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
  • 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"

  
• There must be at least one asymmetric carbon to have stereoisomers
• Asymmetric carbon

  4 different things are attached to it
• Stereoisomers

  Spatial isomers
• Fischer projection


  Used to represent stereochemistry of monosaccharides
• Stereoisomers of carbohydrates have different biological effects
• 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

  
• D and L forms

  Determined by the configuration at the highest-numbered chiral center (penultimate carbon or second to the last carbon), L isomers have the hydroxy (-OH) group attached to the left side of the asymmetric carbon, D isomers have the hydroxy group (-OH) on the right 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

  
• Anomeric carbon


  The hemiacetal (or carbonyl) carbon atom, 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)
  • Rearrangement of ketose to aldose
  • Benedict's test - used to determine the presence of an oxidation reaction that occurs with sugars

  
• Sorbitol


  The sugar alcohol produced by the reduction of glucose, can contribute to cataracts in diabetics
• 1. glucose


  The most abundant monosaccharide, the building block for polysaccharides like starch and cellulose, provides energy for cells when metabolized, regulated by insulin
• 1. galactose


  A stereoisomer of glucose, a component of milk sugar, individuals with galactosemia lack an enzyme needed to metabolize it
• 1. fructose


  One of two monosaccharides that form the disaccharide sucrose, the sweetest of all the sugars, found in honey
• complications

  Including cardiovascular disease, chronic renal failure, and 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 a variety of physical problems, including 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

  • Condensation and Hydrolysis—Forming and Breaking Glycosidic Bonds
  • The –OH group that is most reactive in a monosaccharide is the one on the anomeric carbon
  • When this hydroxyl group reacts with another hydroxyl group on another monosaccharide a glycosidic bond is formed

  
• Glycosidic Bond

  Carbon 1 is the anomeric carbon

  
• Naming Glycosidic Bonds

  It is necessary to name the configuration as well as the carbons involved in the bond formation

  
• Maltose

  • Maltose is known as malt sugar
  • It is 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
  • It is found in milk and milk products
  • An intolerance to lactose can occur in people who inherit or lose the ability to produce the enzyme lactase that hydrolyzes lactose into its monosaccharide units
  • 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
  • It is the most abundant disaccharide found in nature
  • Sucrose is 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