Urea cycle and amino acid metabolism

Biochemistry for NursingSummer semester, 2015

Dr. Mamoun Ahram

Protein digestion

• In mouth: The breakdown of protein (large pieces of food are converted into smaller, more digestible portions).

• In stomach: – Proteins are denatured by the acidic

environment– Proteins are hydrolyzed by pepsin (its zyomgen

= pepsinogen)• In small intestine:

– Pepsin is inactivated by the neutral pH– Proteins and peptides are hydrolyzed by other

proteases (trypsin, chymotrypsin, and carboxypeptidase) into free amino acids.

• Amino acids are absorbed and released into bloodstream.

Protein hydrolysis

• The end result of protein digestion is hydrolysis of all peptide bonds to produce a collection of amino acids.

AMINO ACID METABOLISM

Fates of nitrogen and carbons of amino acids• Our bodies do not store nitrogen-containing compounds and

ammonia is toxic to cells. Therefore, the amino nitrogen from dietary protein and amino acids has just two possible fates:– either be incorporated into urea and excreted– Be used in the synthesis of new nitrogen-containing

compounds• The carbon atoms of amino acids are converted to

compounds that can enter the citric acid cycle, gluconeogenesis, fatty acid synthesis, or acetyl CoA.

Examples of compounds derived from amino acid carbons

• Intermediates of citric acid cycle.– About 10 20% of our energy is normally produced in

from amino acids. • Triacylglycerols (via lipogenesis)• Glycogen (via gluconeogenesis and glycogen

synthesis). • Ketone bodies.

Examples of nitrogen-containing compounds• Nitric oxide (NO, a chemical messenger)• Hormones• Neurotransmitters• Nicotinamide (NADH and NADPH)• Heme (in red blood cells)• Purine and pyrimidine bases (for nucleic acids)

The first step in amino acid catabolism is removal of the amino group.

Transamination reaction

• Transamination reactions interconvert amino acid amino groups and carbonyl groups.

• They are reversible and go easily in either direction, depending on the concentrations of the reactants.

• In this way, amino acid concentrations are regulated by keeping synthesis and breakdown in balance.

Alanine transaminase

• Alanine aminotransferase (ALT) is abundant in the liver.• Above-normal ALT concentration in the blood is an indication

of liver damage due to leakage into the bloodstream.

Production of ammoniaglutamate dehydrogenase

• The glutamate from transamination is as an amino group carrier.

• Most of the glutamate is recycled to regenerate –ketoglutarate.

• This process, known as oxidative deamination.• The ammonium ion formed in this reaction enters into the urea

cycle where it is eliminated in the urine as urea.

Features of the urea cycle

• This cycle is active in the liver.• The liver cleans the blood.• It removes with this cycle ammonia (NH4 +) from the

body in the form of urea.• Urea gets excreted through the kidneys in the urine.• The cycle occurs in the mitochondria and cytoplasm.

The cycleStart

The first reaction

• Carbamoyl phosphate is formed in the mitochondria of the cell from CO2 (in the form of bicarbonate), NH4

+ and ATP.

Building up reactive intermediates

• A carbamoyl group of carbamoyl phosphate ( ) is transferred to the ornithine forming citrulline.

• Exergonic reaction

High-energy bond

Building up reactive intermediates

• Citrulline and aspartate are condensed together into argininosuccinate.– Energy is provided by splitting ATP into AMP and

pyrophosphate.

More energy is produced

X2

Cleavage

• Argininosuccinate is split into arginine and fumarate.

Hydrolysis

• Arginine is hydrolyzed into urea and ornithine.• Ornithine can then enter the cycle again.

Net result

Remember: fumarate

Oxaloacetate

Transamination reaction

Atomic sources of urea

Hyperammonemia

• A complete block of any step in the urea cycle is fatal.

• Inherited disorders result in hyperammonemia which can lead to mental retardation.

• Extensive ammonia accumulation leads to vomiting in infancy, lethargy, irregular muscle coordination (ataxia), and mental retardation, extensive liver damage, and death.

• Treatment:– Dialysis– Low protein diet (long-term)

Gout

• A small amount of our waste nitrogen is excreted in urine and feces as urate rather than urea.

• Because the urate salt is highly insoluble, any excess of the urate anion causes precipitation of sodium urate in joints.

• The pain of gout results from a cascade of inflammatory responses to these crystals in the affected tissue.

Metabolism

• Uric acid is an end product of the breakdown of purine nucleosides, and loss of its proton (H+) gives ureate ion.

Causes of gout

• Metabolic (accelerated breakdown of ATP, ADP, or AMP)– Alcohol abuse generates acetaldehyde that must be metabolized

in the kidney by a pathway that requires ATP and produces excess AMP.

– Inherited fructose intolerance and glycogen storage diseasesaccelerate uric acid production.

– Circulation of poorly oxygenated blood • With low oxygen, ATP is not efficiently regenerated from ADP in

mitochondria, excess ADP is disposed.• Renal

– Conditions that diminish excretion of uric acid include kidney disease, dehydration, hypertension, lead poisoning, and competition for excretion from anions produced by ketoacidosis.

Treatment

• Allopurinol inhibits the enzyme which produces uric acid.– Since hypoxanthine and

xanthine are more soluble than sodium urate, they are more easily eliminated.

• Rasburicase catalyzes enzymatic oxidation of poorly soluble uric acid into amore soluble metabolite allantoin.– It is still under investigation for

treating gout.

Essential and nonessential amino acids

Nonessential EssentialAlanine HistidineArginine IsoleucineAsparagine LeucineAspartate LysineCysteine MethionineGlutamate PhenylalanineGlutamine ThreonineGlycine TryptophanProline ValineSerineTyrosine

• Human beings cannot make 9 amino acids. These are known as essential amino acids

• The other 11 amino acids are termed nonessential amino acids

The importance of essential amino acids and effects of deficiencies• Histidine: A deficiency can cause joint pain (linked to rheumatoid

arthritis)• Lysine: formation of collagen for bones and cartilage.• Methionine: a primary source of sulfur when cysteine intake is

limited.• Phenylalanine: a source of tyrosine.• Tryptophan: the metabolic starting material for serotonin; it is a

natural relaxant that can relieve insomnia. Its deficiency leads to serotonin deficiency syndrome.

• Arginine: the majority of arginine is synthesized in the urea cycle where it is cleaved to form urea and ornithine; a certain amount must be taken daily, especially for men of reproduction age, since 80% of the amino acid composition of male seminal fluid is made of arginine.

Foods with complete amino acids

Foods with incomplete amino acids

Complementary sources of amino acids

Fate of amino acids

White boxes: ketogenic amino acids

Green boxes: glucogenic amino acids

Both ketogenic and glucogenic amino acids are able to enter fatty acid biosynthesis via acetyl-SCoA

Biosynthesis of nonessential amino acids Glutamate

• The amino group of glutamate is the precursor of all nonessential amino acids

• Glutamate is made from -ketoglutarate by reductive deamination

Other precursors of amino acids

Glutamine and asparagine

• Glutamine is made from glutamate

• Asparagine is made from glutamine

Phenylketonuria

• Tyrosine is made from phenylalanine.

• Inability to drive this reaction causes accumulation of phenylalanine, a condition known as phenylketonuria.

Metabolites

Symptoms and treatment of phenylketonuria

• Mental retardation

• Treatment: diet with low phenylalanine (low-protein grain)

• Individuals with PKU must avoid foods sweetened with aspartame (Nutrasweet, for example), which is a derivative of phenylalanine.

Test for phenylketonuria

Alkaptonuria

Symptoms