CHEMISTRY OF BIOMOLECULES CHAPTER 1 M N CHATTERJEA

MAJOR CONCEPTS • A. What are carbohydrates? Their general properties and biomedical importance. • B. List the monosaccharides of biological importance and learn their properties. • C. List the disaccharides of biological importance and learn their properties. • D. Study the chemistry and properties of various polysaccharides. • E. Study the chemistry and functions of proteoglycans.

CARBOHYDRATES • Carbohydrates are defined chemically as Aldehyde or ketone derivatives of the higher polyhydric alcohols, or compounds which yield these derivatives on hydrolysis. CLASSIFICATION Carbohydrates are divided into four major groups— monosaccharides, disaccharides, oligosaccharides and polysaccharides.

BIOMEDICAL IMPORTANCE OF • CARBOHYDRATES Chief source of energy. • Constituents of compound lipids and conjugated proteins. (A conjugated protein is a protein that functions in interaction with other (non-polypeptide) chemical groups attached by covalent bonding or weak interactions. ) • Degradation products act as “promoters” or ‘catalysts’. • Certain carbohydrate derivatives are used as drugs like cardiac glycosides/antibiotics. • Lactose principal sugar • Degradation products utilized for synthesis of other substances such as fatty acids, cholesterol, amino acid, etc

• Constituents of mucopolysaccharides (occurring chiefly as components of connective tissue. They are complex polysaccharides containing amino groups) which form the ground substance of mesenchymal tissues. (mesodermal cells that give rise to such structures as connective tissues, blood, lymphatics, bone, and cartilage) • Inherited deficiency of certain enzymes in metabolic pathways of different carbohydrates can cause diseases, e. g. galactosemia, glycogen storage diseases (GSDs),

• Lactose intolerance, etc. • Derangement of glucose metabolism is seen in diabetes mellitus.

MONOSACCHARIDES OF BIOLOGICAL IMPORTANCE • Trioses: Both D-glyceraldehyde and dihydroxy-acetone occur in the form of phosphate intermediates in glycolysis. • They are also the precursors of glycerol, which the organism synthesizes and incorporates into various types of lipids. • Tetroses: Erythrose-4 -P occurs as an intermediate in hexose monophosphate shunt which is an alternative pathway for glucose oxidation.

PENTOSES • D-ribose is a constituent of nucleic acid RNA; also as a constituent of certain coenzymes, e. g. FAD, NAD, coenzyme A. • D-2 -deoxyribose is a constituent of DNA. • Phosphate esters of ketopentoses—D-ribulose and • D-xylulose occur as intermediates in HMP shunt.

• L-xylulose is a metabolite of D-glucuronic acid and is excreted in urine of humans afflicted with a hereditary abnormality in metabolism called pentosuria. • L-fucose (methyl pentose): occurs in glycoproteins • D-Lyxose: It forms a constituent of lyxoflavin isolated from human heart muscle whose function is not clear.

HEXOSES • D-Glucose: (Synonyms: Dextrose, Grape Sugar) • It is the chief physiological sugar present in normal blood continually and at fairly constant level, i. e. about 0. 1 per cent. • • All tissues utilize glucose for energy. • Erythrocytes and Brain cells utilise glucose solely for energy purposes.

• Occurs as a constituent of disaccharide and polysaccharides. • Stored as glycogen in liver and muscles mainly. • D-galactose: Seldom found free in nature. In combination it occurs both in plants and animals. • Occurs as a constituent of milk sugar lactose and also in tissues as a constituent of galactolipid and glycoproteins.

• It is an epimer of glucose and differs in orientation of H and OH on carbon-4. • It is less sweet than glucose and less soluble in water. • On oxidation with hot HNO 3, it yields dicarboxylic acid, mucic acid; which helps in its identification, since the crystals of mucic acid are not difficult to produce and have characteristic shape.

D-FRUCTOSE: • It is a ketohexose and commonly called as fruit sugar, as it occurs free in fruits. • It is very sweet sugar, much sweeter than sucrose and more reactive than glucose. • It occurs as a constituent of sucrose and also of the polysaccharide inulin. • It is laevorotatory and hence is also called laevulose.

D-MANNOSE: • It does not occur free in nature but is widely distributed in combination as the polysaccharide mannan, e. g. in ivory nut. • In the body, it is found as a constituent of glycoproteins. Sedoheptulose: • It is a ketoheptose found in plants of the sedum family. • Its phosphate is important as an intermediate in the HMP-shunt and has been identified as a product of photosynthesis.

IMPORTANT PROPERTIES OF MONOSACCHARIDES • Iodocompounds: • An aldose when heated with conc. HI loses all of its oxygen and is converted into an iodocompound.

ACETYLATION OR ESTER FORMATION • The ability to form sugar esters • e. g. acetylation with acetyl chloride (CH 3 – COCl) indicates the presence of alcohol groups. • Due to alcoholic –OH groups, it can react with anhydrides and chlorides of many organic and inorganic acids, like acetic acid, phosphoric acid, sulphuric and benzoic acids to form esters of corresponding acids

OSAZONE FORMATION: • It is a useful means of preparing crystalline derivatives of sugars. • Osazones have characteristic Melting points Crystal structures Precipitation time and thus are valuable in identification of sugars.

3 -STEPS OF OSAZONE FORMATION • They are obtained by adding a mixture of phenylhydrazinehydrochloride and sodium acetate to the sugar solution and heating in a boiling water bath for 30 to 45 minutes. • The solution is allowed to cool slowly (not under tap) by itself. • Crystals are formed.

MECHANISM • The reaction involves only the carbonyl carbon (i. e. aldehyde or ketone group) and the next adjacent carbon. • Reactions that take place with an aldosugar • First phenyl hydrazone is formed and then the hydrazone reacts with two additional molecules of phenylhydrazine to form the osazones. • The reaction with a ketose is similar (Fig 3. 8) pg 46

OXIDATION TO PRODUCE SUGAR ACIDS: • When oxidised under different conditions, the aldoses may form: • Monobasic Aldonic acids • Dibasic Saccharic acids • Monobasic uronic acids containing aldehyde groups thus possessing reducing properties.

1. Aldonic acids: • Oxidation of an aldose with Br 2— water converts the aldehyde group to a – COOH group aldonic acid Br 2/H 2 O • Glucose D-Gluconic acid

Saccharic or aldaric acid: • Oxidation of aldoses with conc. HNO 3 under proper conditions converts both aldehyde and primary alcohol groups to –COOH groups, forming dibasic sugar acids, the saccharic or aldaric acids. Examples • D-Glucose D-Glucaric acid • D-Galactose D-Mucic acid

Uronic acids: • When an aldose is oxidised in such a way that the primary alcohol group is converted to – COOH group, without oxidation of aldehyde group, a uronic acid is formed. • They exert reducing action due to presence of free –CHO group. Examples • D-Glucose D-Glucouronic acid • D-Galactose D-Galacturonic acid

BIOMEDICAL IMPORTANCE OF D-GLUCURONIC ACID • In the body D-Glucuronic acid is formed from Glucose in liver by uronic acid pathway, an alternative pathway for glucose oxidation. • It occurs as a constituent of certain mucopolysaccharides. • In addition, it is of importance in that it conjugates toxic substances, drugs, hormones and even bilirubin (a break down product of Hb) and converts them to a soluble nontoxic substance, a glucuronide, which is excreted in urine.

REDUCTION OF SUGARS TO FORM SUGAR ALCOHOLS: • The monosaccharides may be reduced to their corresponding alcohols by reducing agents such as Na-Amalgam. • Similarly, ketoses may also be reduced to form ketoalcohol. • Examples • D-Glucose yields D-Sorbitol. • D-Galactose yields D-Dulcitol. • D-Mannose yields D-Mannitol. • Ketosugar D-Fructose yields D-Mannitol and D-Sorbitol

OTHER SUGAR DERIVATIVES OF BIOMEDICAL IMPORTANCE Deoxy sugars: • Deoxy sugars represent sugars in which the oxygen of a – OH group has been removed, leaving the hydrogen. Thus, –CHOH or –CH 2 OH becomes –CH 2 or –CH 3. • Several of the Deoxy sugars have been synthesized and others are natural products.

• Deoxy sugars of biological importance are: • 2 -deoxy-D-Ribose is found in nucleic acid (DNA). • 6 -deoxy-L-Galactose is found as a constituent of glycoproteins, blood group substances and bacterial polysaccharides.

AMINO SUGARS (HEXOSAMINES): • Sugars containing an –NH 2 group in their structure are called amino sugars. Types: • Two types of amino sugars of physiological importance are: • Glycosylamine: The anomeric –OH group is replaced by an –NH 2 group.

• Example: A compound belonging to this group is Ribosylamine, a derivative of which is involved in the synthesis of purines. • Glycosamine (Glycamine): In this type, the alcoholic – OH group of the sugar molecule is replaced by – NH 2 group. Two naturally occurring members of this type are derived from glucose and galactose, in which – OH group on carbon 2 is replaced by – NH 2 group, and forms respectively Glucosamine and Galactosamine

BIOMEDICAL IMPORTANCE • N-acetyl derivative of D-Glucosamine occur as a constituent of certain mucopolysaccharides (MPS). • Glucosamine is the chief organic constituent of cell wall of fungi, and a constituent of shells of crustaceae (crabs, Lobsters, etc), where it occurs as Chitin, which is made of repeating units of N-acetylated glucosamine. • Hence Glucosamine is often called as Chitosamine.

• Galactosamine occurs as N-acetyl-Galactosamine in chondroitin sulphates which are present in cartilages, bones, tendons and heart valves. • Hence Galactosamine is also known as Chondrosamine.

Antibiotics: • Certain antibiotics, such as Erythromycin, carbomycin, contain amino sugars. • Erythromycin contains dimethyl amino sugar and carbomycin 3 -amino-D-Ribose. • It is believed that amino sugars are related to the antibiotic activity of these drugs.

AMINO SUGAR ACIDS • Neuraminic acid: • It is an amino sugar acid and structurally an aldol condensation product of pyruvic acid and D-Mannosamine. • Neuraminic acid is unstable and found in nature in the form of acylated derivatives known as Sialic acids (Nacetyl Neuraminic acid —NANA).

• Muramic acid: • Another amino sugar acid which is structurally a condensation product of DGlucosamine and Lactic Acid.

GLYCOSIDES DEFINITION: • Glycosides are compounds containing a carbohydrate and a no carbohydrate residue in the same molecule. • In these compounds the carbohydrate residue is attached by an acetal linkage of carbon-I to the non carbohydrate residue. • The non carbohydrate residue present in the glycoside is called as Aglycone.

AGLYCONE.

• The aglycones present in glycosides vary in complexity from simple substances as methyl alcohol, glycerol, phenol or a base such as adenine to complex substances like sterols, hydroquinones and anthraquinones. • The glycosides are named according to the carbohydrate they contain. • If it contains glucose, forms glucoside. If galactose, it forms galactoside and so on.

BIOMEDICAL IMPORTANCE • Glycosides are found in many drugs, spices and in the constituents of animal tissues. • They are widely distributed in plant kingdom. Cardiac glycosides: • It is important in medicine because of their action on heart and thus used in cardiac insufficiency. • They all contain steroids as aglycone component in combination with sugar molecules. • They are derivatives of digitalis, strophanthus and squill plants, e. g

• Ouabain: • A glycoside obtained from strophanthus sp. is of interest as it inhibits active transport of Na+ in cardiac muscle in vivo (Sodium Pump inhibitor).

PHLORIDZIN: • A glycoside obtained from the root and bark of Apple tree. • It blocks the transport of sugar across the mucosal cells of small intestine and also renal tubular epithelium; it displaces Na+ from the binding site of ‘carrier protein’ and prevents the binding of sugar molecule and produces glycosuria