1 - Urea

Amino Acid Catabolism

You want to get rid of the N so that you can use the Carbons it is attached to for energy and catabolism. Must eliminate nitrogen somehow. Urea is a neutral substance that can store the N until you are ready to eliminate it (urine).

Degradation of Amino Acids

Important under three circumstances:
(1) Normal dynamic turnover of body proteins
(2) Amino Acid ingested > Body's needs
(3) Fasting or in Diabetes mellitus
We eliminate all the excess N as urea in the urine. We don't store excess N.
Liver is the site of

Normal dynamic turnover of body proteins

Constant turnover of proteins at all times.
2% of your body's proteins (i.e. muscles) are being broken down to amino acids.
Meanwhile 75% of amino acids (from body-protein breakdown and dietary sources) are being reuitilized to build body proteins.
25% of

When Amino Acid ingested > Body's needs

...Carbons are stored for later use. You must get rid of the excess Nitrogen, leads to higher [urea] in your body due to increased breakdown of amino acid.
(eliminate Nitrogen and Store Carbons)

In Fasting or in Diabetes mellitus

Body is in a state of starvation or perceived starvation (diabetes). Glucose cannot get into the muscles due to lack of insulin. Glucagon is increased bc of low insulin. Overall => catabolic state.
Results in state of gluconeogenesis
- protein and F.A. br

Major Circulating forms of Amino Acids

Alanine and Glutamine are the major circulating amino acids in blood.
Protein breakdown from skeletal muscle is the prominent source of amino acids during starved state.

Amino Acid Source in Starvation...

Protein breakdown from skeletal muscle is the prominent source of...

Major circulating amino acids in blood...

Alanine and Glutamine are the...

Source of N & C for Urea Synthesis

Nitrogen from amino acid breakdown is converted to Urea. Urea consists of 2 ammonias (one from Aspartate and one from free ammonia from glutamate) and a CO (from bicarbonate).

Source of (free) Ammonia

The only significant reaction in humans to produce ammonia from a.a's is the mitochondrial conversion of Glutamate to Ammonia + ?-ketogluarate. (uses H2O and an NAD or NADPH).
Enzyme: Glutamate dehydrogenase

Glutamate Dehydrogenase

catalyzes the mitochondrial conversion of Glutamate to Ammonia + ?-ketogluarate. (uses H2O and an NAD pr NADP).

Liver and Urea

Liver must pool together all N as Glutamate and Aspartate.
Any a.a. that is NOT Asp or Glu must be converted to Asp or Glu.
1) Donates ammonia group from an a.a. to ?-ketoglutarate
2) Aminotransferase (requires PLP) makes Glutamate + ?-keto acid

Aminotransferase

Converts ?-ketoglutarate + Amino Acid --> Glutamate + ?-Keto Acid
Cofactor is PLP (Pyridoxal Phosphate)
Occurs in liver mitochondria.
Can also be reversed to make new a.a.'s for protein synthesis.

Transamination

The major mechanism for removing alpha-amino groups from amino acids is via transamination (Fig. 25-3).
Transaminases (aminotransferases) catalyze the transfer of
an alpha-amino group of an amino acid to the carbon of an alpha-keto acid. The most common a

Pyridoxal Phosphate (PL)

Co-factor for aminotransferase enzyme
Vitamin B6

Fate of Glutamate produced by aminotransferase...

Some Glutamate is used to produce ammonia for urea (N-disposal) and carbons.
The rest is combined with Oxaloacetate to produce Aspartate and ?-Ketoglutarate.

Glutamate NOT used for ammonia production...

Some Glutamate is used to produce ammonia. The rest...
Glu + Ox.acetate ---> ?-Ketoglutarate + Aspartate
?-Ketoglutarate - can be reused to produce more Glutamate from other a.a.'s.
Aspartate provides the second ammonia in urea. (see prev card)

Amino Acid to Urea (basic cycle)

a.a. + ?-Ketoglutarate --> Glutamate --> Aspartate + ?-Ketoglutarate AND NH4 ---> Urea

Aspartate Aminotransferase

Converts Glutamate + Oxaloacetate to ?-Ketoglutarate and ASPARTATE

Reversible Transamination of Asp <-> Glu (via Aspartate Aminotransferase)

Aspartate Aminotransferase Enzyme reaction
Example of a Transaminase reaction:
1) Aspartate donates its amino group, becoming the a-keto acid oxaloacetate.
2) a-Ketoglutarate accepts the amino group, becoming the amino acid glutamate.

Tyrosine Transaminase..

Aminotransferase activity
Production of Glutamate from Tyrosine. Catalyzed by....
(uses PLP co-factor)

Amino Acids that do NOT undergo Transamination

Lys, Thr, & Pro
...are each converted to Glutamate via several steps (NOT a transanimation reaction).

Lysine

...is converted to Glutamate via several steps (NOT a transanimation reaction).

Threonine

...is converted to Ammonia via several steps (NOT a transanimation reaction).

Proline

...is also converted to Glutamate via several steps (NOT a transanimation reaction).

Urea Synthesis (in Liver)

NH3 + Asp + HCO3- + 3 ATP --> Urea
3 molecules of ATP but 4 high energy Pi (one from Carbamoyl Phosphate) are used in urea synthesis. Expensive!
Urea synthesis uses the "Urea Cycle", involving liver mitochondria and cytosol.
Think of this as a base (ammon

Pre-Urea Cycle Cytosol

Amino Acids + ?-ketoglutarate --> Glutamate ---> NH4 and Aspartate + ?-Ketoglutarate ------> Urea

Pre-Urea Cycle Mitochondria

1) Glutamate ---> NH4 and
2) Glutamate + Oxaloacetate ---> Aspartate + ?-Ketoglutarate
3) Aspartate leaves the mitochondria to enter Urea Cycle
4) Ammonia (NH4) from Glutamate is transferred to Carbamoyl Phosphate (CPS-1)
5) Carbamoyl Phosphate + Orinithi

OTC (ornithine transcarbamoylase)

Carbamoyl Phosphate + Orinithine ---> Citruline + Pi
(#1 in picture)

Urea Cycle Overview (Products)

Citrinine and Orthinine cycle through, producing a Urea with each cycle.
Uses 1 ATP (Plus 2 ATP and a Pi from Carbamoyl Phosphate).
Also produces 1 Fumarate per cycle

Urea Cycle Steps

1) Carbamoyl Phosphate + Orthinine --> Citruline (Citruline leaves mitochondria and enters cytosol)
2) Citruline + ATP (adds AMP) ---> Citrullyl-AMP Intermediate +PPi
3) Citrullyl-AMP Intermediate + ASPARTATE ---> Arginosuccinate
4) Arginosuccinate --> Fu

Urea Cycle (Srini's)

FIGURE 18-10 Urea cycle and reactions that feed amino groups into the cycle. The enzymes catalyzing these reactions (named in the text) are distributed between the mitochondrial matrix and the cytosol. One amino group enters the urea cycle as carbamoyl ph

Arginase

Cleaves Urea from Arginine, releasing one Urea and Ornithine (which is reused for another urea cycle)

Committed step of the Urea Cycle, and is subject to regulation

Carbamoyl Phosphate Synthase (CPS or CPS I)

N-acetylglutamate

Allosteric activator of Carbamoyl Phosphate Synthase 1.
CPS has an ABSOLUTE REQUIREMENT for this activator.

Activation of CPS-1

Cellular [Glutamate] is high, signals an excess of free a.a.'s due to protein breakdown or dietary intake. ---> Synthesis of N-acetylglutamate (from Acetyl CoA + Glutamate) ---> Activation of CPS-1

Synthesis of N-acetylglutamate

Acetyl CoA + Glutamate
Activates CPS-1
INCREASES Urea Production

Short Term Regulation of Urea Synthesis

The intrahepatic concentration of N-acetylglutamate is high after a protein-rich meal.

Long Term Regulation of Urea Synthesis

High protein diet --> elevated serum glucagon --> stimulation of the transcription of the genes for urea cycle enzymes

Elimination of N by Kidney

No more than 10% N may be eliminated as ammonium ion by kidney. [Remember that excess N circulates in blood mostly as Glutamine (Gln) & Alanine (Ala)]
Gln + H2O --> NH3 + Glu
Enzyme: Glutaminase
Important in acid-base balance.

Glutaminase

Catalyzes: Glutamine + H2O --> NH3 + Glutamate
Ammonia released contributes to the 10% ammonia being secreted by the kidneys.

Glutamate vs Gutamine

Glutamine has an EXTRA Amino group. (2 aminos per Glutamine)
Glutamine is BASIC
Glutamate is ACIDIC

Glutaminase in Acid-Base Balance of Blood

Conversion of Glutamine --> Glutamate + Ammonia
Ammonia (base) can act as a buffer to fine tune blood pH.

Cirrhosis (definition)

Chronic disease of the liver with gradual destruction of cells and formation of scar tissue; commonly caused by alcoholism

Normal Ammonia Processing

In the stomach ("gut"):
1) Intestinal bacteria digest dietary protein, releasing ammonia.
2) Ammonia enters portal vein to liver and is converted to Urea.
This causes the ammonia concentration [NH3] in the Portal Blood to be GREATER than that of the Syste

Cirrhosis Ammonia Processing

Severe cirrhosis leads to collateral communications between Portal and Systemic veins.
Results in increased [NH3] in Systemic Blood ---> leads to Ammonia Toxicity.

Congenital Hyperammonemia

Deficiency in any urea cycle enzymes (particularly deficiency of OTC) ---> mental retardation, and in severe cases coma and death.
Clinical: In blood [Ammonia]?, [Glutamine] ? presence of pyrimidine metabolites (orotic acid) in urine (due to increase in C

CPS I deficiency

autosomal recessive
results in Congenital Hyperammonemia
Clinical: In blood [Ammonia]?, [Glutamine] ? presence of pyrimidine metabolites (orotic acid) in urine.

Increase in Glutamine due to Congenital Hyperammonemia

Caused by conversion of Glutamine from Glutamate and Ammonia (see Glutaminase for reverse reaction in normal conditions) in an attempt to eliminate excess ammonia.
This can't keep up and ammonia builds up anyways.

Treatment of Congenital Hyperammonemia

Specific treatments depend on which urea cycle enzyme is deficient. More general treatments include:
1) limiting protein intake to the amount barely adequate to supply amino acids for growth, while adding to the diet the a-keto acid analogs of essential a

Hyperammonemia (Adults vs Newborns)

Newborns: due to genetic defect in urea cycle enzyme
Adults: due to diseased liver (alcoholism, cancer, hepatitis, etc)

Vitamin B6

PLP (Pyridoxal Phosphate)
essential for all Transamination reactions

PLP (Vit B6) is Essential for...

- Serotonin synthesis (Trp -> serotonin +CO2)
- Glycogen phosphorylase
- Heme synthesis
- Cys (Cysteine) synthesis
- Sphingolipid synthesis
- Norepinepherine synthesis
(SERa GLYdes HEr Cyster's SPHynx NORth)

PLP (Vit B6) Deficiency vs Excess

Mild deficiency: Irritability, Nervousness, depression, Hair loss, Insomnia
Severe deficiency: Peripheral neuropathy, anemia, Muscle weakness
Amount of Vitamin B6 proportional to protein intake
Excess vitamin is toxic

NO Synthase

Converts Arginine to Citrulline, releasing NO (Nitrous Oxide)
Ca2+ is a Cofactor

NO Action

NO increases conversion of GTP -> cGMP ("c-gimp")
cGMP can cause smooth muscle relaxation or can be converted to GMP (used in other reactions)

Result of NO Vasodilation

NO relaxes vascular smooth muscle --> increased blood flow --> male sexual arousal

Viagra (Sildenafil)

Inhibits phosphodiesterase (PDE5) --> prevents conversion of cGMP to GMP and diverts more cGMP to smooth muscle relaxation.
This causes increased vasodilation --> increased blood flow --> increased boner.