Biomolecules: Carbohydrates

• A living system grows, sustains, and reproduces
• Biochemistry studies chemical processes within living systems
• Living systems have complex biomolecules like Carbohydrates, proteins, nucleic acids, lipids
• Proteins and carbohydrates are essential in food and interact in life processes
• Vitamins and mineral salts have roles in organism functions

Carbohydrates

• Plants primarily produce carbohydrates as a large group of organic compounds
• Carbohydrates were once considered hydrates of carbon, having the formula Cₓ(H₂O)ᵧ
• Glucose (C₆H₁₂O₆) fits the formula C₆(H₂O)₆
• Not all compounds fitting Cₓ(H₂O)ᵧ are carbohydrates
  • Acetic acid (CH₃COOH) fits C₂(H₂O)₂ but isn't a carbohydrate
  • Rhamnose (C₆H₁₂O₅) is a carbohydrate not fitting the definition
• Chemically, carbohydrates are optically active polyhydroxy aldehydes or ketones or hydrolysis products
• Sugars are carbohydrates with a sweet taste
• Sucrose is common table sugar
• Lactose is milk sugar
• Saccharides is another name for carbohydrates

Classification of Carbohydrates

• Carbohydrates are classified by hydrolysis behavior
• Monosaccharides: Cannot be further hydrolyzed to simpler polyhydroxy aldehyde or ketone units
  • ~20 occur in nature
  • Examples: glucose, fructose, ribose
• Oligosaccharides: Yield 2-10 monosaccharide units upon hydrolysis
  • Further classified as di-, tri-, tetrasaccharides
  • Disaccharides are most common
  • Hydrolysis yields identical or different monosaccharides
    • Sucrose yields one glucose and one fructose
    • Maltose yields two glucoses
• Polysaccharides: Yield large numbers of monosaccharide units on hydrolysis
  • Examples: starch, cellulose, glycogen, gums
  • They are non-sugars, and not sweet
• Can be reducing or non-reducing sugars
• Reducing sugars reduce Fehling's solution and Tollens' reagent
• All monosaccharides, whether aldose or ketose, are reducing sugars
• Classified based on carbon atoms and functional groups
  • Aldehyde = aldose
  • Keto = ketose

Types of Monosaccharides

• Triose has 3 carbon atoms
• Tetrose has 4 carbon atoms
• Pentose has 5 carbon atoms
• Hexose has 6 carbon atoms
• Heptose has 7 carbon atoms
• Aldotriose is a triose with and aldehyde
• Ketotriose iis a triose with a ketone

Preparation of Glucose

• Glucose occurs freely and combined in nature
• Found in sweet fruits and honey
• Ripe grapes contain glucose
• Can be prepared from sucrose or starch
• From Sucrose.
  • Glucose and fructose are from boiling sucrose with dilute HCl or H₂SO₄ in alcoholic solution
• From Starch
  • Glucose is commercially from starch hydrolysis by boiling with dilute H₂SO₄ at 393 K under high pressure

Structure of Glucose

• Glucose is an aldohexose, also known as dextrose
• Monomer of larger carbohydrates like starch and cellulose
• Most abundant organic compound on Earth, with molecule formula C₆H₁₂O₆
• Prolonged heating with HI forms n-hexane, suggesting a straight chain of six carbon atoms
• Reacts with hydroxylamine to form an oxime and adds a molecule of hydrogen cyanide to give cyanohydrin
  • Reactions confirm a carbonyl group (>C=O)
• Oxidized to a six-carbon carboxylic acid (gluconic acid) with a mild oxidizing agent
  • Indicates the presence of an aldehydic group
• Acetylation with acetic anhydride gives glucose pentaacetate
  • Confirms five –OH groups attached to different carbon atoms creating a stable compound
• Oxidation with nitric acid yields dicarboxylic, saccharic acid
  • Indicates a primary alcoholic (–OH) group

Glucose Configuration

• Correctly named as D(+)-glucose
• 'D' represents configuration
• '(+)' represents dextrorotatory nature, no relation to optical activity
• 'D' or ‘L' indicate relative configuration to glyceraldehyde isomers
  • (+) Isomer of glyceraldehyde has “D” configuration, -OH lies on the right
  • ‘L’ (−) Isomer of glyceraldehyde, -OH group is on the left
• In monosaccharides, the lowest asymmetric carbon atom is compared
• In (+) glucose, the lowest asymmetric carbon has —OH on the right side
  • Assigned D-configuration
• Most oxidized carbon written at the top of the structure

Cyclic Structure of Glucose

• Structure (I) of glucose does not explain some reactions
  • Does not give Schiff's test
  • Does not form hydrogensulphite addition product with NaHSO₃
  • Pentaacetate of glucose does not react with hydroxylamine
  • Glucose exists in two crystalline forms: α and ß
• Proposed that one of the —OH groups may add to the —CHO group forming a cyclic hemiacetal structure
• Glucose forms a six-membered ring involving —OH at C-5 explaining —CHO group absence with 2 forms in equilibrium
• Two cyclic hemiacetal forms differ only in the hydroxyl group configuration at C1, carbon called anomeric carbon
• Isomers are α-form and ß-form anomers and the cyclic structure is a pyranose structure

Fructose

• Fructose is an important ketohexose obtained with glucose from sucrose hydrolysis
• Natural monosaccharide in in fruits, honey and vegetables
• Used as a sweetener
• A ketohexose molecule with molecular formula C₆H₁₂O₆
• Ketonic functional group at carbon number 2 and six
• Belongs to D-series as a levorotatory compound, written as D-(-)-fructose
• Exists in two cyclic forms from —OH addition at C5 to the (C=O) group
• Forms a five-membered ring named furanose

Disaccharides

• Disaccharides hydrolyze with dilute acids or enzymes to give two molecules of same/different monosaccharides
• Monosaccharides are joined by an oxide linkage by water molecule loss, called glycosidic linkage
• If reducing groups are bonded, disaccharides are non-reducing sugars
  • Sucrose
• If functional groups are free, disaccharides are reducing sugars
  • Maltose and lactose
• Sucrose hydrolysis yields equimolar mixture of D-(+)-glucose and D-(-) fructose
  • C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆
• Bonded by glycosidic linkage between C1 and a-D-glucose & C2 of ß-D-fructose, so sucrose is a non reducing sugar
• Sucrose is dextrorotatory but in hydrolysis gives dextrorotatory glucose and laevorotatory fructose
• Laevorotation of fructose (-92.4°) exceeds glucose dextrorotation (+ 52.5°), so mixture is laevorotatory
• Sucrose hydrolysis changes rotation sign from dextro (+) to laevo (–)
  • Product is called invert sugar
• Maltose has two a-D-glucose units with C1 of one glucose (I) linked to C4 of another glucose unit (II)
  • Free aldehyde group can be produced at C1, shows reducing properties
• Lactose is composed of β-D-galactose and β-D-glucose
  • Linkage between C1 of galactose and C4 of glucose
  • Free aldehyde may be produced at C-1, hence it is also a reducing sugar

Polysaccharides

• Polysaccharides contain a large number of monosaccharide units joined by glycosidic linkages
• Most commonly encountered carbohydrates in nature and act as food storage or structural materials
• Starch: Main storage polysaccharide of plants and dietary source with high content of starch
  • Polymer of α-glucose with two components: amylose and amylopectin
• Amylose is water soluble and about 15-20% of starch
  • Long, unbranched chain with 200-1000 α-D-(+)-glucose units
• Amylopectin is water insoluble and about 80-85% of starch
  • Branched chain polymer of α-D-glucose units with C1-C4 glycosidic linkage, Branching occurs by C1-C6 glycosidic linkage
• Cellulose exclusively in plants
  • Most abundant organic substance in plant kingdom and cell wall constituent
  • Composed of β-D-glucose units combined by glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit
• Glycogen: The carbohydrates are stored in animal body as glycogen with similar structure to amylopectin
  • Present in liver, muscles, and brain
  • Body breaks down glycogen to glucose by enzymes if needed
• Found in yeast and fungi

Importance of Carbohydrates

• Carbohydrates are essential for life in both plants and animals, Forming a major portion of the food
• Honey has been as instant energy by 'Vaids' in ayurvedic system of medicine
• Used by storage molecules as starch in plants and glycogen in animals
• Cell walls of bacteria and plants consist of cellulose
• Used to build furniture
• Wood and clothe consists of cellulose
• Providing raw materials for many important industries; textiles, paper, lacquers and breweries
• Two aldopentoses viz. D-ribose and 2-deoxy-D-ribose are present in nucleic acids
• Found in biosystem in combination with many proteins and lipids

Proteins

• Proteins are most abundant biomolecules in the living system
• Milk, cheese, pulses, peanuts, fish, meat are the chief sources
• Occurring in every part of the body to support structure and functions of life
• Required for growth and maintenance of body
• Derived from Greek word, "proteios" which means primary/prime importance
• Polymers of α-amino acids
• Amino acids contain amino (-NH2) and carboxyl (-COOH) functional groups
  • Can be α, β, γ, δ based on relative position of amino group regarding carboxyl group
  • Only α-amino acids are obtained on hydrolysis of proteins
  • Names reflect property/source while generally represented by a three letter symbol, sometimes one letter symbol

Classification of Amino Acids

• Amino acids are usually colorless, crystalline solids exhibiting properties of salts over amines or carboxylic acids
• Amphoteric behavior in zwitter ionic form by reactions with acids and bases
• Except glycine, naturally occurring a-amino acids are optically active
  • Existing in 'D' and 'L' forms but most amino acids have L-configuration
  • L-Aminoacids are represented with the –NH2 group writing writing the –NH2 group on left hand side
• Classified as acidic, basic, or neutral depending upon amino and carboxyl groups in their molecule
  • Equal number of amino and carboxyl groups makes it neutral
  • More amino groups makes it basic
  • More carboxyl groups makes it acidic
• Amino acids that can be can be synthesized in the body are non-essential - On the other hand, essential amino acids synthesised in the body must be obtained through diet

Structures of Proteins

• Proteins are polymers of a-amino acids connected by peptide bonds or linkages (an amide)
  • In reaction between similar/different amino acids
  • Forming a dipeptide (-CO–NH-)
  • Elimination of water creates peptide bond
• Example: glycine + alanine = glycylalanine
• Third amino acid to a dipeptide crates a tripeptide
  • Tripeptide created an amino acids creates tetra, penta, hexa peptide
  • Polypeptides form from ten amino acids
  • Proteins form from polypeptide chain of hundred amino acid
• Proteins can be classified base don their chemical shape
  • Fibrous type
  • Globular type
• Fibrous proteins consist of polypeptide chains held
  • By hydrogen and disulfide bonds
  • The resulting fiber-like structure produces water-insoluble proteins, such as keratin or myosin
• Globular protein have coiled polypeptide chains
  • The resulting spherical shape produces water-soluble proteins, such as insulin or albumins
• Proteins and their shapes and studied on 4 levels: primary, secondary, tertiary and quaternary
  • Primary structures have 1 or more polypeptide chains and specific arrangement
  • Any change in primary creates a different protein
  • Secondary structure represents structure in polypeptide chain
  • Secondary in 2 structures : α-helix and β-pleated sheet
    • Backbones from regular folding
• Alpha helix makes all hydrogen bonds by twisting into the a right handed screw (helix)
• Beta-pleated sheets all peptide chains are all peptide chains that side-by-side
• Tertiary structures produces two major molecular shapes
  • Fibrous and globular
  • Stabilized by hydrogen bonds, disulfide linkages, van der Waals, and electrostatic forces
  • Quaternary: some proteins are composed of two or more polypeptide chains (sub-units structure)

Native Proteins

• Native proteins have a unique and biological activity
  • Disturbed by temperature/ chemical changes,
  • Cause hydrogen bonds to be disturbed
  • Globules unfold, helix uncoils and causes Denaturation
• Denaturation tertiary and secondary structures are destroyed and primary one remains
  • Cooking or boiling will cause coagulation
  • Curdling from lactic acid

Enzymes

• Enzymes speed up biochemical reactions, which creates a more manageable process for the cells
• Enzymes are have extremely specifically related to reactions and enzyme structures
• Most enzymes are globular proteins
  • Maltase is the enzyme that catalyses hydrolysis of maltose into glucose
• Named for reactions
  • Enzymes catalyzing oxidation/reduction known as oxidoreductase
• Enzyme are just needed in small doses
  • Work like catalysts, decreasing activation energy

Vitamins

• Vitamins in small amounts is a must in our diet because their deficency leads to disease
• Most can't be synthesized in our body
  • Bacteria can help us and those bacteria are essential for survival
• Vitamins have a wide range of structual and chemical classes
  • Accessory food factors: organic compounds in small amounts with optimum biological functions
• Vitamins are classified via solubility of it's compound
  • Fat and oil soluble are known to be kept in fat tissues
  • Water-soluble are not stored and excreted

Nucleic Acids

• Every Generation's characteristics transmits one generation to next within nucleus - In nucleus called chromosomes made of nucleic protein acids ( Deoxy/ribonucleic)
• Nucleic acid classified for long chains by nucleotide

Chemical Composition

• Complete hydrolysis for RNA/DNA yields a pentose sugar
• (RNA contains β-D-ribose for sugar molecules versus Dna molecules which are deoxyrobose - DNA contains four: adenine , cyanine, gunaine with Thymine molecule - The others contains similar acids however it does not have Thymine
• Unit formed by the attachment of base from position of nucleocide sugar carbons are ordered to determine position of base
• Sugar bonds phosphate with phosphate bond

Biological DNA Functions

• Chemcial heredity code maintained in species, stores genetic information
• Replication in cell duplcation for synthesis by various RNA molecules

Hormones

• Molecules act as intercellular messengers in the blood
• Endoctrine act on sites of the body in chemcial nature (Steriod) Polypeptides exist in others (ex.Insulin and endings) Derivatives of the body are epinephrine/norepinephrine
• Hormones are balanced through functions of the body -Insulin keeping blood levles within normal bounds

Study Questions From The Text

1. What are monosaccharides?
2. What are reducing sugars?
3. Write two main functions of carbohydrates in plants.
4. Classify the following into monosaccharides and disaccharides: Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose
5. What do you understand by the term glycosidic linkage?
6. What is glycogen? How is it different from starch?
7. What are the hydrolysis products of (i) sucrose and (ii) lactose?
8. What is the basic structural difference between starch and cellulose?
9. What happens when D-glucose is treated with the following reagents? (i) HI (ii) Bromine water (iii) HNO3
10. Enumerate the reactions of D-glucose which cannot be explained by its open chain structure.
11. What are essential and non-essential amino acids? Give two examples of each type.
12. Define the following as related to proteins (i) Peptide linkage (ii) Primary structure (iii) Denaturation.
13. What are the common types of secondary structure of proteins?
14. What type of bonding helps in stabilising the α-helix structure of proteins?
15. Differentiate between globular and fibrous proteins.
16. How do you explain the amphoteric behaviour of amino acids?
17. What are enzymes?
18. What is the effect of denaturation on the structure of proteins?
19. How are vitamins classified? Name the vitamin responsible for the coagulation of blood.
20. Why are vitamin A and vitamin C essential to us? Give their important sources.
21. What are nucleic acids? Mention their two important functions.
22. What is the difference between a nucleoside and a nucleotide?
23. The two strands in DNA are not identical but are complementary. Explain.
24. Write the important structural and functional differences between DNA and RNA.
25. What are the different types of RNA found in the cell?