Biochemistry
Synthesis of Glycosides, Lactose, Glycoproteins, and Glyolipids
Ch 30
I. Interconversions involving nucleotide sugars
-activated sugars attached to nucleotides are converted to other sugars, oxidized to sugar acids, and joined to proteins, lipids,
or other sugars through glycosidic bonds
Blood typing uses antibodies that recog. either the A, B or Rh(D) antigens. Each Ag is unique b/c of the diff carb chains
attached to the proteins. Process:
-Blood sample is split and mixed with each Ab individually.
-If agglutination (clumping) occurs , then the RBC has the Ag that is recog’d by the Ab
-Abs are bivalent. Each arm binds to a diff cell agglutination
-If neither A nor B Abs cause agglutination, the blood is type O
A. Rxns of UDP-Glucose
-UDP= Uridine diposphate
UDP-Glucose= acitivated sugar nucleotide; a precursor of glycogen and lactose, UDP-glucuronides and the carb chains in
proteoglycans, glycoproteins, and glycolipids.
Sugar transferases- move sugar from the nucleotide sugar to an –OH (or other nucleophile) to form the glycosidic bonds
-transferases also involved in forming glycosidic bonds in bilirubin glucuronides, proteoglycans, and lactose
B. UDP- Glucuronate
Major route of UDP-glucose metabolism- the formation UDP-Glucuronate; serves a precursor of other sugars and of
glucuronides
Glucuronate- formed by oxidation of –OH on C-6acid (by dehydrogenase)
- obtained from diet
- can be formed by degrading Inositol (the sugar in IP3 and a 2nd messenger)
C. Formation of Glucuronides
-fxn of glucuronate- adding it to groups with a hydroxyl group makes them more soluble:
used in:
o Bilirubin- degradation product of heme; is only slightly soluble in plasma
 Transported to liver bound to albumin glucuronate is transferred from UDP-glucuronate
(twice) bilirubin monoglucuronate  bilirubin diglucuronate (now considered
“conjugated”) transported to bile for excretion
o Drugs
o Xenobiotics- pharmacologically, endocrinologically, or toxicologically active substances that are not
produced endogenously/ are foreign to the organism (ex:drugs)
o Other hydroxyl-containing groups
-the other hydroxyl-containing components are converted in ways similar to that of bilirubin using
glucuronyltransferases present in the endoplasmic reticulum and cytoplasm of liver and kidney
- this is one of the major cojugation pathways for excretion of these compounds
-other fate of glucuronate-can re-enter the pathways of glucose metabolism thru rxns that convert it to D-xylulose 5-phosphate (an
intermediate in the pentose phosphate system)
Icterus= Jaundice; failure of the liver to conjugate bilirubin accumulation of unconjugated bilirubin in th blood. (plasma is
supersaturated at >2 to 2.5 mg/dL)
When measuring bilirubin, get 3 readings; total alone is not enough
Direct- conjugated, soluble form
Indirect-unconjugated, unsoluble form
Total lvls-… um, total
60% of full-term babies born with neonatal jaundice; underdeveloped liver and increased hemolysis (fetuses have very high
[RBC]) are the cause. Sunlight or light therapy can be used. Biliruib absorbs the light, undergoes chemical changes, and
becomes more water soluble. Within one week, most babies livers are developed enough to handle the load.
D. Synthesis of UDP-Galactose and lactose from glucose
Lactose made from glucose+ UDP-Galactose
Biochemistry
Synthesis of Glycosides, Lactose, Glycoproteins, and Glyolipids
Ch 30
1) conversion of glucose to galactose
-galactose and glucose are epimers; only differ in stereochemical position of one hydroxyl group at carbon 4… the
conversion is an epimerization
2) lactose synthesis
-only synth’d in the adult female, in the mammary gland during lactation
-Lactose synthase
- found in the ER
-catalyzes transfer of galactose from UDP-Galactose to glucose
-has 2 subunits:
-galactosyltransferase
--Lactalbumin- a modifier protein, made after parturition in response to prolactin; lowers the Km of the
other subunit from 1200 to 1mM… drastically increasing the rate of lactose synthesis. Without it, galactosyltransferase
transfers galactosyl units to glycoproteins
Difference btwn 6-phosphogluconate and glucuronic acid?
Glucuronic acid- is oxidized at C-6 to the caboxylate form
6-phosphogluconate- C-1 is oxidized to a carboxylate
inc. lvls galactose 1-phosphateInhibition of phosphoglucomutase  causes hypoglycemia by:
-interfering with formation of uridine diphosphate (UDP)-Glucose (precursor to glycogen)
-phosphoglucomutase used to convert glucose 6-phosphate  to glucose 1-phosphate required to make UDPGlucose required for UDP-glucuronate needed to conjugate bilirubin so icterus results
-degrading glycogen back to glucose 6-phosphate (G6P)
-can only be converted to glucose by phosphoglucomutase
-glycogen is only 10% efficient at raising blood glucose lvls hypoglycemia
-So you’re losing the precursor to glycogen, and decreasing it’s efficiency for inc. glucose in blood, and have jaundice to top it
off. O_o
Lactose intolerant woman with newborn. Can she breast-feed?
-yes. Dietary lactose is not needed to produce new lactose in the mammary glands. UDP-galactose is epimerized from glucose.
Should increase dietary Calcium intake though.
E. Formation of sugars for glycolipid and glycoprotein synth
-transferases that make oligo and poly saccharide side chains of glycolipids and attach sugar residues to proteins are
specific for the sugar moiety and for the donating nucleotide ( UDP, CMP, GDP)
-Reason for the variety: they have specific and different fxns:
-targeting proteins toward membranes
-recognition sites on cells, hormones, viruses etc
-act as lubricant or molecular sieves
-many steps are reversible: the sugars can enter a common pool from which they are made
-Amino sugars are all derived from glucosamine 6-phosphate
-to synth glucosamine 6-phosphate:
fructose 6-phosphate + amide of glutamine
Glucosamine- is an animo sugar; can be N-acetylated by an acetyltransferase: found in the ER and the cytosol and is
another mode of chemically mod’ing sugars, metabolites, drugs and xenobiotics. Individuals vary in their capacity for
acetylation rxns.
Mannose: in small amts in diet; an epimer of glucose, can be epimerized at C-2 either from:
-fructose 6-phosphate to mannose 6-phosphate
-derivatized sugars (interconversion of sugars)
look at page 550 for the diagram
-N-acetlymannosamine is the precursor of N-acetylneuraminic acid (NANA, a sialic acid)
- negative charge from the addition of a 3 carbon carboxyl from phosphoenolpyruvate
-GDP-massose is the precursor to GDP-fructose
Biochemistry
Synthesis of Glycosides, Lactose, Glycoproteins, and Glyolipids
Ch 30
II. Glycoproteins
A. Structure and fxn
-Glycoproteins- have short carb chains covalently linked to either serine/threonine or aspargine in the protein
-they are oligosaccharide chains; branched; do not contain repeating disaccharides
-most proteins in the blood are glycoproteins
-serve as hormones, antibodies, enzymes (including the clotting cascade) and structural components of the
ECF
-Collagen has galactosyl units & disaccharides composed of galactosyl-glucose attached to hydroxylysine
-products of mucus-producing cells (like salivary mucin) are also glycoproteins
Glycoproteins in the cell:
-can be excreted
-can be in lysosomes, and act like degradation enzymes
-can be receptors for hormones, transport proteins, cell attachment, cell-cell recog and binding sites for bacteria and viruses
Antigenic determinants are located in the oligosacchariades of the glycoproteins and glycolipids of the membrane.
-peeps with A Ag are type A
-peeps with B Ag are type B
-Both and none Type AB, or O, respectivly
B. Synthesis
-protein portion synth’d in ER
-carb chains are attached to the protein in the lumen of the ER and the Golgi complex
-In some cases, initial sugar is added to serine or threonine in the protein then extended by sequential addn of sugars
to the nonreducing end
-UDP-sugars: precursors for the addition of 4 of the 7 sugars that are usually found in glycoprotein
-glucose
-galactose
-N-acetylglucosamine
-N-acetylgalactosamine
-GDP-sugars: precursors for additon of:
-mannose
-L-fucose
-CMP-NANA precursor for:
-NANA
-dolichol- involved in transferring branched sugar chains to the amide N of asparagine
-Sugars are removed and added as the glycoprotein moves from ER thru Goli
-Carb chains used as a targeting marker for lysosomal enzymes
-I-cell- inclusion cell= dz where lysosomal enzyme don’t have the mannose phosphate marker that sends them to lysosomes
-lacking phosphotransferase (which recg’s lysosomal proteins and tags them for transport)=no mannose phosphate
proteins get secreted out of the cell instead of going to the lysosome no degradative enzymes in lysosomes things that
should get broken down, build-up instead= inclusion bodies
III. Glycolipids
A. Fxn and structure
Glycolipids= derivative of sphingosine lipid (eg: cerebrosides, and gangliosides)
-contain ceramide + carb portions attached to its hydroxyl group
-involved in intercell communication
-in membrane as cell recog factors
-ex: carb residues in these oligosaccharides are antigens of the ABO blood group substances
B. Synth
Ceramide+UDP-glucose + UDP-galactose= Cerebrosides
-contain monosaccharide
ceramide+UDP-sugars + CMP-NANA= gangliosides
-contain oligosaccharides
-CMP-NANA= precursor for the NANA parts that branch from the linear chain
-Defects in shingolipid degradation sphingolipidoses (aka gangliosidoses)
Biochemistry
Synthesis of Glycosides, Lactose, Glycoproteins, and Glyolipids
Ch 30
-produced in the Golgi (released on the trans side sent to cell membrane)
-vesicle released from golgi becomes outer/extracellular face of membrane w/ carb sticking out
-carb portion can be used as a recognition signal for foreign proteins
-ex: cholera toxin binds to the carb portion of a ganglioside to gain access into the cell
Clinical
Erna Nemdy- Blood typing lady at hospital
Seriously, we know blood types. If you need more info, check out page 555.
Second group:
-Rh group
-one of the antigenic determinants is the D antigen- stimulates productino of a large number of Abs.
There is one genetic locus with 2 alleles that determine blood type. They encode glycosyltransferases involved in the synth of
oligosaccharides of the blood group substances.
Most people can make H substance- oligosaccharide that contains fructose linked to galactose
Type A= Hsubstance+ N-acetylgalactosamine (added to the galactose of H)
-the A gene encodes the N-acetylgalactosamine transferase
Type B= H substance +galactose (added to the galactose of H)
-the B gene encodes galactosyltransferase
Type AB= have both
Type O= have only H substance
-produce defective transferases
Jay Sakz, age 2
Dx: Tay-Sachs disease
Sx: Obviously mentally retarded, pyschomotor development becoming more abnormal, nearly blind, progressive muscle
weakness (cannot sit up or crawl), weak cough reflex, repeated resp. infections
Autosomal recessive disorder; gen population: prevalence
1 in 300,000; in Eastern European Jews= 1 in 3,600 (1 in 28 Ashkenazi Jews carry the gene)
-can test by measuring the tissue lvl of the protein produced by the gene (hexosaminidase A) or by recomb. DNA techniques.
-skin fibroblasts of parents can be used
-Fxn affected by defective gene:
-hydrolyzing a specific bond btwn N-acetyl-D-galactosamine and a D-galactose in the polar head of the ganglioside.
-dz is one of the gangliosidoses- multifaceted
-the sphingolipidoses (like Fabry and Gaucher dzs)- affect mainly the brain, skin and reticuloendothelial system (liver, spleen)
-Complex lipids build up; all of them have ceramide; they are synth’d at the right rate, they are just not broken down.
The lysosomal enzyme required, is not very active bc it is made in lower quantites, as a result of a gene mutation, or bc an
activating enzyme is deficient.  build up in the affected tissues progressive malfunction sich as psychomotor issues when
the CNS is involved.
-Tx: none available. Enzyme replacement has been difficult since it cannot cross the BBB
Biochemical Comments:
Hexosaminidase A= defective enzyme in Tay-Sachs; has 2 subunits:  and , encoded by HexA and HexB
In Tay Sachs- -subunit defective, but  is active hexosaminidase B cleaves glycolipid Globoside build up of GM2
(requires hexosaminidase A to break-down) but no build up of globoside
In Sandhoff dz- defective -subunit, inactivates both A and B enzymes; dz like Tay Sachs, but with faster progression.
3rd version: Sandhoff Activator dz:
Tay- Sachs sx. But Hexosaminidase A and B activities are normal in a test tube.
Mutation in the protein needed to activate Hexosaminidase A. Hexosaminidase B activity is normal.
In 70% of Tay-Sachs caes among Ashkenazi jews, exon 11 of the gene for Hexosaminidase A contains a mutation of a 4-base
insertion that inserts a premature stop codon.