It is all about carbohydrates, Slides of Biochemistry

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Carbohydrates

BCHM 211 Biochemistry 2 nd^ Semester AY 2020- Natural Science Department College of Arts and Sciences Our Lady of Fatima University /jlp

Importance of Carbohydrates

• Carbohydrates are compounds of tremendous biological importance:
• they provide energy through oxidation
• they supply carbon for the synthesis of cell components
• they serve as a form of stored chemical energy
• they form part of the structures of some cells and tissues
• Carbohydrates, along with lipids, proteins, nucleic acids, and other compounds are known as biomolecules because they are closely associated with living organisms.

Carbohydrates

• Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis

Classes of Carbohydrates

Stereochemistry of CArbohydrates

• Glyceraldehyde, the simplest carbohydrate, exists in two isomeric forms that are mirror images of each other:
• These forms are stereoisomers of each other.
• Glyceraldehyde is a chiral molecule — it cannot be superimposed on its mirror image. The two mirror image forms of glyceraldehyde are enantiomers of each other.

Chirality

• Chiral molecules have the same relationship to each other that your left and right hands have when reflected in a mirror.
• Achiral objects can be superimposed on the mirror images — for example, drinking glasses, spheres, and cubes.
• Any carbon atom which is connected to four different groups will be chiral, and will have two nonsuperimposable mirror images; it is a chiral carbon or a center of chirality.
• if any of the two groups on the carbon are the same, the carbon atom cannot be chiral.
• Many organic compounds, including carbohydrates, contain more than one chiral carbon.

n

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 2 n possible stereoisomers. Maximum number of possible stereoisomers = 2 n

Naming isomers

• When there is more than one chiral center in a carbohydrate, look at the chiral carbon farthest from the carbonyl group: if the hydroxy group points to right when the carbonyl is “up” it is the D-isomer, and when the hydroxy group points to the left, it is the L-isomer.

Chirality

Stereochemistry matters!

Monosaccharides

• The monosaccharides are the simplest of the carbohydrates, since they contain only one polyhydroxy aldehyde or ketone unit.
• Monosaccharides are classified according to the number of carbon atoms they contain:
• The presence of an aldehyde is indicated by the prefix aldo- and a ketone by the prefix keto-.

Physical Properties of Monosaccharides

• • Most monosaccharides have a sweet taste (fructose is sweetest; 73% sweeter than sucrose).
• They are solids at room temperature.
• They are extremely soluble in water:
• Despite their high molecular weights, the presence of large numbers of OH groups make the monosaccharides much more water soluble than most molecules of similar MW.
• Glucose can dissolve in minute amounts of water to make a syrup (1 g / 1 ml H 2

O).