Protein Metabolism....Introduction

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Mir Muzaffar,


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PROTEIN METABOLISM An Introduction Pharmacy College


Objectives Define proteins and classify them, giving the most important examples for each class . Define amino acids and describe their chemical nature Classify amino acids nutritionally and metabolically. List the essential , non essential, glucogenic , ketogenic & mixed amino acids . Define deamination , transamination and transdeamination of amino acids giving an example for each . Outline the fates of each of the carbon skeleton and ammonia derived from amino acid catabolism

Proteins Introduction and Definition:

Proteins Introduction and Definition are the most abundant biological macromolecules , occurring in all cells and all parts of cells and are polymers of amino acids joined to each other by peptide bonds. Constitute about 50% of dry weight of cells . Proteins can be broken down (hydrolyzed) to their constituent amino acids by a variety of methods

Classification of proteins:

Classification of proteins Based on chemical nature and solubility there are three major groups of proteins Simple proteins Conjugated proteins Derived Proteins Simple proteins are made up of only amino acids and are subdivided into: Globular proteins ; soluble in water and dilute salt solutions and are coagulated by heat. Examples are : serum albumin, egg albumin, serum globulins etc. Fibrous proteins :fiber like in shape, insoluble in water and resistant to digestion : examples are : collagen , elastin , keratins etc

Classification of proteins—contd..:

Classification of proteins—contd.. Conjugated Proteins : In addition to amino acids these proteins also contain non-protein groups known as prosthetic groups or conjugating groups . They can be further subdivided into : Nucleoproteins…. e.g. histones , nucleoprotamines Glycoproteins ….. e.g. immunoglobulin, mucin Lipoproteins ….. e.g. serum and membrane lipoproteins Phosphoproteins ….. e.g. casein, vitellin Chromproteins ….. e.g. hemoglobin, cytochromes Mettaloproteins ….. e.g. ceruloplasmin , carbonic anhydrase

Classification of proteins—contd..:

Classification of proteins—contd.. Derived Proteins: These are also of two subtypes Primary derived proteins are denatured, coagulated or first hydrolyzed products of proteins e.g. cooked protein, coagulated egg white. Secondary derived proteins are the progressive hydrolytic products of protein hydrolysis due to breakdown of peptide bonds. e.g proteoses , peptones, peptides

Classification of proteins—contd..:

FUNCTIONAL CLASSIFICATION OF PROTEINS Structural proteins e.g. collagen of bones, keratin of hair and nails Catalytic proteins or enzymes : e.g. hexokinase, pepsin etc Transport proteins : e.g. Hemoglobin, serum albumin Hormonal proteins: e.g. Insulin, growth hormone Contractile proteins: e.g. Actin , myosin Storage proteins: e.g. ferritin Genetic proteins: e.g. Nucleoproteins Defense proteins: e.g. antibodies. Snake venom Receptor proteins : e.g hormone receptors Classification of proteins—contd..

Amino acids :

Amino acids Amino acids are small organic compounds from which proteins are synthesized . Each amino acids has the following 4 groups or atoms attached to α carbon : Amino group (NH 2 ) Carboxyl group (COOH) Hydrogen atom (H) Side chain or radical group (R) All amino acids in humans are : α - amino acids L- amino acids At physiological pH - 7.4 - carboxyl group ( COOH) is dissociated forming a negatively charged carboxylate ion [ COO ] - , Amino group accepts a proton and is posi tively charged as NH3 + .


AMINO ACID STRUCTURE Side chain Amino group ,Basic . Carboxylic group , Acidic . There are about 300 different types but only 20 are found commonly in proteins

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Names and abbreviations of Amino Acids


AMINO ACIDS CLASSIFICATION Two systems of classification: Nutritional Metabolic Nutritional : Essential amino acids Non-essential amino acids

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Nonessential Essential Alanine Arginine * Asparagine Histidine * Aspartate Isoleucine Cysteine Leucine Glutamate Lysine Glutamine Methionine Glycine Phenylalanine Proline Threonine Serine Tyrptophan Tyrosine # Valine Arg and His are essential in diets of juveniles not adults # Tyr is classified as non essential because it is readily formed from Phe AMINO ACIDS CLASSIFICATION


Metabolic : based on Fate of carbon skeleton of amino acids Glucogenic The other 14 Amino acids that are degraded to pyruvate , a - ketoglutarate , succinyl CoA , fumarate , or oxaloacetate are termed glucogenic amino acids. The net synthesis of glucose from these amino acids is feasible because the above TCA cycle intermediates and pyruvate can be converted into phosphoenolpyruvate and then into glucose . AMINO ACIDS CLASSIFICATION


Ketogenic Amino acids that are degraded to acetyl CoA or acetoacetyl CoA are termed ketogenic amino acids because they can give rise to ketone bodies or fatty acids. Of the basic set of 20 amino acids only leucine and lysine are solely ketogenic Mixed P henylalanine, Isoleucine , tryptophan, and tyrosine are both ketogenic and glucogenic . Some of their carbon atoms emerge in acetyl CoA or acetoacetyl CoA , whereas others appear in potential precursors of glucose. AMINO ACIDS CLASSIFICATION

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This classification is not universally accepted, because different quantitative criteria are applied . Whether an amino acid is regarded as being glucogenic , ketogenic , or both depends partly on the eye of the beholder . Glucogenic Mixed Ketogenic Alanine Phenylalanine Leucine Asparagine Isoleucine Lysine Aspartate Tyrptophan Arginine Tyrosine Cysteine Glutamate Glutamine Glycine Histidine Methionine Proline Serine Threonine Valine AMINO ACIDS CLASSIFICATION


Deamination Deamination : It is the removal of amino group from α - amino acid in the form of ammonia -NH 3 . Sites : Mainly in liver & kidney. Enzyme : L-Glutamate dehydrogenase . Coenzyme : NAD + or NADP + . It is a reversible reaction . Used in both, catabolism & biosynthesis . Glutamate + NAD(P) + ↔ α - Ketoglutarate + Ammonia + NAD(P)H + H +


Deamination α - Ketoacid

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Definition: Transamination is a reversible reaction in which an α – NH3 group of one amino acid is transferred to an α – keto acid resulting in the formation of a new amino acid and a new keto acid. It is a process of combined Deamination and Amination . The general reaction is : The donor amino acid thus becomes a new keto acid and the recipient keto acid becomes a new amino acid after receiving the amino group . Transamination

Transamination :

Transamination Sites : principally in liver, kidney, intestine & muscles . Enzymes: Transaminases also called as aminotransferases Coenzyme : Pyridoxal phosphate ( Intermediate carrier ) Examples: Alanine transaminase [ ALT] Aspartate transaminase [ AST] Glutamate Transaminase [GTA}

Importance of Transamination:

Importance of Transamination It is important in both catabolism and biosynthesis of amino acids . Alanine and Glutamate transaminase (GTA) are present in most tissues, transfer amino groups from most amino acids to form Ala (from Pyr ) and Glu [from α -KG]. Ala being also a substrate for GTA, finally transfers its NH 3 → Glu . Formation of ammonia from amino acids through α –amino nitrogen of glutamate.

Transdeamination :

Transdeamination It is Transamination of most amino acid with α - ketoglutarate to form glutamate, then glutamate is deaminated to give ammonia. The net conversion of amino groups to ammonia requires the concerted action of glutamate transaminase [GTA ] & glutamate dehydrogenase [GLDH] . The combined action of GTA & GDH is called Transdeamination .

Catabolic & Anabolic roles of Transdeamination :

Catabolic & Anabolic roles of Transdeamination Anabolic : Formation of non-essential amino acids . Formation of proteins & peptides . Formation of specialized products. Catabolic : Removal of amino group from amino acid as ammonia. Remaining α - keto acid can be catabolized to gives ketone bodies or glucose.

Sources and Transport of ammonia :

Sources and Transport of ammonia Sources Amino acids . Glutamine, mainly in kidneys . Action of intestinal bacteria , through urease enzyme Amines Amino groups of purines and pyrimidines Transport Ammonia is transported efficiently between various tissues and liver in the form of : Glutamine or Alanine . Glutamine is the “ Storehouse of Ammonia ”, highest concentration in blood out of all amino acids. Glutamine is synthesized from Glutamate by Glutamine synthase . Glutamine is deaminated by enzyme Glutaminase , mostly in Kidney and Intestine.

Fates of ammonia:

Fates of ammonia Converted mainly to urea (Urea cycle ) Formation of glutamine Formation of non essential amino acids Formation of Purines and pyrimidines Formation of amino sugars

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Thank you Recommended books : Textbook of Biochemistry with Clinical Correlations By Thomas M. Devlin . L ehninger Principles of Biochemistry

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