Physical Properties of Carbohydrates.

 

• Definition:

• Carbohydrates are hydrates of carbon, which on hydrolysis produce polyhydroxy aldehydes or polyhydroxy ketones.

OR

• Carbohydrates are the derivatives of the polyhydroxy aldehydes or ketones or the products derived from them.

Physical Properties of Carbohydrates:

Asymmetric carbon atom

• A carbon atom to which four different atoms or groups of atoms are attached is said to be an asymmetric carbon atom.

• Many biochemicals contain two or more asymmetric carbon atoms.

• The presence of asymmetric carbon atom allows formation of isomers. 

• The compounds which have the same structural formula, but differ only in spatial configuration are called stereo-isomers or geometric isomers. 

• More the number of asymmetric carbon atoms, more are the number of isomers; 

• e.g. glucose with four asymmetric carbon atoms has 2"= 2 = 16 isomers; n indicates the number of asymmetric carbon atoms.

• If the molecule contains one double bond with two different atoms or groups of atoms linked with every carbon, then there exists cis-trans isomerism. 

• Since the double bond is rigid, the atoms attached to it are not free to rotate like those attached to a single bond. 

• The example of cis-maleic acid and trans-fumaric acid on a planar form are show here. These isomers have different chemical and physiological properties; e.g. fumaric acid is physiologically active.

 

Classification of Carbohydrates Isomers:

• Isomers of carbohydrates are classified as follows:

• D and L isomers.

• Optical isomers.

• Epimers.

• Anomers.

• D and L isomers: 

• When the -OH group around the carbon atom adjacent to the terminal primary alcohol carbon (carbon 5 in case of glucose) is on the right side, the sugar belongs to the D-series. 

• If the hydroxyl group is on the left side, it is a member of the L-series. 

• Majority of monosaccharides in mammals are of D configuration.

• Optical isomers: 

• When a beam of polarized light is passed through a solution exhibiting optical activity, it will be rotated to the right or left in accordance with the presence of the optical isomer. 

• A compound which causes rotation of polarized light to the right is said to be dextro-rotatory and is designated with a plus (+) sign. 

• Rotation of the beam to the left is termed as laevo-rotatory and is designated by the minus (-) sign.

• Stereoisomerism and optical isomerism are independent properties.

• Epimers: 

• Isomers formed as a result of interchange of -OH and -H on carbon atoms 2, 3 and 4 of glucose are known as epimers. 

• In the body, epimerization takes place by the enzyme epimerase.

• Anomers:

• The cyclic structure of glucose is retained in solution, but isomerism takes place about position 1. 

• This is accomplished by optical rotation (mutarotation) by which the positions of -H and -OH groups are changed around carbon 1. 

• Inter-conversion of A and B glucose in solution with change of optical activity is called mutarotation. 

• It is explained by the A and B form of glucose requiring the presence of asymmetric carbon C.

Commonly Asked Questions.

1. Define carbohydrates and write a note on different Isomers of Carbohydrates.

Biological Role of Carbohydrates.

 

• Definition:

• Carbohydrates are hydrates of carbon, which on hydrolysis produce polyhydroxy aldehydes or polyhydroxy ketones.

Biological Role of Carbohydrates:

• Carbohydrates are the chief energy source in many animals; they are an instant source of energy. 

• Glucose is broken down by glycolysis/ krebs cycle to yield ATP.  

• Glucose is the source of storage of energy. It is stored as glycogen in animals and starch in plants.  

• Stored carbohydrates act as an energy source instead of proteins.  

• Carbohydrates are intermediates in biosynthesis of fats and proteins.  

• Carbohydrates aid in regulation of nerve tissue and are the energy source for the brain.  

• Carbohydrates get associated with lipids and proteins to form surface antigens, receptor molecules, vitamins and antibiotics.  

• They form structural and protective components, like in the cell wall of plants and microorganisms.  

• In animals they are an important constituent of connective tissues.  

• They participate in biological transport, cell-cell communication and activation of growth factors.  

• Carbohydrates that are rich in fiber content help to prevent constipation.  

• Also they help in modulation of the immune system.

Commonly Asked Questions.

1. Define Carbohydrates and give their biological importance.

Elimination: Introduction to Biotransformation.

 

• Elimination is the major process for removal of drugs from the body and termination of the drug action.

• It causes irreversible loss of the drug from the body.

• Eliminations occur two process:

• Biotransformation (metabolism)

• Excretion.

Biotransformation:

• It is defined as the conversion of a chemical form of drug into another form using biological machinery.

• The products of biotransformation may vary,

• Some may retain activity.

• Some effectless.

• Some are even more effective.

Xenobiotics:

• Any foregin chemical that is not a nutrient to the body and enters the body by ingestion, inhalation and absorption is called a xenobiotic.

Organs of Biotransformation:

• Major site for biotransformation of xenobiotics is “Liver”.

• Other organs where biotransformation takes place are as follows, (Descending order of extent),

• Liver

• Lungs 

• Kidneys

• Intestine

• Placenta

• Skin.

• Other organs are brain, muscles, spleen etc.

Drug metabolizing Enzymes:

• The enzymes that cause biotransformation of the xenobiotics are different from the enzymes that cause metabolism of the nutrients, they are of following types.

1. Microsomal Enzymes:

• The microsomal enzymes catalyze a majority of drug biotransformation reactions. 

• A microsome is a fragment of endoplasmic reticulum and attached ribosomes.

• A large variety of microsomal enzymes catalyze a number of oxidative, reductive and hydrolytic and glucuronidation reactions.

• Some important characteristics of the microsomal enzyme system are: 

• Lipoidal membrane-bound enzymes of the microsomes are essential for its selectivity towards lipid-soluble substrates.

• The lipid-soluble substrate is biotransformed into a water soluble metabolite by the microsomal enzymes which can be easily excreted.

• e.g. Cytochrome P450.

2. Non Microsomal Enzymes:

• The non-microsomal enzymes include those that are present in soluble form in the cytoplasm and those attached to the mitochondria but not to endoplasmic reticulum. 

• These are also non-specific enzymes that catalyze few oxidative reactions, a number of reductive and hydrolytic reactions and conjugation reactions other than glucuronidation.

• e.g. oxidases, peroxidases, dehydrogenases, esterases, etc.

CHEMICAL PATHWAYS OF DRUG BIOTRANSFORMATION

• R.T.Williams, the leading pioneer in drug biotransformation research divided the pathways of drug metabolism reactions into two general categories

• Phase I reactions,

• Phase II reactions.

1. Phase I Reactions

• These reactions generally precede phase II reactions and include oxidative, reductive and hydrolytic reactions. 

• Generally the polar groups are added to the xenobiotics.

• Called “Asynthetic Reactions” as product is not totally altered.

• The primary objectives phase I reactions are,

• 1. Increase in hydrophilicity

• 2. Reduction in stability

• 3. Facilitation of conjugation (Phase II).

• Outcome of the Phase I reactions may be active, more active or inactive metabolites.

2. Phase II reactions.

• Also called “Conjugation Reactions”.

• Endogenous high molecular weight substances like glucuronic acid and glycine are attached to the xenobiotics to form high molecular weight conjugates hence called “Conjugation reactions”.

• As new compounds are formed they are called “Synthetic Reactions”.

• The outcome of the reactions is mostly inactive compounds and hence these reactions are considered as true detoxification reactions.

• Enzymes involved are “Transferases”.

Commonly Asked Questions:

1. Define biotransformation of Drugs.

2. Define Xenobiotics.

3. Write a note on enzymes involved in biotransformation of drugs.

4. Why are drugs called Xenobiotics?

Carbohydrates Definition and Classification.

 

• Definition:

• Carbohydrates are hydrates of carbon, which on hydrolysis produce polyhydroxy aldehydes or polyhydroxy ketones.

Introduction:

• Carbohydrates are products of plant origin and are a part of an extremely large group of naturally occurring organic compounds. 

• e.g.  Sugar Cane, Starch 

• Carbohydrates are a good source of energy.

• Carbohydrates are also known as saccharides, the word saccharide comes from the Greek word saccharon which means sugar.

Classification of Carbohydrates:

• Carbohydrates can be classified by following ways,

• Depending on the product of hydrolysis.

• Depending on Physical properties.

• On the basis of test with chemical reagents

1. On the basis of Hydrolysis:

1. Monosaccharides: 

1. The simple form of carbohydrates.

2. A carbohydrate that can be hydrolyzed only once to break down into simpler units of polyhydroxy aldehyde or ketone is called monosaccharide. 

3. e.g. glucose, fructose, mannose, etc. 

2. Oligosaccharides: 

1. Sugars that on hydrolysis yield two or nine molecules of monosaccharides are called oligosaccharides. 

2. These are further classified as di-, tri- or tetrasaccharides, etc. 

1. (a) Disaccharides: These are sugars that produce two molecules of the same or different monosaccharides on hydrolysis. Examples are sucrose, maltose, and lactose. 

2. (b) Trisaccharides: Sugars that yield three molecules of the same or different monosaccharides on hydrolysis are called trisaccharides. An example of trisaccharides is Raffinose.

3. Polysaccharides:

1. On hydrolysis, polysaccharides yield a large number of monosaccharides. 

2. If monosaccharides are the same then they are called Homopolysaccharides.

3. If monosaccharides are different, then they are called Heterpolysaccharides

4. Examples of polysaccharide are starch, glycogen, Dextrin, Cellulose etc. 

2. On the basis of Physical Characteristics:

• On the basis of Physical Characteristics carbohydrates are classified as,

• Sugars.

• Non Sugars.

1. Sugars:

• Sugars are crystalline substances, taste sweet and readily water soluble. 

• Because of their fixed molecular weight, sugars have sharp melting points. 

• e.g.  glucose, fructose, sucrose, lactose, etc. 

2. Non Sugars:

• Characteristics of non-sugars are Amorphous, Tasteless, water-insoluble substances with variable melting points.

• E.g. Starch, Glycogen.

C) On the basis of testing  with chemical reagents:

• Depending on chemical reactions with certain chemical reagents the carbohydrates are classified as ,

• Reducing Sugars.

• Non reducing sugars.

1. Reducing Sugars:

• These have a free aldehyde (-CHO) or ketone (C=O) group. 

• These have the ability to reduce the cupric ions (Cu++; blue) in Fehling’s or Benedict’s Solution to cuprous ions (Cu+; reddish) that separate out as cuprous oxide (Cu2O) from the solution. 

• e.g.  Glucose, lactose, melibiose, gentiobiose, cellobiose, mannotriose.

2. Non-reducing sugars:

• A free aldehyde or ketone group is absent.  

• No cuprous oxide (Cu2O) producing chemical reaction takes place.  

• Examples are sucrose, trehalose, raffinose, gentianose, melezitose.

Commonly Asked Questions.

1. Define and classify Carbohydrates.