Al-Turath University Biochemistry Lecture Notes PDF

Al-Turath University Optics Department st 1 Stage BIOCHEMISTRY Assistant Lecturer: Ruqaya Raad  Lecture – 1 CARBOHYDRATE S 1 Biochemistry: Biochemistry is the study of the chemical substances found in living organisms and the chemical interacti...

Al-Turath University Optics Department st 1 Stage BIOCHEMISTRY Assistant Lecturer: Ruqaya Raad  Lecture – 1 CARBOHYDRATE S 1 Biochemistry: Biochemistry is the study of the chemical substances found in living organisms and the chemical interactions of these substances with each other. It deals with the structure and function of cellular components, such as proteins, carbohydrates, lipids, nucleic acids, and other biomolecules. Carbohydrates: Carbohydrates may be defined chemically as aldehyde or ketone derivatives of polyhydroxy alcohols or as compounds that yield these derivatives on hydrolysis, Carbohydrates are: 1- A major source of energy from our diet. 2- Composed of the elements C, H, and O. 3- Also called saccharides, which means “sugars.” 4- Carbohydrates are produced by photosynthesis in plants. glucose is synthesized in plants from CO2, H2O, and energy from the sun then oxidized in living Carbohydrates include not only sugar, but also the starches that we find in foods, such as bread, pasta, and rice. 2 Biological importance of carbohydrates: 1- They provide energy through oxidation. 2- They supply carbon for the synthesis of cell components. 3- They serve as a form of stored chemical energy. 4- They form part of the structures of some cells and tissues. Classification of Carbohydrates: Depending on the number of sugar units in their molecules, carbohydrates can be divided into: 1- Monosaccharides contain a single polyhydroxy aldehyde or ketone unit (e.g., glucose, fructose). 2- Disaccharides consist of two monosaccharide units linked together by a covalent bond (e.g., sucrose). 3- Oligosaccharides contain from 3 to 10 monosaccharide units (e.g., raffinose). 4- Polysaccharides contain very long chains of hundreds or thousands of monosaccharide units, which may be either in straight or branched chains (e.g., cellulose, glycogen, starch). 3 The 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. Isomers means: Chemical compounds that have the same chemical formula but differ in properties and the arrangement of atoms in the molecule. 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. 4 Chirality and Handedness: Chiral molecules have the same relationship to each other that your left and right hands have when reflected in a mirror. Chiral objects cannot be superimposed on their mirror images — e.g., hands, gloves, and shoes. Achiral objects can be superimposed on the mirror images — e.g., drinking glasses, spheres, and cubes. 5  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. 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. Maximum number of possible stereoisomers = 2n 6 Fischer Projections Fischer projections are a convenient way to represent mirror images in two dimensions. A striking feature of carbohydrate structure is the presence of chirality centers. The simplest aldose, glyceraldehyde, has one chirality center— one carbon atom bonded to four different groups. Thus, there are two possible enantiomers—mirror images that are not superimposable. To distinguish the two enantiomers, the prefixes d and l precede the name. Thus, the naturally occurring enantiomer is labeled d- glyceraldehyde, while the unnatural isomer is l-glyceraldehyde. Using a Fischer projection formula, place the carbonyl group at or near the top and the last achiral CH 2OH at the bottom. Therefor d- glyceraldehyde becomes: 7 In summary: 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. 8  Molecules which are enantiomers of each other have exactly the same physical properties (melting point, boiling point, index of refraction, etc.) but not their interaction with polarized light. Polarized light vibrates only in one plane; it results from passing light through a polarizing filter. Optical Activity A levorotatory (–) substance rotates polarized light to the left [e.g., l-glucose; (-)-glucose]. A dextrorotatory (+) substance rotates polarized light to the right [e.g., d-glucose; (+)-glucose]. Molecules which rotate the plane of polarized light are optically active. Many biologically important molecules are chiral and optically active. Often, living systems contain only one of the possible stereochemical forms of a compound, or they are found in separate systems. – L-lactic acid is found in living muscles; D-lactic acid is present in sour milk. – In some cases, one form of a molecule is beneficial, and the enantiomer is a poison (e.g., thalidomide). – Humans can metabolize D- monosaccharides but not L-isomers; only L-amino acids are used in protein synthesis. 9 Assignments: 1- Define Biochemistry? 2- Why are carbohydrates so important? 3- Classify carbohydrates with examples? 4- True or false: a- The simplest carbohydrate is glyceraldehyde. b- Glyceraldehyde is a chiral molecule. c- Carbohydrates include sugar only. d- Glucose is a disaccharide. e- Chiral objects can be superimposed on their mirror images. f- Cubes are achiral object 5- Draw the structure of L-Glucose: 10