Protein Metabolism Lecture Notes (PDF)

Metabolism of Individual Amino Acids Medical Biochemistry and Molecular Biology Department  Glycine Glycine is non-essential, glucogenic amino acid. Importance and fate of glycine: - 1. Purine bases:- – Carbon atoms No 4, 5 and nitrogen atom No 7 are...

Metabolism of Individual Amino Acids Medical Biochemistry and Molecular Biology Department  Glycine Glycine is non-essential, glucogenic amino acid. Importance and fate of glycine: - 1. Purine bases:- – Carbon atoms No 4, 5 and nitrogen atom No 7 are derived from glycine. 2. Glutathione:- – It is a tripeptide formed of 3 amino acids (glutamate, cysteine and glycine) – It acts as a hydrogen carrier. 3. Creatine (methyl-guanido-acetate):- – Creatine is used by muscles in the form of creatine phosphate as a source of energy during muscle exercise. – It is synthesized from 3 amino acids:- glycine, arginine and methionine 4. Heme synthesis:- – Glycine reacts with succinyl CoA to form heme. 5. Collagen synthesis:- – Collagens has a triple helical structure. – Glycine is present in every third position in each helix. Gly -X- Y- Gly - X - Y- Gly -X - Y ---------- etc 6. Bile salts:- Glycine is conjugated with primary bile acids e.g with cholic acid to form glycocholic acid. 7. Formation of serine:- by Serine hydroxymethyl transferase 8. Hippuric acid:- Glycine conjugates with toxic benzoate to form the non-toxic hippuric acid. 9. Neurotransmitter: Glycine acts as inhibitory transmitter in spinal cord. 10. Formation of glyoxylic acid.  Phenylalanine Phenylalanine:- Is an essential, mixed glucogenic, ketogenic amino acid. Functions:- – Enter in the structure of body proteins – Formation of tyrosine mainly in the liver Phenylalanine hydroxylase. - The reaction is irreversible Deficiency of any of the component of the reaction leads to a disease called phenylketonuria. O2 PA Hydroxylase H2O Phenylalanine Tyrosine H4biopterin H2biopterin DHB reductase NADP+ NADPH+H+ DHB = Dihydrobioptren  Tyrosine It is a non-essential amino acid being synthesized from Phenylalanine. It is mixed glucogenic, ketogenic. Important derivatives of tyrosine:-Tyrosine is the precursor of; 1. Catecholamines: - in adrenal medulla and adrenal neurons. It includes dopamine, noradrenaline and adrenaline. 2. Thyroid hormones: - Tyrosine is the precursor of thyroid hormones (T3,T4) in thyroid gland. 3. Melanin pigment: - Melanins are pigments present in many tissues particularly in the eye, hair and skin. 4. Phenol, cresol and tyramine: - These are putrefactive substances produced by the action of bacteria present in large intestine on tyrosine.  Tryptophan It is essential amino acid It is mixed glucogenic, ketogenic amino acid Catabolism and important derivatives of tryptophan:- 1. Biosynthesis of alanine and acetoacetyl CoA. 2. Biosynthesis of niacin which is a member of vitamin B-complex 3. Biosynthesis of serotonin ( 5-hydroxy tryptamine) – It is a stimulatory neurotransmitter 4. Biosynthesis of melatonin:- – It is a hormone produced by pineal gland (body) 5. Synthesis of indol and skatol.  Arginine It is a semi-essential, glucogenic amino acid. Micro-organisms can synthesis arginine from glutamate. Importance of arginine:- arginine enters in the synthesis of: 1. α-ketoglutarate (a-KG):- 2. Creatine 3. Synthesis of streptomycin in streptococcus 4. Conversion via ornithine to putriscine, spermine and spermidine. 5. Synthesis of arginine phosphate in invertebrates 6. Synthesis of urea 7. Synthesis of nitric acid  Serine It is non-essential, glucogenica.a. Fate and importance of serine:- It participate in the synthesis of:- 1. Glycine:- by serine hydroxymethyltransferase. 2. Cysteine:- by trans-sulfhydration with homocysteine. 3. Ethanolamine and choline. 4. Sphingosine:- Formed of serine and palmitoyl CoA. 5. Purine and pyrimidine bases:- The beta-carbon is a source of the methyl groups of thymine and of C2 and C8 of purine bases. 6. Synthesis of phosphoproteins e.g casein.  Methionine It is an essential, glucogenic a.a. Functions:- It enters in the synthesis of 1. Methyl donor:- known as S-adenosyl methionine (SAM). – SAM:- Acts as methyl donor to the following 1) Guanidoacetate Creatine 2) Noradrenaline Adrenaline 3) Ethanolamine Choline 4) Carnosine Anserine 2. Glucose. 3. Cysteine. 4. Polyamines: - that include spermine and spermidine.  Glutamic acid It is non-essential, glucogenic a.a. Biosynthesis:- It is synthesized from α-ketoglutarate. Functions of glutamate:- 1. Synthesis of α-ketoglutarate that gives glucose 2. Synthesis of glutamine by glutamine synthase. 3. Biosynthesis of proline and hydroxyproline 4. Biosynthesis of ornithine 5. Biosynthesis of glutathione 6. Biosynthesis of gamma-amino butyric acid (GABA) by decarboxylation. GABA is an inhibitory transmitter in the brain.  Aspartic acid It is non-essential, glucogenic amino acid. Biosynthesis: - It is synthesized from oxaloacetate by transamination. Importance and fate of aspartic acid: - 1. Biosynthesis of Purines:- It is the source of N1 of purines 2. Biosynthesis of pyrimidines. 3. It is the source of N1, C4, C5 & C6. 4. Biosynthesis of urea. 5. Biosynthesis of β-alanine by decarboxylation. 6. Biosynthesis of asparagine by asparagine synthase.