CARBOHYDRATES

• Carbohydrates


  Commonly referred to as sugars and starches, are polyhydroxy aldehydes and ketones, or compounds that can be hydrolyzed to them

  
• Carbohydrates have C=O and -OH functional groups
• Carbohydrates


  Refers to hydrates of carbon
• Classification of carbohydrates


  • Monosaccharides
  • Disaccharides
  • Polysaccharides

  
• Carbohydrates are the largest group of organic molecules in nature, comprising approximately 50% of the earth's biomass
• Importance of carbohydrates


  • They provide energy
  • They serve as a form of stored chemical energy
  • They form part of the structures of some cells and tissues
  • Carbohydrates linked to proteins function as biomarkers

  
• Biological functions of carbohydrates

  • Storehouse of chemical energy (glucose, starch and glycogen)
  • Precursor for other biomolecules or supply carbon atoms for the synthesis of other biochemical substances (proteins, lipids, and nucleic acids)
  • Structural framework/building blocks of nucleotides (DNA and RNA molecules)
  • Supportive structural building blocks/components in plants and animals (cellulose in plants, chitin in insects)
  • Carbohydrates linked to lipids are structural components of cell membranes
  • Carbohydrates linked to proteins function as markers in a variety of cell–cell and cell–molecule recognition processes and binding (example by other cells, hormones, and viruses)
• Types of carbohydrates


  • Monosaccharides
  • Disaccharides
  • Oligosaccharides
  • Polysaccharides

  
• Monosaccharides


  The simplest carbohydrates, contain a single polyhydroxy aldehyde or ketone unit (e.g., glucose, fructose), contain 3 to 7 carbons, cannot be broken down into simpler units via hydrolysis reaction, pure monosaccharide are water soluble, white crystalline solids

  
• Classification of monosaccharides


  • Aldoses - carbonyl carbon at one end
  • Ketoses - carbonyl carbon in the middle

  
• Nomenclature of monosaccharides


  Carbohydrate nomenclature is unique to "sugar chemistry", monosaccharides are not named using the IUPAC rules, all monosaccharides have names that end in -ose except the trioses glyceraldehyde and dihydroxyacetone, those with a ketone group is usually indicated by using the suffix "ulose"

  
• Monosaccharides


  • Aldoses: aldopentose, aldohexose
  • Ketoses: ketopentose, ketohexose

  
• Stereochemistry of monosaccharides


  Stereoisomers of carbohydrates have different biological effects, aldoses with at least three carbons and ketoses with at least four carbons contain chiral centers, nomenclature must specify the configuration about each asymmetric center, and drawings must be based on a system that clearly specifies these configurations

  
• 2n rule


  When a molecule has more than one chiral carbon, each carbon can possibly be arranged in either the right-hand or left-hand form, thus if there are n chiral carbons, there are 2n possible stereoisomers

  
• Classification based on stereochemistry

  • D isomers - hydroxy group (-OH) on the right side
  • L isomers - hydroxy group (-OH) on the left side

  
• Naturally occurring sugars are D isomers
• Enantiomers


  Stereoisomers that are mirror images of each other
• Diastereomers


  Pairs of isomers that have opposite configurations at one or more of the chiral centers but that are not mirror images of each other
• Epimers


  Two sugars that differ in configuration at only one chiral center
• Cyclization


  Monosaccharides with five or more carbon atoms in the backbone occur as cyclic structures either as a furan ring or pyran ring, cyclization occurs as a result of intramolecular reaction of forming a hemiacetal or hemiketal between the alcohol group and the aldehyde or ketone groups, cyclic sugars are drawn using the Haworth projection

  
• Anomers


  Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom, the hemiacetal (or carbonyl) carbon atom is called the anomeric carbon, the α and β anomers of D-glucose interconvert in aqueous solution by a process called mutarotation

  
• Reactions of monosaccharides


  • Oxidation: aldonic acid, aldaric acid, alduronic acid
  • Reduction: yields sugar alcohol (alditols)

  
• Benedict's test


  A useful test to determine the presence of an oxidation reaction that occurs with sugars, aldose sugars are oxidized by Cu2+ ion, while the Cu2+ ion is reduced to Cu+ ion, the product of this reaction, copper(I) oxide (Cu2O), is not soluble and forms a brick red precipitate in solution, can be used in urine dipsticks to determine the level of glucose in urine
• Sorbitol


  The sugar alcohol produced by the reduction of glucose, can contribute to cataracts in diabetics due to high levels in the blood stream
• 1. glucose


  • The most abundant monosaccharide, the building block for polysaccharides like starch and cellulose, provides energy for cells when metabolized, normal blood glucose levels are in the range of 70–110 mg/dL, excess glucose is converted to the polysaccharide glycogen, insulin regulates blood glucose levels
• 1. galactose


  • A stereoisomer of glucose, a component of milk sugar, individuals with galactosemia lack an enzyme needed to metabolize galactose causing various physical problems
• 1. fructose

  • One of two monosaccharides that form the disaccharide sucrose, found in honey, almost twice as sweet as normal table sugar

  
• Complications of poorly controlled diabetes

  • Cardiovascular disease
  • Chronic renal failure
  • Blindness
• 1. galactose


  • Galactose is a stereoisomer of glucose
  • A component of milk sugar
  • Brain sugar
• Galactosemia


  • Rare inherited disease where individuals lack an enzyme needed to metabolize galactose
  • Galactose accumulates, causing physical problems like cataracts, cirrhosis, and mental retardation
  • Can be detected in newborn screening and affected infants must be given soy-based formula to avoid all milk products with lactose
  • When diagnosed early, individuals who eliminate all lactose- and galactose-containing foods can lead a normal life
• 1. fructose


  • Fructose is one of two monosaccharides that form the disaccharide sucrose
  • Fructose is a ketohexose found in honey and is almost twice as sweet as normal table sugar with about the same number of calories per gram
  • Sweetest of all the sugars; the fruit sugar

  
• HFCS-High fructose corn syrup

  Monosaccharide derivative
• Monosaccharide derivatives


  • Amino sugars - hydroxyl at C2 of the parent compound is replaced with an amino group
  • Deoxy sugars – one or more hydroxyl groups replaced with hydrogen
  • Sugar esters - Condensation of phosphoric acid with one of the hydroxyl groups of a sugar forms a phosphate ester, as in glucose 6-phosphate
• Disaccharides


  • Formed by the condensation and hydrolysis of monosaccharides through glycosidic bonds
  • The -OH group on the anomeric carbon is the most reactive in a monosaccharide

  
• Glycosidic Bond


  Formed when the hydroxyl group on the anomeric carbon of one monosaccharide reacts with another hydroxyl group on another monosaccharide

  
• Naming Glycosidic Bonds


  Need to name the configuration as well as the carbons involved in the bond formation

  
• Maltose


  • Maltose is known as malt sugar
  • Formed by the breakdown of starch
  • Malted barley, a key ingredient in beer, contains high levels of maltose
  • During germination of barley seeds, the starch goes through hydrolysis to form maltose
  • One of the anomeric carbons is free, so maltose is a reducing sugar
• Lactose


  • Lactose is known as milk sugar
  • Found in milk and milk products
  • Intolerance can occur in people who inherit or lose the ability to produce the enzyme lactase that hydrolyzes lactose
  • The glycosidic bond is β(1→4)
  • One of the anomeric carbons is free, so lactose is a reducing sugar
• Sucrose


  • Sucrose is known as table sugar
  • The most abundant disaccharide found in nature
  • Found in sugar cane and sugar beets
  • The glycosidic bond is α,β(1→2)
  • Both anomeric carbons of the monosaccharides in sucrose are bonded, therefore, sucrose is not a reducing sugar
• Reducing vs Non Reducing Sugars

  • Reducing sugars have at least one free anomeric carbon that can be oxidized
  • Non-reducing sugars have both anomeric carbons involved in glycosidic bonds