Carbohydrates

Are carbohydrates polyfunctional molecules?

Yes (carbonyl and multiple alcohol groups)

Classification of glucose

aldohexose

Classification of fructose

ketohexose

What kinds of monosaccharides are the most common in living cells?

pentoses and hexoses

General formula of monosaccharides

(CH₂O)_n

aldotriose

Glyceraldehyde

Aldopentose

Ribose

Aldohexoses

Glucose and galactose

Ketotriose

Dihydroxyacetone

Ketopentose

Ribulose

Ketohexose

Fructose

Fructose facts

found in fruits, used by the food industry as a sweetening agent

Ribose facts

Component of RNA nucleotides

Glucose facts

most abundant monosaccharide in nature, used as primary fuel by living cells

Galactose facts

used to synthesize biomolecules (glycolipids, glycoproteins)

How is a disaccharide formed?

By a dehydration reaction between two monosaccharides, forming a glycosidic bond

Composition of maltose (doesn’t exist freely in nature)

2 glucose joined by a 1-4 glycosidic bond (hydrolysed my maltase)

Composition of sucrose

glucose and fructose joined by a 1-2 glycosidic bond (hydrolysed by sucrase)

Composition of lactose (found in milk)

glucose and galactose joined by a 1-4 glycosidic bond (hydrolysed by lactase)

Oligosaccharides

small polymers of 3-20 monosaccharide residues, don’t occur freely in nature

Glycolipid

Oligosaccharide covalently bonded to a lipid

Glycoprotein

Oligosaccharide covalently bonded to a protein

Kinds of covalent bonds in a glycoprotein

• O-glycosidic linkage between an oligosaccharide and the O atom of serine or threonine

• N-glycosidic linkage between an oligosaccharide and the N atom of asparagine

Functions of oligosaccharides

protect the cell surface from chemical and mechanical damage, cell-to-cell recognition and binding, blood types

which glycoproteins enable the HIV virus to attach to T-cells?

gp120 (external) and gp41 (across the membrane)

which protein lines the inside of the HIV virus envelope?

p17

which proteins compose the HIV virus capsid?

p6 and p24

which protein is bound the the RNA of the HIV virus?

p7

Which enzymes are associated with the viral genome of HIV?

RT, integrase and protease

HIV binding process

gp120 binds to CD4 receptors (on the target cell surface) and CCR5. it changes conformation exposing gp41 which inserts its hydrophobic terminus into the target cell membrane and folds on itself bringing the virus closer allowing fusion

Composition of A antigen

O Antigen + GalNac (N-Acetylgalactosamine) thanks to GalNac transferase

Composition of B antigen

O Antigen + Gal (Galactose) thanks to galactose

Polysaccharides

Hundreds to thousands of monosaccharides residues linked by glycosidic bonds

Homopolysaccharides

Saccharides only composed by one type of monosaccharide

Heterosaccharides

Polysaccharides composed by 2 or more types of monosaccharides

Examples of storage polysaccharides

starch, glycogen

Starch

storage polysaccharide found in chloroplasts (plants), consists only of glucose monomers, hydrolysed by alpha-amylase

Amylose (starch)

20-30%, unbranched, 1000s of glucose residues, long tight helices

Amylopectin (starch)

70-80%, branching every 20-25 residues, no helix formation

Glycogen

storage polysaccharide found in vertebrates, mainly in liver and muscle cells, branching every 8-12 residues