Fatty And Amino Acids

Fatty and Amino acid Introduction The elements of fatty acid structure are quite simple. There are two essential elements, a carboxylic acid group and a long hydrocarbon chain whose length ranges from 4 to 30 carbons; 12-24 is most common. The chain is typically linear, and usually contains an even number of carbons. Many fatty acids which occur naturally arise primarily through variation of chain length and degree of saturation. Hence fatty acids are basically carbolic acids. The carbon chain of fatty acid may be saturated or unsaturated. Fatty acids are generally nonpolar due to long carbon chain. Biological fatty acids are present in the form of esters of glycerol as triacylglycerol. The structure is shown below. The group R, R1, R2, are long chain alkyl group.

Triacylglycerol which are liquid at room temperature are known as oil and those which are solid are called fats.

Functions of Fatty acid a)

Serves

as

energy

storage

device

in

triacylglycerol form. b) They serve as hormones. c) They transmit message from one cell to another cell. d) They are components of phospholipids and glycolipid. Saponification It is the process of manufacturing of soap. Chemically it is the alkaline hydrolysis of triacylglycerols. The reaction produces mixture of corresponding fatty acid salts and glycerol. Fats and oils are boiled with solution of hydroxide (sodium hydroxide) until hydrolysis is complete. Sodium chloride is than added to precipitate soap.

The lipolysis is the process of hydrolysis of triacylglycerol which produces a mixture of corresponding fatty acids.

Triacylglycerols are stored in special animal cell called adipocytes. Lipolysis takes place in adipose cell. The product of lipolysis is transported to different cell for utilizing their energy. Inside the cell, they are carried into mitochondria where they are oxidized by two carbons per oxidation with each oxidation yielding an NADH, FADH2 and an acetyl CoA. Each CoA enters Kerbs cycle for oxidation. Amino Acid Amino acids are the basic structural units of proteins. An alpha-amino acid consists of an amino group, a carboxyl group, a hydrogen atom, and a distinctive R group bonded to a carbon atom. This carbon atom is called as the alpha-carbon because it is adjacent to the carboxyl (acidic)

group. An R group is referred to as a side chain. Amino acid structure is shown below:

In cytosol amino acid always exists in one of the three forms shown below:

Amino acid chain is bonded to each other by peptide bond. A protein may contain several peptide bond hence the name polypeptide. A peptide bond creates an amide. The condensation of two molecules of

amino acid creates peptide bond. Reverse reaction is hydrolysis of peptide bond.

Peptide bond has a partial double bond character. This can be explained on the basis that nitrogen has capacity to form four bonds including lone pair. Oxygen can carry partial negative charge due to this electron delocalizing takes place and peptide bond acquires a partial double bond property. The partial double bond does not allow the double bond to rotate and effect secondary and tertiary structure of the peptides. Amino acid can be synthesized by all living organisms. In human body ∀-amino acid is present. The acid is called ∀-amino acid as the amino group is attached to ∀-carbon of carbonyl group. Alkyl group is present on each amino acid and is called side chain of the amino acid. The amino acids are classified based on structure of their

side chain. There are 22 ∀-amino acid that can be derived from proteins. Only 20 amino acids are used by cell for synthesis of proteins. Two amino acids are synthesized after polyamide chain is developed. These are hydroxyproline and cystine. Essential amino acids are those acids which are required by the body, but cannot be synthesized in the body. Hence they have to be supplemented in the food. Each amino is different due to difference in their R group. The R group can be categorized into a) Acidic —The amino acid which contains a carboxylic acid group. b) Basic—the amino acid which has an amine group. c) Polar d) Nonpolar A list of all the 20 amino acid is tabulated.

Polar side of the group is hydrophilic and is attracted towards water and will turn towards water. The nonpolar side of the group is hydrophobic is not water loving and it will turn away from water. Thee properties affect tertiary protein structure. Isoelectric Point In acid solution an amino acid is protonated and exists as cation. In basic solution the amino acid is deprotonated and exists as anion. The intermediate pH at which the amino acid exists exactly balanced between anionic and cationic forms and exists as neutral dipolar zwitter ion is called isoelectric point ‘pI’. This phenomenon can be observed during the titration of any amino acid or protein. Let us take an amino acid and titrate it. A strong base is taken. Initially pH is low but as pH increases acid starts losing its proton and its conjugate base starts increasing until a point is reached where acid loses all its protons. This is the isoelectric point and maximum number of zwitterions is present at this point. The titration continues and we reach a point where the solution is completely basic.

Isoelectric point is controlled by side group ‘R’. Lower pI means acidic group and higher value of pI means basic ’R’ group. Carbohydrates Carbohydrates are the compounds which contain water molecule along with carbon and hydrogen molecules. The general formula of carbohydrates is Cx(H2O)y Simple carbohydrates are known as sugar saccharides. Some of most common sugars are sucrose - ordinary table sugars, glucose - the principal blood sugar, fructose - sugar in fruit, and maltose for malt sugar. The simplest carbohydrate that cannot be further hydrolyzed is called monosaccharides. The carbohydrates that produce two molecules of monosaccharides

on

hydrolysis

are

called

disaccharides

and

the

carbohydrates which yield three molecules of monosaccharides are called trisaccharides and so on. Carboydrates that yield between 3-10

monosaccharides are called oligosaccharides and carbohydrates which give more than 10 monosaccharides are called polysaccharides. Carbohydrate are also defined as polyhydroxy aldehyde or polyhydroxy ketone or the substances which yield polyhydroxy aldehyde or ketone on hydrolysis Fischer projection of fructose and glucose are shown (can also be drawn on other format)

Fischer projection of fructose and glucose shows that fructose is ketone and glucose is aldehyde. General name for the sugars like glucose which contain aldehyde are called aldoses. The general name of sugar which possesses ketone like fructose is called ketose. General name of carbohydrates depending upon number of carbon they posses are called triose , tetrose, pentose, hexose and so on. Some time names are combined like glucose may be called aldohexose: e.g. the fructose may be called as ketohaxose while glucose may be called as aldohexose. D and L designation The simplest monosaccharide compounds are glyceraldehydes which has a stereocenter. Hence glyceraldehydes exist in two enantiomeric forms.

Fischer projection is shown for both enantiomers. These two compounds serve as configurational standards for saccharides. A monosaccharide which has its highest numbered stereocenter and same configuration as D(+) glyceraldehydes is designated as D-sugar. The sugar which has same configuration as L-(-) glyceraldehydes is called L-sugar. See figure below:

Anomeric carbon Sugars or carbohydrates or saccharide open chain structure exists in equilibrium with cyclic structures. The cyclic structure is formed by intermolecular reaction between carbonyl carbon (from aldehyde or ketone group) and alcohol group at highest numbered carbon. The cyclic structure of D-(+) – glucose are called hemiacetals.

In the structure shown below, alcohol group on chiral carbon marked number 5 acts as a nucleophile and attacks carbonyl group. This causes a ring structure to be formed along with formation of hemiacetals. Carbon marked as number 1 in the diagram is called anomeric carbon. It is the only carbon in the molecule which is bonded to two oxygen atoms one from the alcohol group and other from the carbonyl group.

The anomeric carbon is called ∀-anomeric carbon if –OH of glucose is opposite to ---CH2OH at carbon number 5. It is called ∃-anomeric carbon if ---OH of glucose is on the same side of ----CH2OH of carbon no.5. The cyclic structures are also named depending number of members on the ring including oxygen. A six member ring is called pyranose. A five member ring is called furanose. Hence the full name of a glucose ring structure is glucopyranose. Glycosides The reaction between monosaccharide hemiacetal with alcohol yields an acetal in which anomeric –OH is replaced by –OR. These acetals are called glycosides.

The reaction between ∃-glucopyranose and methanol produces ∀ and ∃ methyl glycopyranosides (Structure shown below). The group which is attached to anomeric carbon of a glycoside is called aglycone.

Methyl ∀- glycopyranoside Oxidation of sugars Sugars contain aldehyde and ketone in their open chain structure. Hence like other aldehydes, aldoses (sugars) are easily oxidized by oxidizing agents. Aldoses react with Tollens reagent, Fehling reagent, Benedict reagent. These reactions serve as tests for reducing sugars. All aldoses are reducing sugars because they contain aldehyde carbonyl group so as some ketoses also. Ketoses reduce because they are isomerized in basic solution to aldoses by a keto-enol tautomeric effect. Glycosides do not react with Tollens reagent as glycosides do not have an open chain form.

Disaccharides Disaccharides contain a glycosidic acetal bond between the anomeric carbon of one sugar and an -OH at any position on the other sugar. Most common bonds are between C1 of the first sugar and C4 of the second sugar. There are other combination also such as C1 - C1 and C1 - C6. These linkages are called glycosidic linkages. Disaccharides will give Tollens reagent test if there is an anomeric carbon free to react and does not involve in glycosidic bond.

Some common disaccharides and polysaccharides are listed with their linkage.

The process of hydrolysis breaks the glycosidic bond. The presence of an enzyme speeds up the breaking process. Animals do not posses the enzymes to break the cellulose. Some human lack the enzymes which breaks lactose. **************************************************