Carbohydrates Quiz: Structure and Functions

Biochemistry of Carbohydrates

• Carbohydrates are substances containing carbon, hydrogen, and oxygen.
• They are aldehyde (CHO) or ketone (C=O) derivatives of polyhydric alcohols (with more than one OH group) or compounds that yield these derivatives on hydrolysis.

Importance of Carbohydrates

• Chief source of energy.
• Important structural components in animal and plant cells.
• Important part of nucleic acids, free nucleotides, and coenzymes.
• Major antigens (e.g., blood group substances) are carbohydrates.
• Have a biological role as parts of hormones, their receptors, and enzymes.

Classification of Carbohydrates

• Monosaccharides: Contain one sugar unit (cannot be hydrolyzed).
• Disaccharides: Contain two sugar units.
• Oligosaccharides: Contain 3-10 sugar units.
• Polysaccharides: Contain more than 10 sugar units.

Monosaccharides

• The simplest units of carbohydrates containing one sugar unit.
• General formula: C_n(H₂O)_n
• Naming:
  • Based on functional groups: Aldoses (aldehyde group -CHO) and Ketoses (ketone group -C=O).
  • Based on the number of carbon atoms: Trioses (3C), Tetroses (4C), Pentoses (5C), Hexoses (6C).
    • Aldotrioses/Ketotrioses, Aldotetroses/Ketotetroses, Aldopentoses/Ketopentoses, Aldohexoses/Ketohexoses

Classification of Monosaccharides

• Trioses:

  • Aldotrioses: Glyceraldehyde ("glycerose").
  • Ketotrioses: Dihydroxyacetone.

• Tetroses:

  • Aldotetroses and ketotetroses. (e.g., Erythrose, Erythrulose)

• Pentoses:

  • Aldopentoses (e.g., Ribose, arabinose, xylose, lyxose).
  • Ketopentoses (e.g., Ribulose, xylulose).
    • Important functions of pentoses:
      • Component of nucleic acids (RNA and DNA).
      • Component of ATP, GTP.
      • Component of coenzymes NAD, NADP, and FAD.

• Hexoses:

  • Aldohexoses: glucose, mannose, galactose.
  • Ketohexose: fructose.
    • Important functions of glucose:
      • Most important carbohydrate sugar; "grape sugar."
      • Main sugar in blood.
      • Major energy source.
      • Converted to glycogen, galactose, etc.
    • Important functions of galactose:
      • Converted to glucose in the liver.
      • Major component of lactose. -Important functions of fructose:
      • "Fruit/semen sugar."
      • Sweetest sugar.
      • Part of honey and fruits. -Important functions of mannose -Part of glycoproteins and sialic acid -Found in plant mannans

Ring (Cyclic) Structure of Sugars

• Due to reaction between C=O (carbonyl) of an aldehyde group in aldoses or of a ketone group in ketoses with an alcoholic hydroxyl group.
• Furanose: 4-carbon ring.
• Pyranose: 5-carbon ring.

Optical Activity

• Ability of a substance to rotate plane-polarized light.
• Dextrorotatory (d or +): rotates light to the right.
• Levorotatory (l or -): rotates light to the left.
• Glucose is dextrorotatory, some use the name dextrose.
• Fructose is levorotatory, sometimes called levulose.

Optical Isomerism

• Ability of a substance to exist in more than one form (isomer).
• Structural isomerism involves different atoms or groups' arrangement into different patterns.
• Stereoisomerism involves the spatial configuration (arrangement) of atoms or groups in space.

Structural Isomerism

• Different arrangement of atoms and groups forming the molecule (e.g., ring vs. straight chain).

Stereoisomerism

• D- and L- isomerism (enantiomers): Differ in distribution of H and OH.
• Anomers: Stereoisomers differing in arrangement around anomeric carbon after cyclization
• Epimers: Stereoisomers differing in arrangement around a single non-anomeric carbon.

Sugar Derivatives

• Sugar acids (e.g., gluconic, glucuronic, saccharic acids).
• Sugar alcohols (e.g., sorbitol).
• Deoxy sugars (e.g., deoxyribose).
• Amino sugars (e.g., glucosamine, galactosamine).

Disaccharides

• Maltose, isomaltose, cellobiose, lactose, sucrose, trehalose.

• Reducing disaccharide: Maltose, Isomaltose, Cellobiose, and Lactose.

• Non-reducing disaccharide: Sucrose & Trehalose

• Important properties of reducing disaccharides include acting as a reducing agent, and forming osazone crystals.

Polysaccharides

• Contain more than 10 monosaccharide units, and yield monosaccharides upon acid hydrolysis.
• Classified into homopolysaccharides (one type of monosaccharide) and heteropolysaccharides (more than one type of monosaccharide).
  • Homopolysaccharides include:
    • Glucosans: Starch, Dextrins, Dextran, Glycogen, Cellulose.
    • Fructosans: Inulin.
    • Galactosans: Agar-agar.
    • N-acetyl-glucosan: Chitin.

Heteropolysaccharides

• Contain different sugar units.
• Include glycosaminoglycans (mucopolysaccharides).
  • Criteria for glycosaminoglycans: repeating disaccharide units w/ acidic sugar - amino sugar.
  • Often contain sulfate groups.
  • Unbranched.
  • Mostly extracellular (except heparin).
  • Components of connective tissue (bone, elastin and collagen). -Act as lubricants and cushions
    • Classification into sulfur free & containing (e.g., hyaluronic acid, chondroitin, keratan sulfate, heparin, and heparan sulfate).

Conjugated Carbohydrates

• Simple protein + non-protein group (prosthetic group)
  • Glycoproteins, Glycolipids, and Proteoglycans.

Derived Carbohydrates

• Result of protein denaturing or hydrolysis -Primary & Secondary

Biochemistry of Proteins

• Objectives
  • Amino acids
  • Peptides
  • Protein.
  • Methods of precipitation of proteins
  • Separation techniques for proteins and amino acids

Amino acids

• Structural units of proteins.
• Obtained from proteins by hydrolysis.
• General formula:
  • α (alpha) carbon
  • Amino group (-NH₂)
  • Carboxyl group (-COOH)
  • Hydrogen atom (H)
  • Side Chain/Radical group (R)
• About 300 amino acids exist in nature, but only 20 polymerize to form proteins in mammals.
• All amino acids in mammals are L-amino acids. D-amino acids are found in bacteria.
• Amino acids are optically active (except glycine).
• Amino acids are amphoteric (react as both an acid and a base).

Classification of Amino Acids

• Chemical Classifications based on R group Properties

  • Neutral
  • Acidic
  • Basic
  • Special amino acids

• Nutritional Classifications based on essential/non-essential needs

  • Essential
  • Non essential
  • Semi-essential

• Metabolic classifications based on ability to be catabolized

  • Ketogenic & Glucogenic

Peptides

• Formed from linking amino acids via peptide bonds.
• Dipeptide (2 amino acids).
• Tripeptide (3 amino acids)
• Oligopeptide (3-10 amino acids).
• Polypeptide (10-50 amino acids).
• Glutathione (tripeptide):
  • Glutamyl-cysteinyl-glycine
  • Functions in detoxification, maintaining redox balance, and amino acid transport.

Proteins

• Polymers formed predominantly by amino acids linked via peptide bonds.
• Molecular weight usually greater than 50 amino acids.
• Can contain other atoms/ions in addition to N, C, H, O (e.g., S, P, metals).

Methods of Precipitation of Proteins

• At the isoelectric point (proteins have neutral net charge).
• By various concentrations of salt solutions (salting out).
• By heavy metals (protein denaturation).
• By alkaloid reagents (protein denaturation).
• By alcohol precipitation (protein denaturation).
• By heat coagulation (protein denaturation).

Separation techniques for proteins and amino acids

• Electrophoresis: Movement of charged molecules in an electric field.
• Chromatography: Separation based on different properties.
• Precipitation: Removal by precipitation with various agents.
• Ultracentrifugation: Separation by density.
• Dialysis: Separation by size/weight through a semi-permeable membrane.
• Precipitation by antibodies: Utilizing the specificity of antibodies to precipitate particular protein.