Amino acids metabolism new

GENERAL PATHWAYS OF AMINO ACIDS METABOLISM

Digestion and absorbtion of proteins in the gastrointestinal tract.

Nitrogenous balance.

Proteins function in the organism.Proteins function in the organism.

All enzymes are proteins.All enzymes are proteins. Storing amino acids as nutrients and as building Storing amino acids as nutrients and as building blocks for the growing organism.blocks for the growing organism. Transport function (proteins transport fatty acids, Transport function (proteins transport fatty acids, bilirubin, ions, hormones, some drugs etc.).bilirubin, ions, hormones, some drugs etc.). Proteins are essential elements in contractile and Proteins are essential elements in contractile and motile systems (actin, myosin).motile systems (actin, myosin). Protective or defensive function (fibrinogen, Protective or defensive function (fibrinogen, antibodies).antibodies). Some hormones are proteins (insulin, somatotropin).Some hormones are proteins (insulin, somatotropin). Structural function (collagen, elastin). Structural function (collagen, elastin).

Amino acids

Proteins of food

Metabolites of glycolysis and Krebs cycle

Anabolic ways Catabolic ways

Synthesis of cell and extracell proteins

Synthesis of peptide physiologi-cally active substances

Trans-ami-nation

Deami-nation

Decar-boxila-tion

Urea, CO2, H2O

Amines

Proteins and peptides of the organism

GENERAL PATHWAYS OF AMINO ACIDS METABOLISM

Nitrogen balance is a comparison between Nitrogen intake (in the form of dietary protein) and Nitrogen loss (as undigested protein in feces,NPN as urea, ammonia, creatinine & uric acid in urine,sweat & saliva & losses by hair, nail, skin).

NB is important in defining1.overall protein metabolism of an individual2.nutritional nitrogen requirement.

Nitrogen Balance (NB):

Nitrogenous balanceNitrogenous balance It may be It may be positive, negative and neutral (zero). positive, negative and neutral (zero). Positive nPositive nitrogenous balanceitrogenous balance – the amount of nitrogen entered the – the amount of nitrogen entered the organism is more than amount of nitrogen removed from the organism is more than amount of nitrogen removed from the organism. It occurs in young growing organism, during organism. It occurs in young growing organism, during recovering after severe diseases, at the using of anabolic recovering after severe diseases, at the using of anabolic medicines pregnancy, lactation and convulascencemedicines pregnancy, lactation and convulascence

Negative nNegative nitrogenous balanceitrogenous balance – the amount of nitrogen removed – the amount of nitrogen removed from the organism is more than amount of nitrogen entered the from the organism is more than amount of nitrogen entered the organism. It occurs in organism. It occurs in senile age, destroying of malignant tumor, senile age, destroying of malignant tumor, vast combustions, poisoning by some toxinsvast combustions, poisoning by some toxins. High loss of tissue . High loss of tissue proteins in wasting diseases like proteins in wasting diseases like burns, hemorrhage & kidney burns, hemorrhage & kidney diseases with albuminurea (High breakdown of tissue proteins ) diseases with albuminurea (High breakdown of tissue proteins ) in hyperthyroidism, fever, infectionin hyperthyroidism, fever, infection

Zero nZero nitrogenous balanceitrogenous balance – – the amount of nitrogen removed from the the amount of nitrogen removed from the organism is equal to the amount of nitrogen entered the organism. It organism is equal to the amount of nitrogen entered the organism. It occurs in healthy adult peopleoccurs in healthy adult people Normal adult: will be in nitrogen equilibrium, Losses = IntakeNormal adult: will be in nitrogen equilibrium, Losses = Intake

A deficiency of even one amino acid results in a negative nitrogen balance. In this state, more protein is degraded than synthesized.

The normal daily requirement of protein for adults is 0.8 g/Kg body wt. day-1.

• That requirement is increased in healthy conditions:during the periods of rapid growth, pregnancy, lactation and adolescence.• Protein requirement is increased in disease states:illness, major trauma and surgery.• RDA for protein should be reduced in:hepatic failure and renal failure

Protein Requirement for humans in Healthy and Disease Conditions

BV is : a measure for the ability of dietary protein to provide the essential amino acids required for tissue protein maintenance.

•Proteins of animal sources (meat, milk, eggs) have high BV because they contain all the essential amino acids.•Proteins from plant sources (wheat, corn, beans) have low BV thus combination of more than one plant protein is required (a vegetarian diet) to increase its BV.

Biological Value for Protein (BV)

Chemical composition of digestive juices.Chemical composition of digestive juices.

Gastric juice contains water, enzymes, hydrochloric acid, Gastric juice contains water, enzymes, hydrochloric acid, mineral salts and other compounds. About 2,5 l of mineral salts and other compounds. About 2,5 l of gastric juice is secreted per day. gastric juice is secreted per day. The role of hydrochloric acid in digestion.The role of hydrochloric acid in digestion. Denaturate proteins (denaturated proteins easier Denaturate proteins (denaturated proteins easier undergo digestion by pepsin than native proteins).undergo digestion by pepsin than native proteins). Stimulates the activity of pepsin.Stimulates the activity of pepsin. Hydrochloric acid Hydrochloric acid has bactericidial properties.has bactericidial properties. Stimulates the peristalsis.Stimulates the peristalsis. Regulate the enzymatic function of pancreas.Regulate the enzymatic function of pancreas.

Protein digestion

Digestion in Stomach

Stimulated by food acetylcholine, histamine and gastrin are released onto the cells of the stomach

The combination of acetylcholine, histamine and gastrin cause the liberation of the gastric juice.

Mucin - is always secreted in the stomach HCl - pH 0.8-2.5 (secreted by parietal cells) Pepsinogen (a zymogen, secreted by the chief cells)

Proteolytic enzymes and their activation.

Three enzymes are in gastric juice: pepsin, gastricsin and rennin. All these enzymes cleave proteins or peptides.

Pepsinogen (MW=40,000) is activated by the enzyme pepsin, which is already present in the stomach and by hydrochloric acid.

Pepsinogen cleaved off to become the enzyme pepsin (MW=33,000) and a peptide fragment to be degraded.

Pepsin partially digests proteins by cleaving the peptide bond formed by aromatic amino acids: Phe, Tyr, Trp

Optimal pH for Optimal pH for gastricsingastricsin is 2,0-3,0. The ratio is 2,0-3,0. The ratio between gastricsin and pepsin in gastric juice is between gastricsin and pepsin in gastric juice is 1:5,5. This ratio can be changed in some 1:5,5. This ratio can be changed in some pathological states. pathological states.

RRenninennin also also possesses a proteolytic activitypossesses a proteolytic activity and and causes a rapid coagulation of ingested caseincauses a rapid coagulation of ingested casein. But . But this enzyme plays important role only in children this enzyme plays important role only in children because the optimal pH for it is 5-6. because the optimal pH for it is 5-6.

Digestion in the Duodenum

Stimulated by food secretin and cholecystokinin regulate the secretion of bicarbonate and zymogens trypsinogen, chymotrypsinogen, proelastase and procarboxypeptidase by pancreas into the duodenum

Bicarbonate changes the pH to about 7

The intestinal cells secrete an enzyme called enteropeptidase that acts on trypsinogen cleaving it into trypsin

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Enteropeptidase secreted by the mucosa of duodenum initiatesthe activation of the pancreatic proenzymes

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Proteolytic enzymes exhibit the preference for particular types of peptide bonds

Proteinases preferentially attacks the bond after:

Pepsin aromatic (Phe, Tyr) and acidic AA (Glu, Asp)

Trypsin basic AA (Arg, Lys)

Chymotrypsin hydrophobic (Phe, Tyr, Trp, Leu) and acidic AA (Glu, Asp)

Elastase AA with a small side chain (Gly, Ala, Ser)

Peptidases:

Carboxypeptidase A nearly all AA (not Arg and Lys) Carboxypeptidase B basic AA (Arg, Lys)

aminopeptidase nearly all AA

Prolidase proline

Dipeptidase only dipeptides

The splitting of elastin in an intestine is catalyzed by elastase and collagen is decomposed by collagenase.

Digestion of protein takes place not only in the intestinal cavity but also on the surface of mucosa cells.

Mechanism of amino acid absorbtion.

This explanation is called the sodium cotransport theory for amino acid transport; it is also called secondary active transport of amino acid.Absorption of amino acids through the intestine mucosa can occur far more rapidly than protein can be digested in the lumen of the intestine. Since most protein digestion occurs in the upper small intestine most protein absorption occurs in the duodenum and jejunum.

Most proteins are completely digested to free amino acids

Amino acids and sometimes short oligopeptides are absorbed by the secondary active transport

Amino acids are transported via the blood to the cells of the body.

The sources of amino acids:The sources of amino acids: 1) absorption in the intestine; 1) absorption in the intestine; 2) formation during the protein decomposition; 2) formation during the protein decomposition; 3) synthesis from the carbohydrates and lipids. 3) synthesis from the carbohydrates and lipids.

Using of amino acids:Using of amino acids:

1) for protein1) for protein synthesis;synthesis;2) for synthesis of nitrogen containing compounds (creatine, 2) for synthesis of nitrogen containing compounds (creatine, purines, choline, pyrimidine);purines, choline, pyrimidine);3) as the source of energy (oxidation – deamination, 3) as the source of energy (oxidation – deamination, transamination, decarboxilation); transamination, decarboxilation); 4) for the gluconeogenesis;4) for the gluconeogenesis;5) for the formation of biologically active compounds.5) for the formation of biologically active compounds.

Overview of Amino Acid Catabolism:Overview of Amino Acid Catabolism:Interorgan RelationshipsInterorgan Relationships


• Liver– Synthesis of liver and plasma proteins– Catabolism of amino acids

• Gluconeogenesis• Ketogenesis• Branched chain amino acids (BCAA) not

catabolized• Urea synthesis

– Amino acids released into general circulation• Enriched in BCAA (2-3X)


• Skeletal Muscle– Muscle protein synthesis– Catabolism of BCAA

• Amino groups transported away as alanine and glutamine (50% of AA released)

– Alanine to liver for gluconeogenesis– Glutamine to kidneys

• Kidney– Glutamine metabolized to a-KG + NH4

• a-KG for gluconeogenesis• NH4 excreted or used for urea cycle (arginine

synthesis)– Important buffer from acidosis

PROTEIN TURNOVER

How can a cell distinguish proteins that are meant for degradation?

Protein turnover — the degradation and resynthesis of proteins

Half-lives of proteins – from several minutes to many years

Structural proteins – usually stable (lens protein crystallin lives during the whole life of the organism)Regulatory proteins - short lived (altering the amounts of these proteins can rapidly change the rate of metabolic processes)

Ubiquitin - is the tag that marks proteins for destruction ("black spot" - the signal for death)

Ubiquitin - a small (8.5-kd) protein present in all eukaryotic cells

Structure: extended carboxyl terminus (glycine) that is linked to other proteins; lysine residues for linking additional ubiquitin molecules

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Proteasomes degrade regulatory proteins (short Proteasomes degrade regulatory proteins (short

half-life) and abnormal or misfolded half-life) and abnormal or misfolded

proteinsproteins

Ub + short peptides

Protein-Ub

AA

cytosolicpeptidases

regulation of

cell cycle,

apoptosis,

angiogenesis

- hollow cylindric supramolecule, 28 polypeptides- four cyclic heptamers (4 7 = 28)- the caps on the ends regulate the entry of proteins into destruction chamber, upon ATP hydrolysis- inside the barrel, differently specific proteases hydrolyze target protein into short (8 AA) peptides- Ub is not degraded, it is released intact

GENERAL WAYS OF AMINO ACIDS METABOLISM

The fates of amino acids: 1) for protein synthesis;

2) for synthesis of other nitrogen containing compounds (creatine, purines, choline, pyrimidine);

3) as the source of energy;

4) for the gluconeogenesis.

The general ways of amino acids degradation: Deamination Transamination Decarboxilation

The major site of amino acid degradation - the liver. Deamination of amino acids

Deamination - elimination of amino group from amino acid with ammonia formation.

Four types of deamination: - oxidative (the most important for higher animals), - reduction, - hydrolytic, and - intramolecular

Reduction deamination:

R-CH(NH2)-COOH + 2H+ R-CH2-COOH + NH3

amino acid fatty acid

Hydrolytic deamination:

R-CH(NH2)-COOH + H2O R-CH(OH)-COOH + NH3

amino acid hydroxyacid

Intramolecular deamination:

R-CH(NH2)-COOH R-CH-CH-COOH + NH3

amino acid unsaturated fatty acid

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General scheme of oxydative transamination

CH2CH2COOH

O

CHOOC+R CH

NH2

COOH

aminokyselina 2-oxoglutarát

HOOC CH CH2CH2COOH

NH2

+R C

O

COOH

glutamát2-oxokyselina

aminotransferasapyridoxalfosfát

amino acid

2-oxo acid

2-oxoglutarate

glutamate

aminotransferase

pyridoxal phosphate

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Glutamate dehydrogenase (GMD, GD, GDH)

• requires pyridine cofactor NAD(P)+

• GMD reaction is reversible: dehydrogenation with NAD+,

hydrogenation with NADPH+H+

• two steps:

• dehydrogenation of CH-NH2 to imino group C=NH

• hydrolysis of imino group to oxo group and ammonia

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In transaminations, nitrogen of most AA is concentrated in glutamate

Glutamate then undergoes

dehydrogenation + deamination

and releases free ammonia NH3

!

Oxidative deaminationL-Glutamate dehydrogenase plays a central role in amino acid deamination

In most organisms glutamate is the only amino acid that has active dehydrogenase

Present in both the cytosol and mitochondria of the liver

Transamination of amino acids

Transamination - transfer of an amino group from an -amino acid to an -keto acid (usually to -ketoglutarate)

Enzymes: aminotransferases (transaminases).

-amino acid-keto acid

-keto acid -amino acid

There are different transaminases The most common: alanine aminotransferase alanine + -ketoglutarate pyruvate + glutamate

aspartate aminotransferase aspartate + -ketoglutarate oxaloacetate + glutamateAminotransferases funnel -amino groups from a variety of amino acids to -ketoglutarate with glutamate formationGlutamate can be deaminated with NH4

+

release

Mechanism of transamination

All aminotransferases require the prosthetic group pyridoxal phosphate (PLP), which is derived from pyridoxine (vitamin B6).

First step: the amino group of amino acid is transferred to pyridoxal phosphate, forming pyridoxamine phosphate and releasing ketoacid.

Second step: -ketoglutarate reacts with pyridoxamine phosphate forming glutamate

Ping-pong kinetic mechanism

Ping-pong kinetic mechanism of aspartate transaminase

aspartate + -ketoglutarate oxaloacetate + glutamate

Decarboxylation – removal of carbon dioxide from amino acid with formation of amines.

Usually amines have high physiological activity (hormones, neurotransmitters etc).

amine

Enzyme: decarboxylases Coenzyme – pyrydoxalphosphate

Decarboxylation of amino acids

DECARBOXYLATION OF AMINO ACIDS

α-decarboxilation

ω-decarboxilation

Decarboxilation with transamination

Decarboxilation with conjugation of two molecules

Significance of amino acid decarboxylation

1. Formation of physiologically active compounds

glutamate gamma-aminobutyric acid (GABA)

histaminehistidine

1) A lot of histamine is formed in inflamatory place;1) A lot of histamine is formed in inflamatory place;It has vasodilator action;It has vasodilator action;Mediator of inflamation, mediator of pain;Mediator of inflamation, mediator of pain;Responsible for the allergy development;Responsible for the allergy development;

Stimulate HCI secretion in stomach. Stimulate HCI secretion in stomach. -CO2-CO2

2) Tryptophan 2) Tryptophan Serotonin SerotoninVasokonstrictorVasokonstrictorTakes part in regulation of arterial pressure, body Takes part in regulation of arterial pressure, body temperature, respiration, kidney filtration, mediator of temperature, respiration, kidney filtration, mediator of nervous systemnervous system

3) Tyrosine 3) Tyrosine Dopamine DopamineIt is precursor of epinephrine and norepinephrine. It is precursor of epinephrine and norepinephrine. mediator of central nervous systemmediator of central nervous system

4) Glutamate 4) Glutamate -aminobutyrate (GABA)-aminobutyrate (GABA)Is is ingibitory mediator of central nervous system. In Is is ingibitory mediator of central nervous system. In medicine we use with anticonvulsion purpose (action).medicine we use with anticonvulsion purpose (action).

2. Catabolism of amino acids during the decomposition of proteins

ornithine putrescine

lysine cadaverine

Enzymes of microorganisms (in colon; dead organisms) decarboxylate amino acids with the formation of diamines.

Amino acids metabolism new

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