Topic 4 - #2 Carbohydrates Topic 2 Biologically Important Monosaccharide Derivatives There are a large number of monosaccharide derivatives. A variety of chemical and enzymatic reactions produce these derivatives of the simple sugars: Some of the common derivatives are: - Sugar Alcohols - Deoxy sugars - Sugar phosphates - Amino sugars Sugar Alcohols: Structures of some sugar alcohols: - Cannot be digested quickly in blood hence they end up in sugar snacks - No carbonyls no ketones or aldehydes Deoxy sugars Structures of some Deoxy sugars: - Deoxy sugars are monosaccharides with one or more hydroxyl groups replaced by hydrogens. Seen in structures above. - 2-Deoxy-D-ribose is a component of the structure of DNA e.g. Single strand RNA & DNA - presence of the hydroxyl groups determines the stability of the RNA or DNA molecule
Amino sugars - Sugars with an amino group at C-2 are amino sugars - They are found in many oligosaccharides and polysaccharides - The hydroxyl in carbon 2 sugar is replaced by an amine group (-NH 2 ) as seen below in Galatctosamine and Glucosamine molecules. Both are beta-d structure: - Glucose is alpha-d while galactose is beta-d at 1 st residue - Glucose is alpha-d at 4 th residue, while galactose is beta-d at 4 th residue Sugar Phosphates - Phosphate esters of glucose, fructose and other monosaccharides are important metabolic intermediates (energy) - The ribose moiety of nucleotides such as ATP is phosphorylated at the 5 -position - Phosphate group is part of an ester group such as seen in a-d-glucose-1-phosphate: - When glucose is broken down in cells, it undergoes a series of cellular reactions to form high-energy phosphates, such as ATP, BTP, CTP and DTP. - ATP is the major cellular energy unit used by cells responsible for anabolic reactions (creating bonds)
Oligosaccharides Disaccharides - Usually 2 to 10 simple sugar residues - Disaccharides are the simplest Oligosaccharides - two monosaccharides linked by a glycosidic bond - Each unit (monosaccharide) is termed a residue - In order to form glycosidic bonds, the process requires enzymes. There are number of enzymes that are responsible for synthesizing oligosaccharides. - E.g. UDP-galactose + Glucose (lactose synthase enzyme) Betagalactosyl-(1 4)-glucose) molecule + UDP: Glycosidic Bonds Glycosidic bonds refers to the attachment of monosaccharide residues. Covalent bonds between the anomeric hydroxyl of a cyclic sugar and the hydroxyl of a second sugar (or another alcohol containing compound) forms the glycosidic molecules/bonds. There are 2 major types of glycosidic bonds: 1) a (1 4) Glycosidic bond - carbon 1 linking to carbon 4 2) a(1 6) Glycosidic bond - carbon 1 linking to carbon 6 There can be a residue which is in beta-structure, hence the glycosidic bond is beta. Such example is the Lactose (galactose-beta-1,4-glucose) with the b(1 4) glycoside bond.
Carbohydrate digestion Carbohydrates can be synthesized in tissues but can also be broken down (catabolic). Sugar produces a sensation of sweetness. The relative sweetness is indicated in brackets in the diagram below: The process of digestion: Those who are lactose intolerant lack the enzyme lactase Concept of a reducing sugar: A ring structure formed is called a hemiacetal structure. This is for both sucrose and maltose. However, hemi-maltose structure can fold up and also open up, hence double movement. The carbon with 2 oxygen attached to it is the anomeric carbon. Flip the structure to have the anomeric carbon on the right hand side to see if it is alpha or beta ring structure. That hemiacetal can open up and form an aldehyde at the 4 th spot at either of the residues. The opened aldehyde can then act as a reducing agent. Hence if a solution is taken into consideration, such as copper citrate mixture (benedict s reagent blue solution), and is reacted with an aldose (carbohydrate), the reaction produces a carboxylic acid or a carboxylate anion, and a brick-red precipitate.
Some sugars are nonreducing sugars, as this difference is due the position of the anomeric carbon in the ring structure. If the anomeric carbon is not in the glycosidic bond with another anomeric carbon to the other molecule, the carbon cannot open up and form an aldehyde, and so cannot act as a reducing sugar, such as seen in sucrose: Examples of disaccharides: Sucrose-fructose cannot open up, hence not a reducing sugar More on Anomeric Carbons the carbon with 2 oxygens
Trehalose A natural protectant for insects - a disaccharide - not a reducing sugar - the top form is the chair form axial or equatorial axis can be seen shows the accurate representation of the molecule to be seen in aqueous solutions Polysaccharides - Polymers of the simple sugars - Functions: Storage (starch and glycogen), Structural (chitin and cellulose) and recognition (cell surface polysaccharides) - Nomenclature for polysaccharides is based on their composition and structure When looking at polysaccharides there are 2 ways to recognize them as through composition: (a) Homopolysaccharides same monomers - Unbranched - Branched - linkage (b) Heteropolysaccharides different monomers - Two monomer types, unbranched - Multiple monomer types, branched with linkages Starch an energy storage polysaccharide - An important source of energy we consume starch and break it down to release cellular energy for use - Starch is made of 10-30% amylose and 70-90% amylopectin Amylose - Can act as a reducing sugar - alpha(1 4) links
Amylopectin - highly branched, with branches occurring every 12 to 30 residues - alpha(1 4) links - the branches in amylopectin are a(1 6) links Cellulose a structural polysaccharide - cellulose provides physical structure and strength to plants - it is the most abundant natural polymer in the world - cotton is almost pure cellulose - contains beta-1,4-linked d-glucose units, hence contains beta(1 4) links: Comparison of amylose and cellulose: - However, cellulose is a structural polymer, hence its structural properties must come from its Intrachain bonds which are very strong compared to other polymers in the carbohydrate family. - The rotation allowed around the anomeric carbon in cellulose is extremely important as it provides this strength physical property. Hydrogen bonds are formed between hydrogen and oxygen (hydroxyl group to oxygen). Due to arrangement and orientation of the cellulose structure, there is a longitude of hydrogen bonds, hence linkages between adjacent glucose residues, linkages between adjacent chains of glucose residues and linkages between adjacent layers above each chains of glucose residues. The
immense linkages of hydrogen bonds provide extreme strength of the structure of the cellulose molecule: - Amylose does have not the strength property due to its linkages with the bent alpha 1 4 linkage position as it adopts a helical conformation: - Cellulose, with the beta 1 4 linkage position, can adopt a fully extended conformation: Digestion of Starch and Cellulose - Enzymes Amylase: - Catalyses the breakdown of starch into sugars - Present in human saliva and intestine - Not able to breakdown cellulose - The amylase enzyme however cannot break down the beta form of amylose nor can it break down cellulose due to beta linkages Cellulase: - Breaks the beta(1 4) glycosidic bonds in cellulose
- The only vertebrates able to use cellulose as food are ruminants (cattle, sheep, goats, camels, giraffes). The extra stomach compartment (rumen) of a ruminant teems with bacteria and protists that secrete cellulose End of Topic 4 - #2 Carbohydrates Topic 2