Carbohydrate Biochemistry Overview

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Questions and Answers

What is the primary function of lactose in mammals?

Serve as a structural component of cell membranes

Act as a hormone in metabolic regulation

Function as a neurotransmitter

Provide a calorie source for offspring (correct)

What type of bond is formed between sugar and nucleophilic amino acid residues in glycoprotein synthesis?

Hydrogen bond

Phosphodiester bond

Glycosidic bond (correct)

Peptide bond

Which of the following correctly describes glycosyltransferases?

Enzymes that hydrolyze glycosidic bonds

Enzymes that synthesize polysaccharides without a sugar donor

Enzymes involved in glycolysis only

Enzymes that transfer sugars to nucleophilic amino acids (correct)

Which statement about glycolipids is true?

They serve as recognition factors on the cell membrane. Signup and view all the answers

What is UDP-glucose primarily used for in cellular metabolism?

Glycogen synthesis and glycoprotein attachment Signup and view all the answers

What is the primary role of UDP-glucuronate in the body?

Formation of glucuronides for detoxification Signup and view all the answers

Which enzyme is responsible for converting UDP-glucose to UDP-glucuronate?

UDP-glucose dehydrogenase Signup and view all the answers

What happens to bilirubin when glucuronate groups are added?

It becomes more soluble and can be excreted Signup and view all the answers

Which of the following methods is NOT suggested to beat drug testing?

Inhibiting the enzyme UDP-glucose dehydrogenase Signup and view all the answers

What is the first step in the synthesis of lactose from UDP-glucose?

Epimerization to convert glucose to galactose Signup and view all the answers

What type of glycosidic linkage is formed when galactose is attached to glucose by lactose synthase?

B 1,4 glycosidic linkage Signup and view all the answers

Which of the following is NOT a fate of glycoproteins?

Be secreted directly into the bloodstream Signup and view all the answers

Which carbohydrate is identified as a receptor for influenza?

Sialic acid Signup and view all the answers

What determines the blood group type in humans?

Presence of carbohydrate antigens on red blood cells Signup and view all the answers

How does the treatment for influenza function according to the information provided?

By inhibiting the viral neuraminidase enzyme Signup and view all the answers

Which sugars might be involved in the complex branching of glycoproteins?

Fucose and Mannose Signup and view all the answers

What is the significance of glycosyl groups on proteins targeted for lysosomes?

They protect proteins from lysosomal enzymes. Signup and view all the answers

Which blood type can accept donations from A and O blood types?

Type A Signup and view all the answers

Study Notes

Synthesis of Glycosides, Lactose, Glycoproteins & Glycolipids

Carbohydrates have diverse functions beyond being energy sources.
Some carbohydrates aren't found in the diet and must be synthesized.

Carbohydrate Functions

Lactose: A disaccharide synthesized in the mammary gland, providing energy for offspring.
Glycosides: Sugars attached to non-sugar entities (e.g., glycoproteins, glycolipids).
Glycoproteins: Attached to N or O groups on extracellular proteins (N- and O- glycosylation).
Glycolipids: Carbohydrates attached to lipids, located on cell membrane exteriors; act as recognition factors (e.g., glycero- and sphingo-glycolipids).

Precursor UDP-Glucose

UDP-glucose is formed from glucose-1-phosphate (G1P).
It's an "activated sugar" with various roles.
- Glycogen synthesis
- Attachment to proteoglycans, glycoproteins, or glycolipids.
- Conversion to UDP-glucuronate (precursor for polysaccharides).
- Synthesis of lactose.

Glycosyltransferases

Sugars are transferred during glycoprotein/proteoglycan synthesis from nucleotide sugars to nucleophilic amino acid residues (serine, threonine, and asparagine).
The sugar-residue bond is a glycosidic bond.
Glycosyltransferases catalyze these reactions.
Glycosyltransferases have important roles in development and mutations can cause diseases.
Bacteria use glycosyltransferases, making them targets for antibiotic inhibitors.

Metabolic Routes of UDP-Glucuronate

UDP-glucuronate is a crucial product from UDP-glucose.
It's a substrate for proteoglycans and glycoproteins, as well as other sugars and glycosides (steroids and xenobiotics).
- Xenobiotics: Substances not naturally produced by humans (drugs or carcinogens).

Formation of Glucuronides

UDP-glucose is converted to UDP-glucuronate by UDP-glucose dehydrogenase (producing 2 NADH).
UDP-glucuronate is converted to glucuronide by UDP-glucuronate transferase (forming a glycosidic bond).
Glucuronide is water-soluble, facilitating removal of nonpolar substances (drugs, xenobiotics) in urine or bile.

Some Compounds Degraded and Excreted as Urinary Glucuronides

Several compounds are processed into glucuronides for excretion, including steroids, hormones, and xenobiotics.
- Examples: estrogen, progesterone, triiodothyronine, acetylaminofluorene, meprobamate, and morphine.
- Testosterone is also excreted as a glucuronide.

How to Beat Drug Testing

Methods to cheat drug tests, including using alternative urine, adulterating the sample, using diuretics, and inhibiting enzymes that make UDP-drug conjugates are not discussed in detail as they are ethically questionable and illegal.

Steroids

Green and white teas suppress UDP-glucuronosyltransferase UGT2B17, affecting testosterone glucuronidation.

Formation of Bilirubin Diglucuronide

Bilirubin is a heme degradation product (from myoglobin, hemoglobin, cytochromes).
Adding hydrophilic glucuronate groups makes bilirubin soluble and excretable.
Bilirubin accumulation causes jaundice (yellowing of skin and eyes).
Phototherapy converts bilirubin to a more soluble form.

Synthesis of Lactose

Lactose (milk sugar) is a disaccharide of galactose and glucose.
Lactose is generally formed from UDP-glucose in two steps:
- Epimerase converts the glucose moiety to galactose.
- Lactose synthase attaches galactose to the C4 of glucose (forming a β(1→4) glycosidic linkage)

Pathways for Interconversion of Sugars

Complex pathways allow for multiple potential uses of glucose.
Glucose can form glycoproteins, glycolipids, glycosaminoglycans (e.g., heparin), or sialic acid.
- Sialic acid serves as an influenza receptor.

N-Glycosylation

Glycoproteins have complex carbohydrate branches (e.g., N-acetylneuraminic acid, galactose, N-acetylglucosamine, mannose, fucose).
Carbohydrate sequence resembles amino acid sequence.

Fates of Glycoproteins & Proteoglycans

Glycoproteins are created in the endoplasmic reticulum (ER), then secreted from the Golgi to the plasma membrane.
They often act as receptors on cell surfaces.
Some glycoproteins are secreted or transported to lysosomes, with glycosyl groups protecting proteins from lysosomal enzymes.

Carbohydrates & Influenza

1918 flu pandemic caused ~100 million deaths worldwide.
Influenza entry and exit involve sialic acid-containing receptors on cells.
Treatment targets viral neuraminidase (an enzyme that degrades sialic acid and allows egress).

Carbohydrates and Blood Groups

Carbohydrates on red blood cells determine blood type (A, B, AB, or O).
Different blood types have specific carbohydrate antigens and corresponding antibodies.
Blood type compatibility is critical for blood transfusions.

Blood Groups-Continued

The antigenic determinant is the portion of the carbohydrate structure responsible for blood type specificity.
Other blood groups exist, determined by other carbohydrate patterns (e.g., Rh).
- Common terms: Galactose, N-acetylgalactosamine, N-acetylglucosamine, and fucose.

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Description

This quiz covers the synthesis and functions of important carbohydrates such as glycosides, lactose, glycoproteins, and glycolipids. Explore how UDP-glucose serves as a precursor in various biochemical processes, including energy provision and cellular recognition. Test your knowledge on the diverse roles of these complex molecules.