Presented By:

Dr.SHYAM.S.KUNJWAL DEPARTMENT OF ZOOLOGY SCHOOL OF SCIENCES UTTARAKHAND OPEN UNIVERSITY

Programme code:ZO503BLOCK – III: PHYSIOLOGICAL CHEMISTRY

TOPIC:AMINO ACIDS&PEPTIDESTOPIC:AMINO ACIDS&PEPTIDESTOPIC:AMINO ACIDS&PEPTIDESTOPIC:AMINO ACIDS&PEPTIDES

M.Sc. Zoology I YearM.Sc. Zoology I YearM.Sc. Zoology I YearM.Sc. Zoology I Year

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Amino acids are the essential components of all living cell as they are thebuilding blocks of proteins. Although over 300 different amino acids are found in nature, only 20 amino

acids are involved in protein and are called as standard amino acids. Amino acids are also metabolically active and supply substrates for many

other biochemical reactions. Chemically, amino acids have a carboxyl groupand an amino group, bonded to the same carbon atom. The general structuralformula of the amino acid: (fig. 1).

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Fig.1 General Structure of an Amino Acid In protein amino acids, it is always the α-carbon atom which is aminated and hence

they are called α-amino acids.

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A α -amino acid consists of a central carbon atom, called the carbon,linked to an amino group, a carboxyl group, a hydrogen atom, and adistinctive R group. The R group is often referred to as the side chain. With four different

groups connected to the tetrahedral a-carbon atom, a-amino acids arechiral (with the exception of glycine- for which R is the H atom); the twomirror-image forms are called the L- isomer and the D-isomer (fig.2). All the amino acids which occur in proteins belong to L-form however D-

amino acids have been discovered in antibiotics.

Fig 2.Two mirror image forms of amino acid

Peptide Bond, Formation of a Dipeptide

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Peptides are chains of amino acids &two amino acid molecules are linked together

by a covalent bond, called a peptide bond. Peptide bond formation is an example of a condensation reaction, which is formed

by removal of the elements of water from the α-carboxyl group of one amino acidand α-amino group of another. The product formed by a peptide bond is a peptide. Two amino acids can be joined

by peptide bond to form a dipeptide . similarly, amino acids can be linked to formtripeptides, tetrapeptides, pentapeptides and so forth.

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When a few amino acids can be joined in this manner, the structure iscalled an oligopeptide. When many amino acids are joined, the product iscalled a polypeptide. Polypeptides generally have molecular weights below10,000. The polypeptide chains form proteins. A protein may consist of asingle or more polypeptide chains and has higher molecular weight. An amino acid unit in a peptide is often called a residue. In a peptide, the

amino acid residue at the end with a free α-amino group is the aminoterminal (N-terminal) residue, the residue at the other end, which has a freecarboxyl group, is the carboxyl terminal (C-terminal) residue (fig.3).

Fig 3 C-terminal and N-terminal ends in a peptide chain

Amino acids can be classified on the basis of their R groups (side chain) into the followingclasses:

1. Amino Acids with Nonpolar, Aliphatic R groupsThe R groups in this class of amino acids are non-polar and hydrophobic. Thefollowing amino acids are classified as having non-polar side chains

2. Amino Acids with Nonpolar, Aromatic R groups:

The R groups in this class of amino acids are relatively non-polar aromatic andhydrophobic in nature. The following amino acids are classified as having non polararomatic side chains

Amino Acids with Nonpolar, Aromatic R groups

3.Amino Acids with Polar, Uncharged R groupsThe R groups in these amino acids are more soluble in water or more hydrophilic thanthose of the non-polar amino acids as they contain functional groups that formhydrogen bonds with water. The following are classified under this class.

Amino Acids with Polar, Uncharged R groups

4. Amino Acids with Positively Charged Polar R groups The R groups in this class of amino acids are polar and positively charged. These arehydrophilic in nature. The following amino acids are classified as having positivelycharged R groups.

Amino Acids with Positively Charged Polar R groups

5. Amino Acids with Negatively Charged Polar R groups The R groups in this class of amino acids are polar and negatively charged. These arehydrophilic in nature and have a second carboxylic group. The following amino acids areclassified under this class.

Amino Acids with Negatively Charged Polar R groups

ESSENTIAL AND NONESSENTIAL AMINO ACIDS:

PROPERTIES OF AMINO ACIDS:

PHYSICAL PROPERTIES:

1. General: Amino acids are usually colorless. Crystalline solids, with highmelting points (usually > 200 0C). These are relatively smaller moleculeswith average molecular weight about 120 D. They have high boiling points.2. Solubility: Amino acids are usually soluble in water. However, the solubilityvaries from different amino acids.3. Optical Isomerism: All amino acids except glycine have at least one chiral(asymmetric) carbon atom and therefore they are optically active. They existin D- and L-isomeric form.4. Absorption of U.V.: Three amino acids tryptophan, tyrosin andphenylalanine (aromatic amino acids) absorb ultraviolet light. Since mostbiological proteins contain at least a tyrosine residue, the absorption of lightby protein in UV (280 nm) range has been used as a conventional method fordetermining protein content spectrophotometrically.

CHEMICAL PROPERTIES:

1. Amphoteric Nature: Amino acids in aqueous solutions are ionized andcan act as acids or bases. Substances having this dual nature areamphoteric substances and are called ampholytes, since amino acidscontain both amino and carboxyl group, they are capable of both donatingand accepting protons

Amphoteric Nature of an Amino Acid

2. Zwitterions: At a certain pH (isoelectric pH, pI), the amino acidmolecules in aqueous solution carry both the positive and negativecharges in equal amount and exist as zwitterions (dipolar ion). At thispoint the net charge in it is zero. The zwitterions solutions of amino acidsdo not migrate to any pole in an electric field. However, if acid is added tothe solution they become positively charged ions and move to the cathode.Addition of alkali makes them negatively charged and they move to anode.

Formation of Zwitterions

Peptide Bond Formation: Amino acids are joined together in aprotein molecule by peptide bonds (-CO-NH-) formed by thecondensation of α-COOH of one amino acid with α-NH2 groupof another one with the release of one molecule of water .

Formation of Peptide Bond

Ninhydrin Reaction: Amino acids yield coloured productswith ninhydrin (triketohydrindene hydrate)

Ninhydrin Reaction: amino acid yields coloured product with ninhydrin

BIOLOGICAL ROLE OF AMINO ACIDS

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Amino acids participate in a number of biological activities:

They are building blocks of proteins. Proteins form structural as well asfunctional components of the cell.Some amino acids give rise to other compounds, such as:

Tyrosine is part of hormone thyroxin and adrenaline.Glycine participates in the synthesis of heme.Tryptophan produces vitamin nictotinamide and plant hormoneindole-3-acetic acid (IAA).

Β-Alanine is needed in the synthesis of coenzyme-A and vitaminpantothenic acid.Nonprotein amino acids are components of antibiotics.Because of amphoteric nature they act as buffers in the body fluids.They also synthesize glucose by gluconeogenesis.Histamine is an important vasodialator, derived from amino acid

histamines.

BIOLOGICAL ROLE OF PEPTIDES

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Peptides participate in a number of biological activities:They serve as intermediates in the formation of proteins.They appear as constituents in a group of compounds called alkaloids.

Majority of these have been isolated from fungi, although they are alsofound in higher plants. Ergotamine is a peptide alkaloid from rye ergotand has pronounced pharmacological properties.Many of them possess antibacterial activities and are usually present in

fungi and bacteria. Penicillin G is a common antibiotic.Certain other peptides serve as growth factors. Folic acid, a water

soluble vitamin is an example of it. Another group of peptides serving asgrowth factor for a variety of microorganisms is streptogenins.

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Higher animals do synthesize certain peptides serving ashormones e.g. oxytocin, bradykinin etc.Certain peptides like glutathione participate in controlling

the oxidation-reduction potential of the cell. This may alsoserve as a key intermediate in electron transfer systems.Enkephalins (pentapeptides) formed in certain part of brain.

These are associated with the perception of pain andpleasure.Some peptides are components of peptidoglycans which

form bacterial cell wall.

Most Possible long question answer from this topic: 1. What are amino acids? Describe briefly the physical and chemical propertiesof amino acids?2. Classify various amino acids on the basis of their side chains and give theirstructure?3. Describe the general reactions of amino acids and their biological significance?4. What are peptides? Describe the structure and biological importance ofpeptides? Most Possible Short notes:1. Peptide Bond2. Amphoteric Property of amino acids3. Essential and Nonessential Amino Acids4. Non standard amino acids5. Zwitterions

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