BIOLOGICAL MOLECULES BZ 002 Prof. Silvia Díaz

BIOLOGICAL MOLECULES BZ 002 Prof. Silvia Díaz

L4. Carbohydrates: Monosaccharides Objective: By the end of this lecture students should be able to: Explain the structure, properties, isomers and related functions of monosaccharides.

L4. Carbohydrates: Monosaccharides Contents 1. Introduction to sugars. 2. Structures and properties of monosaccharides. 3. Isomerism.

1. Introduction to sugars

Sugars … are important components of our diet. They are present in fruits, honey, table sugar, and syrups. Within our bodies the simple sugar D-glucose is an essential source of energy. It is present in blood at a relatively constant concentration of 5.5 mM and is transported to all tissues.

Polysaccharides … have diverse functions: Reserve of readily available energy. Exs: glycogen, in animal cells; starches and other polysaccharides in plants. Structural functions in nature: cellulose, forms the fibers of cotton, plant cell walls, and wood. Chitin a nitrogen containing polysacharide form the tough exoskeletons of anthropods and the cell walls of fungi.

Polysaccharides functions (cont.) Hyaluronan is a polysaccharide that carry many negative charges, and form a protective layer between animal cells, while pectins play a similar role in plants. A third function of sugar residues (glycosyl groups) is in biological recognition and communication.

2. Structures and properties of monosaccharides

Monosaccharides Monosaccharides are usually colourless, water-soluble, crystalline solids. Some have a sweet taste. A typical sugar, and the one with the widest distribution in nature, is glucose. Examples of other monosaccharides of biological importance include fructose, galactose, ribulose and ribose.

Monosaccharides The simple sugars, or monosaccharides, are polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses).

Monosaccharides All have the composition (CH2O)n, hence the family name carbohydrate. N is an integer whose value ranges from three to seven.

Classification of aldoses according to the number of carbon atoms

Classification of ketoses according to the number of carbon atoms

Some of the factors that give monosaccharides this diversity are: Number of carbon atoms in skeletons of the molecules. The position of the carbonyl group which makes them be classified as aldehydes or ketones-this makes these molecules exhibit different physical and chemical properties. For example, aldehydes are very easily oxidized and are powerful reducing agents as compared to ketones. … and

4. isomerism

Isomerism Structural isomerism. For example, hexoses like glucose, mannose and galactose are structural isomers since they have the same molecular formula (C6H12O6), but different arrangement of atoms.

Stereoisomerism Stereoisomerism. Monosaccharides show stereoisomerism. The simplest aldose, glyceraldehyde, contains only one chiral center (describes a molecule whose arrangement of atoms is such that it cannot be superimposed on its mirror image)

Ring structures of monosaccharides In aqueous solutions, pentoses and hexoses form 5-membered and 6-membered rings (C6 H1206) α-D-Glucopyranose

Ring structures of monosaccharides The most common pentoses occur in ring forms.

Ring structures of monosaccharides The most common hexoses; glucose, fructose and galactose naturally occur in ring forms.

When glucose is in the form of ring it can also form isomers  and β isomers occur in glucose depending on the position of the –OH group on carbon atom 1 in the molecule. It is an α- isomer if the -OH group on C-1 is below the plane of the ring and a β isomer if it is above the plane of the molecule.

When glucose is in the form of ring it can also form isomers The existence of α and β isomers leads to greater chemical variety and is of importance in, for example, the formation of starch and cellulose. Starch is made of α- glucose monomers. Cellulose is made of β -glucose monomers.

Functions of monosaccharides Trioses, (glyceraldehyde and dihydroxyacetone) are intermediates in glycolysis, photosynthesis (dark reactions) and other branches of carbohydrate metabolism. They can also form lipids, for example, triglycerides contain glycerol which is formed from glyceraldehyde.

Functions of monosaccharides Pentoses (ribose and ribulose) have many functions that include: synthesis of nucleic acids, for example RNA nucleotides contain ribose while DNA nucleotides contain deoxyribose, synthesis of some coenzymes like NAD, NADP, coenzyme A, FAD and FMN, synthesis of energy carrier molecules AMP, ADP, and ATP, synthesis of polysaccharides called pentosans, ribulose bisphosphate is the carbon dioxide acceptor in photosynthesis.

Functions of monosaccharides Hexoses (glucose, fructose, galactose and mannose) have the following functions: they are the primary substrate for oxidation to provide energy during respiration with glucose, the most common monosaccharide standing out as the most common respiratory substrate they are the building units for disaccharides, oligosaccharides and polysaccharides, their carbon skeletons serve as raw materials for the synthesis of other small organic molecules including amino acids and fatty acids.

Final questions: What kind of functional groups are present in monosaccharides? Which are the factors that give its diversity to monosaccharides?

Summary of key points: Carbohydrates, are a group of organic molecules that include both sugars and the polymers of sugars. Carbohydrates have the chemical formula (CH2O)n Monosaccharides, the simplest carbohydrate molecules are either aldoses or ketoses and can be clasified according to the number of carbon atoms in trioses, tetroses, pentoses, hexoses…

Summary of key points: Monosaccharides have one or more asymmetric carbon atoms and can therefore exhibit stereoisomerism.

Self assessment: Can you draw the structure of the four stereoisomeric forms of an aldotetrose? How many stereoisomeric forms can be formed by a hexose such as glucose? Write the chemical formulas of Glyceraldehyde Dihydroxyacetone Ribose Ribulose D-glucose D-fructose

Reading suggestions Biological Molecules Module, Unit 2. Alberts et al., Essential Cell Biology, 3rd Ed. 2010. Chapter 2, pp. 50-52. Panel 2-1. Bolsover, S.R., Hyams J.S. and Shephard E.A. (2004). Cell Biology. 2nd Ed. John Wiley & Sons, Inc., Hoboken, New Jersey. Chapter 2, pp. 19-27.

Carbohydrates. Disaccharides and polysaccharides Next lecture Carbohydrates. Disaccharides and polysaccharides

BIOLOGICAL MOLECULES BZ 002 Prof. Silvia Díaz