Biochemistry-Reactions from UDP Glucose Flashcards
What is the building block of glycoside formation?
Activated monosaccharide donors like UDP-glucose
What are enzymes responsible for biosynthesis and breakdown of glycosides?
Biosynthesis: glycosyl transferases. Breakdown: glycosidic hydrolases.
Identify the different monosaccharides shown below
From left to right: Glyceraldehyde, DHA, Glucose, Fructose, Ribose, Ribulose.
Which carbon is the D- and L- reference carbon in carbohydrates?
The chiral carbon furthest from the aldehyde or ketone group
Stereoisomers that are not mirror images of each other
Diastereomers
Stereoisomers that are mirror images of each other
Enantiomers
Two monosaccharides that differ only around ONE asymmetric center
Epimers
Identify the carbohydrates shown below
From left to right: D-glyceraldehyde, D-glucose, D-mannose, D-galactose and L-glucose
How do you cyclize this Fisher projection?
Turn it 90 degrees clockwise and react the aldehyde with the OH group on C5. Note that the carbons on the right of the Fisher projection will be pointing down in the cyclic sugar. This will form the anomeric carbon.
Where do the alpha and beta hydroxyl groups point in D sugars?
Alpha is down and beta is up.
Why is high blood sugar so destructive?
You get a higher concentration of the open aldehyde form of the sugar reacting with amino acids and tearing at basement membranes. The aldehyde also reacts with the N-terminus of Hgb…hence the principle of the A1C test.
How do you cyclize this Fisher projection?
*
How does the cyclic monosaccharide go on to form a glycosidic linkage with another cyclic monosaccharide?
The hemiactetal or hemiketal reacts with the hydroxyl group of the adjacent sugar in a dehydration reaction. Water is lost and the linkage is formed.
Why don’t we undergo mutarotation in our own bodies?
The glycosidic bond is locked in its alpha or beta conformation. You would have to hydrolyze the molecule and resynthesize it to undergo mutarotation. Mutarotation only happens in our bodies when there is no glycosidic linkage.
Why do you require an activated monosaccharide donor in order to form glycosidic bonds?
Dehydration, taking out water, is unfavorable in our bodies w/ 55 M H2O.
How do you form your activated monosaccharide donor?
Phosphoglucomutase takes Gluc-6-P to Gluc-1-P. Gluc-1-P + UTP -> UDP-Gluc + Pyrophosphate, by the enzyme UDP-glucos pyrophosphorylase. This reaction is brought in the forward direction by hydrolysis of the UTP phosphate group.
How do you synthesize lactose from UDP-glucose?
UDP glucose + UDP-glucose 4-epimerase + NAD+ -> UDP-galactose + glucose + galactosyl transferase -> beta-1,4 glycosidic bond forms lactose
Where is lactose synthesized in the body?
Childbirth triggers release of prolactin -> Prolactin triggers synthesis of alpha-lactalbumin (milk protein) -> alpha-lactalbumin increases reactivity of galactosyltransferase -> UDP-galactose is attacked by glucose -> lactose is formed.
What happens in a double displacement reaction? What would happen to the glycosidic linkage if it were a single displacement reaction?
Enzyme-OH attacks the UDP donor. Then the acceptor attacks the donor-enzyme complex to form an alpha 1,4 glycoside. Note that you retain the alpha linkage in this reaction, if it were a single displacement you would invert the glycosidic linkage.
What molecules are used to activate glucose and galactose monosaccharides?
UDP
What molecules are used to activate fucose and mannose monosaccharides?
GDP
What are the monosaccharides commonly used in synthesis of glycoproteins?
beta-D-Gal, beta-D-Man, alpha-L-Fuc, GalNAc, GlcNAc and Sialic acid (NANA)
How do you make amino sugars?
They all start with fructose and glutamine as the nitrogen source. The nitrogen binds to the active C2 carbon of fructose.
How do you make activated GalNAc from D-gluc-6-P?
An acetyl transferase takes the acetyl group from acetyl-CoA and puts in on D-gluc-6-P. Then a mutase transfers the P to C1 to form GlcNAc-1P. This reacts with a pyrophosphorylase to form UDP-GlcNAc which can react with an wpimerase to form UDP-GalNAc
2 major classes of glycoproteins
N linked: uses the asparagine nitrogen to make a linkage with glucosamine; serine or threonine will be on the other side to have a pattern of Asn-X-Ser or Asn-X-Thr. O linked: uses the serine or threonine oxygen to make a linkage with galactosamine, no pattern though.
How do you synthesize o-linked mucin glycoprotein carbohydrate chains?
UDP-GalNAc + HO-Ser on mucin protein -> beta-serine-N-acetyl-galactosamine -> UDP-Gal adds to form a beta-1,3 linkage -> Sialic Acid adds to form an alpha-2,6 linkage -> Fucose adds to form an alpha-1,2 linkage with galactose. This happens one monosaccharide at a time in the golgi.
What are the two classes of N-linked oligosaccharide chains?
High mannose (multiple antennary branches of mannose) and complex (varying branches of sugars). Note that all chains attach to the nitrogen of asparagine.
How does synthesis of N-linked oligosaccharides take place?
1) Sugars are assembled on ER membrane protein dolichol phosphate’s phosphate 2) The whole block of oligosaccharides are transferred to the protein growing on the ribosome
How are N-linked oligosaccharides processed?
3 glucoses are trimmed from the high mannose structure. The high mannose structure goes through the golgi and can be modified to form the complex form.
What are the three locations the golgi sends glycoproteins after they have been processed?
1) Sent out in a vesicle to fuse w/plasma membrane and become a membrane protein 2) Sent out in a vesicle to be secreted into the extra-cellular space 3) Sent to the lysosome to become lysosomal enzymes (carbohydrate on enzyme protein backbone is protective)
How does the body ensure that lysosomal enzymes get into the lysosomes?
1) Mannose reacts and forms a complex with UDP-GlcNAc via the enzyme phosphotransferase 2) N-acetylglucosaminidase pops off the GlcNAc to form mannose-6-P. Mannose-6-P is a target for the lysosome and binds the lysosomal enzymes on its way there.
A 6 year old boy presents with progressive psychomotor retardation, skeletal abnormalities, coarse facial features, gingival hyperplasia and recurrent upper respiratory infections. He dies at the age of 8. What autosomal recessive disorder could be causing his condition?
I-Cell disease. This happens when N-acetylglucosaminyl-1-phosphotransferase is defective and does not put mannose-6-P markers on the enzymes as they leave the golgi and head to the lysosomes. This results in non-functional lysosomes and accumulation of cytoplasmic glycolipids and mucopolysaccharides causing formation of dense cytoplasmic inclusion bodies (I-cells).
What determines if someone has type A, B or O blood?
Type O only has H substance (Galactose + fucose on glycolipid or glycoprotein). Type A has H substance and an additional GalNAc. Type B has H substance and has an extra Gal.
What is a ganglioside?
A glycolipid + Ceramide (sphingosine-based lipid w/amino acids) + Sialic Acid
Tay-Sachs mutation
HEXA gene = hexosaminidase A deficiency. GM2 activator deficiency can also cause similar disease because the activator is necessary for hexA to act on gangliosides.
Formation of glucuronide from UDP glucose
1) Dehydrogenation reaction takes the alcohol group to a carboxylic acid 2) Compounds from phase 1 (bilirubin, drugs and other waste) pop off UDP and conjugate with glucuronide
How many glucuronic acids get conjugated with bilirubin?
- This increases its water solubility so it can be excreted.
