains attached to
lipids); nucleotide sugars used to donate sugar residues for formation of glycosidic bonds in both glycoproteins
and glycolipids
 Glycoproteins contain short chains of carbs (oligosaccharides) that are usually branched; oligosaccharides
generally composed of glucose, galactose, and their amino derivatives
o Mannose, L-fucose, and N-acetylneuraminic acid (NANA) frequently present
o Carb chains grow by sequential addition of sugars to serine or threonine residue of protein
o Nucleotide sugars are precursors
o Branched carb chains may be attached to amide nitrogen of asparagine in protein; these chains
synthesized on dolicholphosphate and subsequently transferred to protein
o Glycoproteins found in mucus, blood, compartments in cell (such as lysosomes), in ECM, and embedded
in cell membrane with carb portion extending into extracellular space
 Glycolipids belong to class of sphingolipids; synthesized from nucleotide sugars that add monosaccharides
sequentially to hydroxymethyl group of lipid ceramide (related to sphingosine)
o Often contain branches of NANA produced from cytidine monophosphate –NANA (CMP-NANA)
o Found in PM with carb portion extruding from cell surface
o Serve as cell recognition factors
 Each antigen in blood typing distinctive, in part because of different carbohydrate chains attached to protein
Interconversions Involving Nucelotide Sugars
 Activated sugars attached to nucleotides converted to other sugars, oxidized to sugar acids, and joined to
proteins, lipids, or other sugars through glycosidic bonds
 UDP-glucose – activated sugar nucleotide that is precursor of glycogen and lactose, UDP-glucuronate and
glucuronides, and cab chains in proteoglycans, glycoproteins, and glycolipids
o In synthesis of many carb portions of above compounds, sugar transferred from nucleotide sugar to
alcohol or other nucleophilic group to form glycosidic bond – use of UDP as leaving group provides
energy for formation of new bond
 Enzymes that form glycosidic bonds are sugar transferases (e.g., glycogen synthase is glucosyltransferase);
transferases also involved in formation of glycosidic bonds in bilirubin glucuronides, proteoglycans, and lactose
 UDP-glucuronate serves as precursor of other sugars and glucuronides; glucuronate formed by oxidation of
alcohol on carbon 6 of glucose to acid (through 2 oxidation states) by NAD+-dependent dehydrogenase
o Glucuronate present in diet and can be formed from degradation of inositol (sugar alcohol that forms
inositol triphosphate [IP3]), which is an intracellular second messenger for many hormones
 Function of glucuronate in excretion of bilirubin, drugs, xenobiotics, and other compounds containing hydroxyl
group is to add negative charges and increase solubility
o Bilirubin is degradation product of heme formed in reticuloendothelial system and is only slightly soluble
in plasma; transported to liver bound to albumin
 In liver, glucuronate residues transferred from UDP-glucuronate to 2 carboxyl groups on
bilirubin, sequentially forming bilirubin monoglucuronide and bilirubin diglucuronide
(conjugated bilirubin)
 More soluble conjugated bilirubin actively transported into bile for excretion
 Failure of liver to transport, store, or conjugate bilirubin results in accumulation of unconjugated
bilirubin in blood; jaundice (icterus) occurs when plasma becomes supersaturated with bilirubin
and excess diffuses into tissues
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When bilirubin levels measure in blood, one can measure indirect bilirubin
(nonconjugated form bound to albumin), direct bilirubin (conjugated, water-soluble
form), or total bilirubin
 If total bilirubin levels high, then determination of direct and indirect bilirubin needed to
appropriately determine caused for elevation of total bilirubin
o Many xenobiotics, drugs, steroids, and other compounds with hydroxyl groups and low solubility in
water converted to glucuronides in similar fashion by glucuronyltransferases present in ER and
cytoplasm of liver and kidney – one of major conjugation pathways for excretion
Glucuronate, once formed, can reenter pathways of glucose metabolism through reactions that eventually
convert it to D-xylulose 5-phosphate (intermediate of pentose phosphate pathway)
6-Phosphogluconate produced by first oxidative reaction in pentose phosphate pathway, in which carbon 1 of
glucose oxidized to carboxylate
Glucuronic acid oxidized at carbon 6 to carboxylate form
Many (60%) full-term newborns develop jaundice (neonatal jaundice) because increased destruction of RBCs
after birth (fetus has unusually large number of RBCs) and immature bilirubin conjugating system in liver
o Leads to elevated levels of nonconjugated bilirubin, which is deposited in hydrophobic (fat)
environments
o If bilirubin levels reach a certain threshold at 48 hours, newborn is candidate for phototherapy, where
child placed under lamps that emit light between 425-475 nm in wavelength; bilirubin absorbs the light,
undergoes chemical changes, and becomes more water-soluble
o Usually within a week of birth, newborn’s liver can handle load generated from RBC turnover
Lactose synthesized from UDP-galactose and glucose; galactose not required in diet for lactose synthesis
because galactose can be synthesized from glucose
o Galactose and glucose are epimers (differ only in sterochemical position of one OH group at carbon 4)
 Formation of UDP-galactose from UDP-glucose is epimerization
 Epimerase doesn’t transfer hydroxyl group; it oxidizes hydroxyl to ketone by transferring
electrons to NAD+, then donates electrons back to reform alcohol group on other side of C
o Lactose is unique in that it is synthesized only in mammary gland for short periods during lactation
 Lactase synthase (enzyme present in ER of lactating mammary gland) catalyzes last step in
lactose biosynthesis (transfer of galactose from UDP-galactose to glucose)
 Has 2 protein subunits: galactosyltransferase and α-lactalbumin (modifier protein
synthesized after parturition in response to hormone prolactin; lowers Km of
galactosyltransferase for glucose, thereby increasing rate of lactose synthesis)
 In absence of α-lactalbumin, galactosyltransferase transfers galactosyl units to
glycoproteins
o Although lactose in dairy products is major source of galactose, ingestion of lactose not required for
lactation; UDP-galactose in mammary gland derived principally from epimerization of glucose
 Dairy products are major dietary source of Ca2+, so breastfeeding mothers need increased Ca2+
from a food source
Inhibition of phosphoglucomutase by galactose-1-phosphate results in hypoglycemia by interfering with both
formation of UDP-glucose (glycogen precursor) and degradation of glycogen back to glucose-6-phosphate
o 90% of glycogen degradation leads to glucose-1-phosphate, which can only be converted to glucose-6phosphate by phosphoglucomutase
o When phosphoglucomutase activity inhibited, less glucose-6-phosphate production occurs, and hence,
less glucose available for export
 Stored glycogen only 10% efficient in raising blood glucose levels, and hypoglycemia results
 UDP-glucose levels reduced because glucose-1-phosphate required to synthesize UDP-glucose,
and in absence of phosphoglucomutase activity, glucose-6-pohsphate (derived from either
glucokinase reaction or gluconeogenesis) can’t be converted to glucose-1-phosphate, preventing
formation of UDP-glucuronate, which is necessary to convert bilirubin to diglucuronide form for
transport into bile (causes jaundice)
Transferases that produce oligosaccharide and polysaccharide side chains of glycolipids and attach sugar
residues to proteins specific for sugar moiety and for donating nucleotide (e.g., UDP, CMP, or GDP)
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Sugar nucleotides used for glycoprotein, proteoglycan, and glycolipid formation include derivatives of
glucose and galactose, acetylated amino sugars, and derivatives of mannose
o Reason for large variety of sugars attached to proteins and lipids is they have relatively specific and
different functions, such as targeting protein toward PM, providing recognition sites on cell surface (for
other cells, hormones, or viruses), or acting as lubricants or molecular sieves
 Many steps for formation of sugars not reversible, so glucose and other dietary sugars enter common pool from
which diverse sugars can be formed
 Amino sugars all derived from glucosamine-6-phosphate; to synthesize glucosamine-6-phosphate, amino group
transferred from amide of glutamine to fructose-6-phosphate
o Amino sugars, such a sglucosamine, can then be N-acetylated by acetyltransferase
o N-acetyltransferases present in ER and cytosol and provide means of chemically modifying sugars,
metabolites, drugs, and xenobiotic compounds
 Mannose found in diet in small amounts; is an epimer of glucose, and mannose and glucose interconverted by
epimerization reactions at carbon 2
o Interconversion can take place either at level of fructose-6-phosphate to mannose-6-phosphate or at
level of derivatized sugars
 N-acetylmannosamin is precursor of NANA (a sialic acid) and GDP-mannose is precursor of GDP-fucose
o Negative charge on NANA obtained by addition of 3-carbon carboxyl moiety from phosphoenolpyruvate
Glycoproteins
 Contain short carb chains covalently linked to either serine/threonine or asparagine residues in protein
o Oligosaccharide chains often branched and do not contain repeating disaccharides
 Most proteins in blood are glycoproteins – serve as hormones, antibodies, enzymes (including those of blood
clotting cascade), and structural components of ECM
o Collagen contains galctosyl units and disaccharides composed of galactosyl-glucose attached to
hydroxylysine residues
o Antigenic determinants of blood type located in oligosaccharides of glycoproteins and glycolipids of PMs
 Secretions of mucus-producing cells, such as salivary mucin, are glycoproteins
 Most glycoproteins secreted from cells, but some segregated in lysosomes, where they serve as lysosomal
enzymes that degrade various types of cellular and extracellular material
 Some glycoproteins produced like secretory proteins, but hydrophobic regions of protein remain attached to
PM, and carb portion extends into extracellular space
o Serve as receptors for compounds such as hormones, as transport proteins, and as cell attachment and
cell-cell recognition sites
o Bacteria and viruses also bind to these sites
 Protein portion of glycoproteins synthesized on ER
o Carb chains attached to protein in lumen of ER and Golgi complex
o In some cases, initial sugar added to serine or threonine residue in protein, and carb chain extrended by
sequential addition of sugar residues to nonreducing end
o UDP-sugars are precursors for addition of 4 of 7 sugars usually found in glycoproteins (glucose,
galactose, N-acetylglucosamine, and N-acetylgalactosamine)
o GDP-sugars are precursors for addition of mannose and L-fucose
o CMP-NANA precursor for NANA
o Dolichol phosphate (synthesized from isoprene units) involved in transferring branched sugar chains to
amide nitrogen of asparagine residues
o Sugars removed and added as glycoprotein move from ER through Golgi complex
o Carb chain used as targeting marker for lysosomal enzymes
 I-cell (inclusion cell) disease – rare condition in which lysosomal enzymes lack mannose phosphate marker that
targets them to lysosomes
o Enzyme deficient in I-cell disease is phosphotransferase located in Golgi apparatus; it usually recognizes
lysosomal proteins because of 3D structure, such that they can all be appropriately tagged for transport
to lysosomes
 As result of lack of mannose phosphate, lysosomal enzymes secreted from cells
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Because lysosomes lack normal complement of enzymes, undegraded molecules accumulate in
membranes inside cells, forming inclusion bodies
Glycolipids
 Derivatives of lipid sphingosine; sphingolipids include cerebrosides and gangliosides
 Contain ceramide with carb moieties attached to hydroxymethyl group
 Involved in intercellular communication
 Oligosaccharides of identical composition present in both glycolipids and glycoproteins associated with PM,
where they serve as cell recognition factors (for example, carb residues in oligosaccharides are antigens of ABO
blood group substances)
 Cerebrosides synthesized from ceramide and UDP-glucose or UDP-galactose
o Contain single sugar (monosaccharide)
 Gangliosides contain oligosaccharides produced from UDP-sugars and CMP-NANA, which is precursor for NANA
residues that branch from linear chain
 Defects in degradation of sphingolipids lead to sphingolipidoses (gangliosidoses)
 Sphingolipids produced in Golgi complex; lipid component becomes part of membrane of secretory vesicle that
buds from trans face of Golgi; after vesicle membrane fuses with cell membrane, lipid component of glycolipid
remains in outer layer of PM and carb component extends into extracellular space
o Sometimes carb component used as recognition signal for foreign proteins (for example, cholera toxin
binds to carb portion of GM1 ganglioside to allow its catalytic subunit to enter the cell
Disease
Enzyme deficiency
Fucosidosis
α-Fucosidase
Generalized gangliosidosis
GM1-β-galactosidase
Tay-Sachs disease
Hexosaminidase A
Tay-Sachs variant (Sandhoff) disease
Hexosaminidase A and B
Fabry disease
α-Galactosidase
Ceramide lactoside lipidosis
Ceramide lactosidase (B-galactosidase)
Metachromatic leukodystrophy
Arylsulfatase A
Krabbe disease
β-galactosidase
Gaucher disease
β-glucosidase
Niemann-Pick disease
Sphingomyelinase
Farber disease
ceramidase
Blood Group Information
 Blood group substances are oligosaccharide components of glycolipids and glycoproteins found in most PMs
o Genes for those in RBCs encode glycosyltransferases involved in synthesis of oligosaccharides of blood
group substances
o Most individuals can synthesize H substance (oligosaccharide that contains fucose linked to galactose at
non-reducing end of blood group substance)
o Type A individuals produce N-acetylgalactosamine transferase (encoded by A gene) that attaches Nacetylgalactosamine to galactose residue of H substance
o Type B individuals produce galactosyltransferase (encoded by B gene) that links galactose to galactose
residue of H substance
o Type AB individuals produce both transferases, and some oligosaccharides contain Nacetylgalactosamine and some contain galactose
o Type O individuals produce defective transferase, and therefore get nothing on their galactose residue
of their H substance
Biochemistry of Tay Sachs Disease
 Hexosaminidase A (defective enzyme in Tay-Sachs disease) composed of 2 subunits (α-chain and β-chain)
o HexA gene encodes α-subunit, and HexB gene encodes β-subunit
 In Tay-Sachs disease, α-subunit defective, and hexosaminidase A activity lost
 β-subunit can form active tetramers in absence of α-subunit, and this activity (hexosaminidase B) cleaves
glycolipid globoside and retains activity in children with Tay-Sachs disease
o Thus children with Tay-Sachs disease accumulate ganglioside GM2, but not globoside
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Mutation of HexB gene, and production of defective β-subunit, leads to inactivation of both hexosaminidase A
and B activity; this leads to Sandhoff disease
o Both activities lost because both activities require functional β-subunit
o Clinical course similar to Tay-Sachs but with accelerated timetable because of initial accumulation of
both GM2 and globoside in lysosomes
Sandhoff activator disease – have Tay-Sachs symptoms, but both hexosaminidase A and B activities normal;
caused by mutation in protein needed to activate hexosaminidase A activity; in absence of activator,
hexosaminidase A activity minimal, and GM2 initially accumulates in lysosomes; has no effect on hexosaminidase
B activity
When glycolipid cannot be degraded because of enzymatic mutation, it accumulates in residual bodies (vacuoles
that contain material that lysosomal enzymes cannot digest); normal cells contain small number of residual
bodies, but in diseases of lysosomal enzymes, large numbers of residual bodies accumulate in cell, eventually
interfering with normal cell function
In 70% of cases of Tay-Sachs disease in persons of Ashkenazi Jewish background, exon 11 of gene for α-chain of
hexosaminidase A contains mutation (4-base insertion, which alters reading frame of protein and introduces
premature stop codon father down protein, so no functional α-subunit can be produced