M.Prasad NaiduMSc Medical Biochemistry, Ph.D,.

Peptides are short sequences of amino acids held by peptide bonds.

Each peptide chain will have 2 ends an amino terminal [N] and a carboxy terminal [C].

Peptide bonds are digested by peptidases and proteases.

1. Glutathione 3 2. TRH 3

3. Enkephalins 54. Angiotensin- II 85. Oxytocin 96. Vasopressin 97. Bradykinin 98. Aspartame 2

Glutathione (GSH)

Glutathione has a Pseudopeptide linkage.

• GS-H is a Tripeptide – 3 amino acids

•It is gama glutamyl – cysteinyl – glycine

•Glutathione is present in RBC and many other tissues.

• Reduced glutathione (GS-H) is the active form

2G – SH G - S – S – G

Reduced Oxidized Active Inactive

Functions of Glutathione

1. Reduced glutathione is essential for maintaining the normal structure of red blood cells.

Glutathione (reduced) performs specialized functions in erythrocytes

It maintains RBC membrane structure and intergrity.

It protects hemoglobin from getting oxidized by agents such as H2O2.

• glutathione keeps iron in ferrous state in haemoglobin there by preventing formation of methaemoglobin.

• RBCs with lowered level of reduced glutathione are more susceptible to haemolysis.

2. Glutathione serves as a coenzyme fro certain enzymes.eg: Prostaglandin PGE2 synthetase.

3. It is essential for the formation of correct disulfide bonds in several proteins.

4. Toxic amounts of peroxides and free radicals produced in the cells are scavanged by glutathione peroxidase (selenium containing enzyme)

Peroxidase2GSH + H2O2 G – S – S – G - + 2H2O

5. Glutathione is involved in the transport of amino acids in the intestine and kidney tubules via γ-glutamyl cycle or meister cycle .

6. It keeps the enzymes in an active state by preventing the oxidation of sulfhydryl (-SH-) group of enzyme to disulfide (-s-s-) group.

7. As a conjugating agent in detoxification (liver)

Conjugation for detoxification:

Glutathione helps to detoxify several compounds by transfering the cysteinyl group

e.g:a. Organo phosphorus compoundsb. Halogenated compoundsc. Nitrogenous substances ( chloro

dinitro benzene)d. Heavy metalse. Drugs.

The reaction is catalyzed by glutathione- s – transferase (GST)

TRH - Thyrotropin-releasing hormoneSecreted by hypothalamus,

causes anterior pituitary gland to release thyrotropic hormone

Methionine enkephalin - Enkephalins (5 Aas)

Opiate-like peptide.Found in brain. Inhibits sense of pain.

Angiotensin- II (8 AAs)

Pressor or hypertensive peptideStimulates release of aldosterone from adrenal cortex.

Oxytocin – (9 AAs)

Oxytocin secreted by posterior pituitary gland

Contains 9 amino acids(nonapeptide)Oxytocin causes contraction of uterus.

Vasopressin (antidiuretic hormone) (9 Aas)

Secreted by posterior pituitary glandcauses kidney to retain water from urine

Plasma bradykinin Bradykinin – 9 AAs

Vasodilator peptideProduced from plasma proteins by snake venom enzymes.

Aspartame -2AAs

It’s a dipeptide produced commercially by combination of aspartic acid and phenylalanine.

It is above 200 times sweeter than sucroseIt is used as a low calorie artificial sweetner in soft drink industry

Iso electric PH (PI) of an amino acid / protein is the PH at which it has both + Ve and – Ve charges in equal quantities and as a whole it is electrically neutral. Hence it does not move in an electric field .protein precipitated.

Iso electric PH (PI) = PK 1 + PK 2

(for Monoamino, Monocarboxylic AA) 2

Ex : for glycine = 2.4 + 9.8 = 6.1

2

Isoelectric PH

Amino acids / Proteins are ampholytes - contain both acidic (-COOH) and basic group(-NH2)

They can donate proton and accept a proton. Hence they are ampholytes.

Zwitterion or Dipolar ion

Amino acids / Proteins act as zwitter ion (dipolar ions) containing positive and negative ionic groups.

All the ionisable groups present in the protein will influence the pI of the protein.

At isoelectric pH proteins will not migrate in electric field .

Proteins have minimum solubility.Hence easily precipitated.Proteins have minimum buffering

capacity .Proteins have low viscosity.

From the graph it is evident that the

buffering action is maximum in and

around pK1 or at pK2 and minimum at plAmino acids can release H+ and act as

weak acids.

It can be quantitatively described by Henderson – Hasselbalch equation :

pH = pKa + log [A-] (Conjugate base)

[HA] (Acid)

It predicts maximum buffering occurs ±1 pH around pKa

ProteinsLysozyme

Pancreatic ribonuclease

Myoglobin

Hemoglobin

Human serum immunoglobulins

Carboxypeptidase

Casein

Fibrinogen

Human serum albumin

Egg albumin

Pepsin

PI11

9.6

7.0

6.8

6.4 – 7.2

6.0

4.6

5.5

4.8

4.6

1.0

Isoelectric pH of some proteins

The pI value is characteristic for each protein. In a

solution at a pH value above its pI, a protein will have a

net negative charge(Anion) below its pI, it has a

positive charge(cation).

By this technique proteins are seperated (immobilized) at Isoelectric pH during electrophoresis.

As the electrophoresis occurs proteins migrate to positions corresponding to isoelectric pH.Serum proteins can be seperated by to 40 different bands.

Curdling of milk – lactic acid formed during curdling of milk brings the pH of milk to 4.6(Iso electric pH of casein) where casein is precipitated.

Heat and Acetic acid Test;Acetic acid is added to urine to bring the pH to

around 4.8 (Iso electric pH of albumin) and then heated to detect albumin in urine.

Non Protein Amino Acidsor

Non standard Amino Acids

Amino acids Functionsα – Amino acidsOrnithineCitrulline Intermediates in the biosynthesis of ureaArginosuccinic acid

ThyroxineTriiodothyronine Thyroid hormones derived from tyrosine.

S-Adenosylmethionine Methyldonor in biological system.Homocysteine Intermediate in methioninen metabolism. A risk factor for coronary heart diseases.

Homoserine Intermediate in threonine, aspartate and methionine metabolism.

3,4-Dihhydroxy phenylanine (DOPA) A neurotransmitter, serves as a precursor of melanin pigment.

Non standard amino acids

Non-α-amino acids

β-Alanine Component of vitamin

pantothenic acid and

coenzyme A

γ-Aminobutyric acid A neurotransmitter produced

(GABA) from glutamic acid

δ- Aminolevulinic acid Intermediate in the synthesis

(ALA) of porphyrin (finally heme)

Taurine Found in association with

bile acids.