Dr. Azin Nowrouzi Tehran University of Medical Sciences Dr. Azin Nowrouzi Tehran University of Medical Sciences TUMS

Many essential biomolecules contain N 2

Major Functions of Nitrogen Derived from Dietary Protein HemeBlood cell CholinePL GlycosamineSugar NucleotidesDNA Protein synthesis Protein Biogenic aminesNeurotransmitters CarnitineHeart Creatine phosphate « Energy » 3

The nitrogen cycle No biomolecules are dedicated to the storage of N excess N is excreted N must be replenished by dietary protein 4

A. Ammonia Is Incorporated into Glutamate Reductive amination of  -ketoglutarate by glutamate dehydrogenase occurs in plants, animals and microorganisms In mammals & plants, located in mitochondria. 5

B. Glutamine Is a Nitrogen Carrier in Many Biosynthetic Reactions A second important route in assimilation of ammonia is via glutamine synthetase It is present in all organisms. In humans it is most active in the liver. Glutamine is transported from the liver to other tissues via the blood. 6

Major Ammonium ion carrier 7 Assimilation of Ammonia

Glutamate synthatase is not present in humans. It is found in Bacteria It is used by blue-green algae and by Rhisobia.

Glutamate provides the amino group for the synthesis of many other amino acids through trasamination reactions Prosthetic group involved in amino transfer 9

Overall mechanism of transamination

Alanine Aminotransferase 11

Central role of the aminotrasferases and glutamate dehydrogenase in nitrogen metabolism 1.Proteins are degraded to amino acids 2.Removal of nitrogen is first step in degradation 3.Ammonium is converted into urea 4.C-skeleton enters known pathways 5.Amino acids are made from intermediates of other pathways

Amino Acid Synthesis The ability of an organism to live and grow is dependent on protein synthesis Therefore, a supply of all 20 aa is necessary. Higher plants are able to synthesize all 20 aas. Many microorganisms and higher animals make fewer Humans make 10 of the 20 aas (these are called nonessential amino acids. The remainder must be supplied in the diet, usually in the form of plant or animal proteins (these are called essential amino acids).

All 20 amino acids are essential for life, They are necessary for protein synthesis – Essential or indispensable: 9 – Nonessential or dispensable: 11 Review complete vs. incomplete All natural, unprocessed animal and plant foods contain all twenty amino acids A lack of any one of them leads to severe metabolic disruption and ultimate death. All 20 amino acids are essential for life, They are necessary for protein synthesis – Essential or indispensable: 9 – Nonessential or dispensable: 11 Review complete vs. incomplete All natural, unprocessed animal and plant foods contain all twenty amino acids A lack of any one of them leads to severe metabolic disruption and ultimate death. Metabolic Classification of the Amino Acids 14

Their synthesis depends on the availability of the appropriate carbon skeletons and a source of ammonia. Glucose is ultimately the source of carbon skeletons for most nonessential aa. Two essential aa, phenylalanine and methionine, are used to make tyrosine and cysteine, respectively. Since ammonia is available in the fed state, amino acids become essential to our diet when we are not able to synthesize their carbon skeletons.

α-keto acids required for synthesis of nonessential amino acids α-keto acidAmino acid Pyruvate Oxaloacetate α-keto glutarate Pyruvate, 3-phosphoglycerate Alanine Aspartate, Asparagine Glutamate, glutamine, Proline, Arginine Serine

Biosynthesis of all amino acids in plants and microorganisms

Biosynthesis of Amino Acids 18

Amino acids are precursors of some other biomolecules 20

Obtained in large amounts in diet as proteins most N is in proteins AA’s are the source of N in the diet Not like carbohydrate and fat Because no large stores of protein in the body Thus, a continuous intake is required If not enough consumption Tissue breakdown occurs Amino Acids 21

Protein Quality Animal vs. Plant protein Important in maintaining N balance Proteins have different biological value (BV) Major reasons why animal protein is called BV protein, whereas plant protein is of lower BV: – Animal protein is “complete” - contains all essential amino acids – Contains essential amino acids in larger amounts and in proper proportion for optimal utilization Note: Soybean protein even though from a plant, is comparable to animal protein o In children leads to kwashiorker 22

28 grams 23

Note The 56-g protein requirement for adult male can be met by 45 g of animal protein Same requirement would necessitate 65 g plant protein Combining plant products (legumes + grains) provides all essential amino acids Mixture of 30% animal protein and 70 % plant protein similar to use of animal protein alone 24

Protein RDA varies in different stages of life cycle years: 1 g/lb years:.71 g/lb 1-6 years:.56 g/lb 7-14 years:.45 g/lb years:.41 g/lb 19+ years:.36 g/lb 1Ib = kg 25

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Protein Turnover Body proteins turn over; t 1/2 = min - wks 400g of protein are synthesized per day and 400g are broken down – Secretory proteins such as digestive enzymes, polypeptide hormones, and antibodies, turn over rapidly – Structural proteins are much more metabolically stable.

Chemical Signals for Turnover ubiquitinatin – A small, heat stable protein (ubiquitin) reacts with other proteins to mark them for destruction Oxidation of amino acid resides- Pro, Arg, Lys Pest sequences- one or more regions rich in proline (P), gltamate (E),serine (S), and threonine (T) 28

Nitrogen Balance 29

Protein Digestion Gastric Digestion – Function of pH Kills bacteria Denatures proteins… – Activation and Action of Pepsin Intestinal Digestion – Pancreatic enzymes – Intestinal enzymes 30

31 Denaturation of Proteins at low pH

Activation of the Gastric and Pancreatic Zymogens 32 Trypsinogen Val-Asp-Asp-Asp-Asp-Lys Trypsin Proelastase Elastase Procarboxypeptidase Carboxypeptidase Chymotrypsinogen Chymotrypsin Enteropeptidase

33 Zymogens Activation

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35 Other details of Intestinal Enzymes

Brush Border 36 Final conversion of peptides to free amino acids

37 Secondary active transport driven by Na + gradient Facilitated diffusion Amino Acid Absorption

Transepithelial amino acid transport. Na+ 38

Gamma-Glutamyl Cycle A metabolic cycle for transporting amino acids into cells. 39

Disorders of Amino Acid transportation or absorption Uptake (transport) systems exist especially in intestine & kidney. Lack of specific transporter results in a disease state. This can be partially overcome through uptake of peptides. Uptake (transport) systems exist especially in intestine & kidney. Lack of specific transporter results in a disease state. This can be partially overcome through uptake of peptides. 40