Dr. Azin NowrouziTehran University of Medical SciencesDr. Azin NowrouziTehran University of Medical Sciences

TUMS

Many essential biomolecules contain NMany essential biomolecules contain N

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Major Functions of NitrogenMajor Functions of NitrogenDerived from Dietary ProteinDerived from Dietary ProteinMajor Functions of NitrogenMajor Functions of NitrogenDerived from Dietary ProteinDerived from Dietary Protein

Heme Blood cell

Choline PL

Glycosamine Sugar

Nucleotides DNA

Protein synthesis

Protein

Biogenic amines Neurotransmitters

Carnitine Heart

Creatine phosphate

« Energy »

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The nitrogen cycle

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

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.

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B. Glutamine Is a Nitrogen Carrier in Many Biosynthetic

Reactions

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.

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Major Ammonium ion carrier

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Assimilation of AmmoniaAssimilation 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 synthetaseGlutamate synthetase

TransaminationReversible transfer of Ammonia between amino acids and -ketoacids

by aminotransferase

TransaminationReversible transfer of Ammonia between amino acids and -ketoacids

by aminotransferase• Glutamate provides the amino group for

the synthesis of many • other amino acids through trasamination

reactions

Prosthetic group involved in amino transfer 9

Aspartate AminotransferaseAspartate AminotransferaseOverall mechanism of transamination

Alanine Aminotransferase Alanine Aminotransferase

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Central role of the aminotrasferases and glutamate dehydrogenase in nitrogen metabolism

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 SynthesisAmino 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

Metabolic Classification of the Amino Acids

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• 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.

What are the nonessential amino acids synthesized from?

What are the nonessential amino acids synthesized from?

α-keto acids required for synthesis of nonessential amino acids

α-keto acids required for synthesis of nonessential amino acids

α-keto acid Amino acid

PyruvateOxaloacetateα-keto glutaratePyruvate, 3-phosphoglycerate

AlanineAspartate, AsparagineGlutamate, glutamine, Proline, ArginineSerine

Biosynthesis of all amino acids in plants and microorganisms

Biosynthesis of Amino Acids

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Synthesis of TyrosineSynthesis of Tyrosine

Amino acids are precursors of some other biomolecules

Amino acids are precursors of some other biomolecules

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Protein QualityAnimal vs. Plant protein

Protein QualityAnimal 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 proteino In children leads to kwashiorker

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28 grams

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NoteNote

• 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

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Protein RDA varies in different stages of life cycle

Protein RDA varies in different stages of life cycle

• 0-0.5 years: 1 g/lb• 0.5-1 years: .71 g/lb• 1-6 years: .56 g/lb• 7-14 years: .45 g/lb• 15-18 years: .41 g/lb• 19+ years: .36 g/lb• 1Ib = 0.4536 kg

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Protein TurnoverProtein Turnover

• Body proteins turn over; t1/2= 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

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)

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Nitrogen BalanceNitrogen Balance

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Protein DigestionProtein Digestion

• Gastric Digestion– Function of pH

• Kills bacteria• Denatures

proteins…

– Activation and Action of Pepsin

• Intestinal Digestion– Pancreatic

enzymes– Intestinal enzymes 30

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Denaturation of Proteins at low pHDenaturation of

Proteins at low pH

Activation of the Gastric and Pancreatic Zymogens

Activation of the Gastric and Pancreatic Zymogens

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Trypsinogen

Val-Asp-Asp-Asp-Asp-Lys

TrypsinProelastase

Elastase

Procarboxypeptidase

Carboxypeptidase

Chymotrypsinogen

Chymotrypsin

Enteropeptidase

slow, nonenzymatic

pepsinogen(zymogen)

fast, enzymatic

pepsin

+

autocatalyticconversion of

pepsinogen to pepsin

denaturedproteins hydrolysis

large peptidefragments 33

Zymogens ActivationZymogens Activation

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Other details of Intestinal Enzymes

Other details of Intestinal Enzymes

di- and tripeptides aa's

di- and tripeptides

amino acids

peptidases

amino-peptidase

Brush Border

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Final conversion of peptides to free amino

acids

Final conversion of peptides to free amino

acids

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• Secondary active transport driven by Na+ gradient

• Facilitated diffusion

Amino Acid AbsorptionAmino Acid Absorption

Na+

Na+

ATPADP Na+

K+

K+

Intestinallumen Amino

acid Brush border

Active transporter

Aminoacid +Pi

Facilitatedtransporter

Amino acid

Serosalside

Portal vein

Di- andTripeptides

peptidasespeptides

Transepithelial amino acid transport.

Na+

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Gamma-Glutamyl CycleGamma-Glutamyl Cycle

• A metabolic cycle for transporting amino acids into cells.

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Disorders of Amino Acid transportation or absorption

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