LESSON 48-52

Intermediary metabolism of amino

acids

Slides 3-14 are the same as in the

„Lessons 5-8” in the first semester

• Repetition:

Structure and classification of amino acids

ALPHA-L-AMINO ACIDS

• All peptides, polypeptides and proteins in the mammalian are polymers of alpha-L-amino acids.

• There are 20 alpha-amino acids that are relevant to the make-up proteins.

• Several other amino acids are found in the body free or in combined states.These non-protein associated amino acids perform specialized functions (such as: ornithine,citrulline, OH-proline, OH-lysine, etc)

Stereoisomers= mirror images of amino

acids: L-isomers in the mammalian proteins

• All objects have mirror images. Like many biomolecules, amino acids exist in mirror-image forms (stereoisomers) that are not superimposable. Only the L-isomers of amino acids commonly occur in nature. (The Mirror of Venus (1898), Sir Edward Burne-Jones/Mueseu Calouste Gulbenkian Lisbon/The Birdgeman Art ) Library

THE GENERAL STRUCTURE

OF AN ALPHA- AMINO ACID

Alpha C atom

GENERAL DESCRIPTION OF

ALPHA- AMINO ACIDS• The alpha-amino acids in peptides and proteins

(excluding proline) consist of a carboxylic

(-COOH) and an amino (-NH2) functional group attached to the same carbon atom. This carbon is the alpha-carbon.

• Distinct R-groups, that distinguish one amino acid from another, also are attached to the alpha-carbon (except in the case of glycine where the R-group is hydrogen). The fourth substitution on the alpha-carbon of amino acids is hydrogen.

OPTICAL PROPERTIES OF THE

AMINO ACIDSA carbon atom with 4 distinct constituents is

said to be chiral (asymmetric).

The one amino acid not exhibiting chirality is glycine since its '"R-group" is a hydrogen atom.

All of the amino acids in proteins exhibit the same absolute steric configuration (based on the arbitrary) as related to the L (levarotatory)-glyceraldehyde.

D-amino acids are never found in proteins, although they exist in nature. D -amino acids are often found in polypetide antibiotics, microbes, plants, insects.

OPTICAL ROTATION ACTIVITY

RELATED TO THE L-AND D-

GLYCERALDEHYDE

TABLE OF ALPHA-AMINO ACID

FOUND IN PROTEINSAmino Acid

Symbol Structure*

pK1 (COOH)

pK2 (NH2)

pK R Group

MONOAMINO MONOCARBOXYLIC ACIDS Amino Acids with Aliphatic R-Groups

Glycine Gly - G

2.4 9.8

Alanine Ala - A

2.4 9.9

Valine Val - V

2.2 9.7

Leucine Leu - L

2.3 9.7

Isoleucine Ile - I

2.3 9.8

TABLE OF ALPHA-AMINO ACID

FOUND IN PROTEINSNon-Aromatic Amino Acids with Hydroxyl R-Groups

Serine Ser - S

2.2 9.2 ~13

Threonine Thr - T

2.1 9.1 ~13

Amino Acids with Sulfur-Containing R-Groups

Cysteine Cys - C

1.9 10.8 8.3

Methionine Met-M

2.1 9.3

TABLE OF ALPHA-AMINO ACID

FOUND IN PROTEINS

MONOAMINO DICARBOXYLIC ACIDS Acidic Amino Acids and their Amides

Aspartic Acid

Asp - D

2.0 9.9 3.9

Asparagine Asn - N

2.1 8.8

Glutamic Acid

Glu - E

2.1 9.5 4.1

Glutamine Gln - Q

2.2 9.1

TABLE OF ALPHA-AMINO ACID

FOUND IN PROTEINS

DIAMINO MONOCARBOXYLIC ACIDS Basic Amino Acids

Arginine Arg -

R

1.8 9.0 12.5

Lysine Lys -

K

2.2 9.2 10.8

Histidine His -

H

1.8 9.2 6.0

TABLE OF ALPHA-AMINO ACID

FOUND IN PROTEINSAmino Acids with Aromatic Rings

Phenylalanine Phe -

F

2.2 9.2

Tyrosine Tyr -

Y

2.2 9.1 10.1

Tryptophan Trp-W

2.4 9.4

Imino Acids

Proline Pro -

P

2.0 10.6

HYDROPHOBIC, HYDROPHILIC, HYDROPHILIC AND

NEGATIVELY CHARGED, HYDROPHILIC AND

POSITIVELY CHARGED AMINO ACIDS

Intermediary metabolism (yellow:

proteins)

Degradation of an amino acid or synthesis of

a new amino acid by transamination

• Transamination as the name implies, refers to the transfer of an amine group from one molecule to another. This reaction is catalyzed by a family of enzymes called transaminases. Actually, the transamination reaction results in the exchange of an amine group on one acid with a ketone group on another acid. It is analogous to a double replacement reaction.The most usual and major keto acid involved with transamination reactions is alpha-ketoglutaric acid, an intermediate in the citric acid cycle. A specific example is the transamination of alanine to make pyruvic acid and glutamic acid.Other amino acids which can be converted after several steps through transamination into pyruvic acid include serine, cysteine, and glycine. http://www.elmhurst.edu/~chm/vchembook/631transam.html

Transamination: GPT=ALAT

Transamination: Schiff-base production

AS1

(-NH2-csoport donor)

piridoxal-P Schiff-base

Schiff-base

piridoxamin-P

α-ketosav1

α-ketosacid2

(-NH2-group

acceptor)

AS2

One of the roles of the transamination:

synthesis of new amino acids

• The transamination reaction works also for the synthesis of amino acids. In this situation alpha-ketoglutaric acid first uses transamination of a different amino acid to make glutamic acid, which then reacts with a keto acid to make a new amino acid.In effect, the interconversion of alpha-ketoglutaric acid and glutamic acid lies at the very heart of nitrogen metabolism. These molecules serve as the "collection and receiving agent" for nitrogen. The subsequent fate of the amino group is in new amino acids, any nitrogen bases, or any other nitrogen containing compounds. http://www.elmhurst.edu/~chm/vchembook/631transam.html

Vetcare's surgeries: Measuring liver

enzymes: ASAT, ALAT…

• Vettest 8008 Biochemistry Analyzer – measures e.g. liver

enzymes in different liver diseases, cholesterol, calcium etc and

performs kidney function tests

Liver disease in dog www.lbah.com/liver.htm

Localization of Transaminases

Enzyme Localization (cytosol or

mitochondrion)

ASAT=GOT

ALAT=GPT

Alpha-ketoglutaric acid and glutamic acid serve as

the "collection and receiving agent" for nitrogen

Synthesis of new amino acids: essential and

non-essential amino acids

• Transamination reactions can be used to synthesize amino acids needed or not present in the diet. An amino acid may be synthesized if there is an available "root" ketoacid with a synthetic connection to the final amino acid. Since an appropriate "root" keto acid does not exist for eight amino acids, (lys, leu, ile, met, thr, try, val, phe), they are essential and must be included in the dietbecause they cannot be synthesized.Glutamic acid usually serves as the source of the amine group in the transamination synthesis of new amino acids. The reverse of the reactions mentioned earlier are the most obvious methods for producing the amino acids alanine and aspartic acid.Several nonessential amino acids are made by processes other than transamination. Cysteine is made from methionine, and serine and glycine are synthesized from phosphoglyceric acid - an intermediate of glycolysis. http://www.elmhurst.edu/~chm/vchembook/631transam.html

Synthesis of certain non-essential amino acids

www.personal.kent.edu/.../amino/nonessential.htm

Essential and non-essential amino

acids• www.wannabebig.com/article.php?articleid=279

semiessential Urea cycle!

Chicken: Gly, Arg

are essential!

Ruminants have no essential amino acidds: production

by rumen microbes!

• Oxidative deamination is also an oxidative reaction that occurs under aerobic conditions in all tissues but especially the liver. During oxidative deamination, an amino acid is converted into the corresponding keto acid by the removal of the amine functional group as ammonia and the amine functional group is replaced by the ketone group. The ammonia eventually goes into the urea cycle.Oxidative deamination occurs primarily on glutamic acid because glutamic acid was the end product of many transamination reactions.The glutamate dehydrogenase is allosterically controlled by ATP and ADP. ATP acts as an inhibitor whereas ADP is an activator. http://www.elmhurst.edu/~chm/vchembook/631transam.html

Metabolism of amino groups of amino

acids: oxidative deamination

Oxidative deamination

Central role for glutamic acid

• Apparently most amino acids may be deaminated but this is a significant reaction only for glutamic acid. If this is true, then how are the other amino acids deaminated? The answer is that a combination of transamination and deamination of glutamic acid=trans-deamination occurs which is a recycling type of reaction for glutamic acid. The original amino acid loses its amine group in the process. The general reaction sequence is shown on the next slide. http://www.elmhurst.edu/~chm/vchembook/631transam.html

Central role for glutamic acid

D-amino acid oxidases

D-amino acid oxidase

D-aminoacid α-ketosacid

Fate of carbon skeleton of amino acids:

keto- and glucogenic amino acids

• Once the keto acids have been formed from the appropriate amino acids by transamination, they may be used for several purposes. The most obvious is the complete metabolism into carbon dioxide and water by the citric acid cycle.

• However, those amino acids converted into acetyl CoA can be converted into ketone bodies (such as acetoacetate), if they do not have oxaloacetic acid (OAC) as partner to enter the citric acid cycle: ketogenic amino acids: Leu, Lys

• If carbohydrates are lacking in the diet, then those amino acids converted into pyruvic acid and OAC or a molecule of citric acid cycle can be converted into glucose or glycogen in GNG: glucogenic amino acids such as Ala, Arg, Asn, Asp, Cys, Glu, Gln, Gly, His, Met, Pro, Ser, Thr, Val.

• If an amino acid can form partly acetyl CoA, partly OAC is called as gluco- and ketogenic amino acid: Ile, Phe, Trp, Tyr

• The hormones cortisone and cortisol from the adrenal cortex stimulate the synthesis of glucose from amino acids (GNG) in the liver and also function as antagonists to insulin.

Fate of carbon skeleton (for

demonstration only)

•

Production and detoxification of

ammonia

• Production:

• Oxidative deamination: L-Glu dehydrogenase, D-amino acid oxidase

• Degradation of biogenic amines

• Absorption from the gastrointestinal tract (mainly at ruminants!)

• Detoxification:

• Urea cycle

• L-Glu synthesis (L-Glu dehydrogenase)

• Gln (transport form) and Asn synthesis

• In the kidney: glutaminase activity, ammonium ion production

• Purine synthesis: uric acid (mainly at birds!)

Detoxification of ammonia: urea cycle

• The main possibility: urea cycle

• Species lacking urea cycle:

• Birds have no arginase enzyme,

that s why no urea cycle!

Az ornitin-cycle

http://en.wikipedia.org/wiki/Hans_Adolf_Krebs

http://www.nap.edu/openbook.php?record_id=9681&page=34

Hans Krebs

(1900-1981)

Nobel-prize:1953

Kurt Henseleit

(1907-1973)

Argininosuccinase

Portosystemic shunt clubs.akc.org/stca/liverShunt.htm

• Extrahepatic portosystemic shunt: blood of the portal vein enters in vena

cava (not in the liver): risk of ammonia toxicosis increases (ammonia can

not enter in the liver and urea cycle)

Portosystemic shunt in yorkshire terrierdogs.y2u.co.uk/Dog_Yorkshire_

terrier.htm

In the United States, Yorkshire terriers have almost a 36 times greater risk of

developing shunts than all other breeds combined.

Glutamine (transport form of ammonia)

production by glutamine synthetase

• www.ucl.ac.uk/~ucbcdab/urea/amtox.htm

+Pin

Glutamin synthesis and

degradation

glutamin synthetase

Glu Gln

H2O

KIDNEY

glutaminase

LIVER

Decarboxylation of amino acids

coenzyme: PALP•

Amino acid Biogenic amine Role

Ser Etanolamine=cholamine Cephaline

Cys Cysteamine HS-CoA

Cysteic acid Taurine Conjugation: bile salts

Glu GABA NTM

Asp Beta-Ala HS-CoA

Tyr Tyramine Vaso- and smooth muscle

contraction, NTM

DOPA Dopamine Catecholamines, NTM,

adrenalin (stress hormone)

Trp Tryptamine NTM

5OH-Trp Serotonine NTM

His Histamine Inflammation, allergy, NTM

Lys Cadaverine Polyamine, putrefaction of

animal tissue

Ornithine Putrescine Polyamine, putrefaction of

animal tissue

NTM=neurotransmitter

Biogenic amine: Ser →

Ethanolamine=cholamine

• Cholamine as an amino alcohol can be the constituents

of phospholipids, such as cephalin

• Serine decarboxylase

Biogenic amines: Cys → Cysteamine and

Asp → Beta-alanine

Panthotenic acid (vitamin) consists of alpha,gamma-dihydroxy

beta,beta-dimethyl butyric acid and beta-alanine. The active form of

panthotenic acid is Coenzyme A (HS-CoA) contains cysteamine.

Cysteamine Beta alanine

Cysteine Aspartate

Cys- and Asp-decarboxylase

Biogenic amine: Cys → cysteic acid →

taurine Lambert IH, Neurochemical Research 29: 27-63, 2004

www2.imbf.ku.dk/CellSignalling/IHLambert/• You have to know its role in bile salt synthesis!

• Bile acids are conjugated with taurine to produce bile salts

• Taurine synthesis and physiological roles in mammalian cellsLambert IH, Neurochemical Research 29: 27-63, 2004

Cysteic acid

Taurine is essential for cats

• Bile acids can be conjugated only by taurine to produce

bile salts

• Synthesis is not enough

• Deficiency: cardiomyopathy, retinal degeneration

Biogenic amine: Glutamate → Gamma-

amino-butyrate=GABA

www.niaaa.nih.gov/.../Neuroscience/gabba.htm

• Glutamate decarboxylase

• Inhibitory neurotransmitter

• GABA receptor subunits

in the brain

Biogenic amine: Tyr → Tyramine

• Tyrosine decarboxylase

• Found in food (cheese, chocolate, smoked meat etc)

• Vasoconstriction (by stimulation of catecholamine secretion), headache,

smooth muscle constriction, neurotransmitter

Biogenic amine: Tyr → DOPA →Catecholamines

(Dopamine-Noradrenaline-Adrenaline)

www.mfi.ku.dk/ppaulev/chapter30/kap30.htm

• Dihydroxyphenylalanine (DOPA) decarboxylase

• Catecholamines: neurotransmitters, dopamine: coordination of movement,

noradrenaline, adrenaline: stress hormones

Biogenic amine: 5-OH-Trp → Serotonin=

5-OH-tryptamine www.drugabuse.gov/.../teaching4/Teaching3.html

• 5-OH tryptophane decarboxylase

• Neurotransmitter, regulation of sleep and wakefulness, etc.

Biogenic amine: His→Histamine

www.jle.com/.../64/article.md?fichier=images.htm

• Inflammation, allergic reactions (proinflammatory signal released from the

mast cells in response to allergic reactions or tissue damage), capillary

dilatator, neurotransmitter

Histamine induced urticaria

Histidine

decarboxylase

Poliamines: ornitine → putrescine,

Lys → cadaverine

• Cadaverine is a foul-smelling molecule produced by protein hydrolysis during putrefaction of animal tissue. It is also produced in small quantities by living beings.

• Putrescine: is produced also by the breakdown of amino acids in living and dead organisms.

• The two compounds are largely responsible for the foul odor of putrefying flesh, but also contribute to the odor of such processes as bad breath and bacterial vaginosis. (Further poliamine produced from putrescine such as spermidine and spermine give among others the odor of sperm.)

• (Ornithine and Lys decarboxylase)

Dipeptides: Carnosine,

anserine

• Carnosine (beta Ala+His) muscle

and brain of mammals

• Anserine (beta Ala+CH3-His) muscle and

brain of birds

• Effects:

• Buffering of lactic acid in muscle and red

blood cells (anti-ishaemic)

• Antioxidant

carnosine

anserine

Tripeptide: Glutathionewww.bio.davidson.edu/.../favorite_gene.html

Tripeptide: glutathione

• Glutathione (γ-glutamyl-cysteinyl-glycine,

GSH)– Found in each cell types (much in the liver and brain)

– Produced in the γ-glutamyl-cycle (needs ATP)

– Reduced glutathione (GSH) -SH(tiol)-group is a H-donator, and

by this way reduces other molecules oxidized glutathione

(GSSG) has a disulphide bond

GSH + HSG GS-SG

Glutathione peroxidase

2H

Glutathione reductase

Ascorbic acidDehydroascorbic acid

NADPH + H+NADP+

Oxidized and

reduced glutathione

Gamma-glutamyl cycle

aims: transport of amino acids and synthesis

of glutathioneMeasured in the clinical praxis

Biochemical role of glutathione

• 2GSH GSSG oxidation: 2 H are

released. Role of released H:– Antioxidant

– Split disulphide bonds of proteins by reduction

– Ribonucleoside-diphosphate reductase enzym needs

H for reduction

– Reduction of thioredoxine

thioredoxine thioredoxine

glutatihone

peroxidase

glutathione

reductase

Main role of glutathione: antioxidant activity

(elimination of reactive oxygen species=ROS,

such as free radicals, H2O2)

• The PPP supplies the RBC with NADPH to maintain the reduced state of glutathione. The inability to maintain reduced glutathione in RBCs leads to increased accumulation of peroxides, predominantly H2O2, that in turn results in a weakening of the cell wall and concomitant hemolysis. Accumulation of H2O2 also leads to increased rates of oxidation of hemoglobin to methemoglobin that also weakens the cell wall. Glutathione removes peroxides via the action of glutathione peroxidase. The PPP in erythrocytes is essentially the only pathway for these cells to produce NADPH. Any defect in the production of NADPH could, therefore, have profound effects on erythrocyte survival.

Hemolytic anaemia in cats and

dogs

Sulphur containing

compounds

Hemolytic

anaemia

Glucose-6-P-

dehydrogenase

activity of PPP

decreases

Less

NADPH+H+

Less reduced

Glutathione

Oxidative

damage of

erythrocytes

https://thehorseaholic.com/red-maple-toxicity-horses/

Gallic acid

Methemoglobin and

–SH group oxidation=

(oxidative damage of

erythrocytes

Hemolysis

RED MAPLE TOXICITY OF HORSES

Metabolic pathways: summary

www2.ufp.pt/~pedros/bq/integration.htm