Nitrogen Elimination and Carbon Chain Metabolism I Flashcards
The free amino acids that are generated in the gut are mobilized by peptide transporters in intestinal epithelial cells, and delivered via the portal system to the
Liver
In the liver, a family of related enzymes removes α-NH3 groups from amino acids and transfers them to an acceptor molecule, typically
α-ketoglutarate
Transfer of α-NH3 to α-ketoglutarate results in the formation of
Glutamate
While this first step does not alter the overall amount of nitrogen, it does reposition the nitrogen for more efficient elimination during the
Second stage
In this second step, glutamate dehydrogenase can release nitrogen (as ammonia), which is then assimilated into
Urea
After the removal of the alpha-amino group, carbon chains can be modified and channeled into the
-generates energy
TCA cycle
The remnant carbon skeleton can also be stored, following their assimilation into
Fat and carbohydrates
The carbon chain of amino acids can be used for generating energy, or stored as fat and carbohydrate after removal of the
Alpha-amino group
Rarely coded in gene sequences, and this amino acid is underrepresented in proteins
Methionine
Methionine is encoded by the single codon
ATG
Degraded to yield amino acids, primarily in the gut, and absorbed by epithelial cells that line the intestine
Dietary Proteins
Because the stability of cellular proteins can vary from seconds to years, their degradation is
Highly regulated
Required for the uptake of amino acids in the gut, and their transfer to the portal system
-In the intestinal wall
Trans-membrane protein channels
Help recover amino acids which are inadvertently released into the urine.
-Mutations in these pumps result in loss of amino acids which can cause amino acid deficiencies
Membrane channels in the kidney
There are three primary sites where proteins are synthesized. These locations are termed
Gastric, pancreatic, and intestinal
The function of the mouth/saliva is to masticate food so that it is easily transferred to the stomach. This initiates the first of three stages of
Proteolysis
Generally named after critical residues in their active site
Proteases
Proteases can also be named after a
Cofactor, substrate, or location
The extreme acidity of the stomach begins protein unfolding and
Denaturation
Digestive proteases are synthesized as inactive precursors, called
Zymogens
The zymogen is catalytically inactivated by an inhibitory
-when removed, enzyme is activated
Prodomain
Synthesized in intestinal mucosal cells, and its key function is to convert trypsinogen to active trypsin
Enteropeptidase
Trypsin then activates the rest of the enzymes. This mechanism of autocatalytic protease activation is activated by hormonal signaling, and occurs primarily in the
Intestinal tract
Secrete dipeptidases that can generate free amino acids
Intestinal cells
Amino acid and dipeptide transporters take up amino acids/peptides into the intestinal cells, where we see complete
Peptide hydrolysis
Free amino acids are transferred to the liver by the
Portal vein
Proteases in the gut. These non-specific and potent enzymes are classified as
Endo- and exopeptidases
These proteases are indiscriminate, because they degrade all, and not specific proteins
Endo- and exopeptidases
May be either amino- or carboxypeptidases
Exopeptidases
A serineprotease that cleaves after basic residues (lysine and arginine).
-The hydroxyl group in serine contributes to the catalytic properties of this endopeptidase.
Trypsin
The first step in amino acid breakdown involves the removal of the
Alpha amino group
A family of aminotransferase enzymes transfers α-NH3 from most amino acids to
α-ketoglutarate
Although this mechanism does not alter the balance in nitrogen (because net nitrogen level is unchanged), it provides an efficient way to concentrate all the
α-NH3 molecules on one or a few molecules
The primary acceptor for NH3 in the liver, and most other tissues
α-ketoglutarate
However, is generated and is to the liver in muscle cells (which produce abundant pyruvate)
Alanine (alanine = pyruvate + NH3)
Are detected in the blood following a high protein meal because they provide a safe way to transfer excess NH3 to the liver for the synthesis of urea, and excretion in the urine
Alanine, Glutamine, and Asparagine
Not transported in its free form because it is highly toxic to the central nervous system
Ammonia
Can be released from glutamate via oxidative deamination to produce ammonia
NH3
Oxidative deamination is mediated by
-present in all tissues
Glutamate dehydrogenase
Alternatively, α-ketoglutarate can enter the
TCA cycle
The amino group can be transferred from glutamate to oxaloacetate by
Aspartate aminotransferase
Both ammonia and aspartate provide a nitrogen in the assimilation of
Urea in the liver
Can add NH3 to glutamate to increase the capture and transport of nitrogen (ammonia) from peripheral tissues to the liver
Glutamine Synthase
Can add NH3 to aspartate (asparagine)
Asparagine synthesis
Under conditions of ammonia toxicity, these reactions can rapidly sequester excess
NH3
The transamination of pyruvate yields
-can be safely transported to the liver
Alanine
Alanine is converted back to pyruvate by transamination, and can then enter the pathway of
Gluconeogenesis
The critical enzyme that initiates the net removal of nitrogen from amino acids (specifically glutamate)
Glutamate dehydrogenase
Glutamate dehydrogenase is regulated by allosteric mechanisms. It is inhibited by
ATP/GTP
Glutamate dehydrogenase is regulated by allosteric mechanisms. It is activated by
ADP/GDP
Stimulation of glutamate dehydrogenase when energy levels are low increases the deamination of glutamate, to yield higher amounts of
α-ketoglutarate
This reaction occurs primarily in the mitochondrion, and requires the cofactor
NAD
NH3 that is released from glutamate is condensed with carbon dioxide (in the mitochondrion) to form
Citrulline
Glutamate transfers NH3 (transamination) to
-forms aspartate
Oxaloacetate
Glutamate transfers NH3 (transamination) to oxaloacetate (forming aspartate), which donates the nitrogen to citrulline, yielding the
2nd nitrogen in urea
This reaction occurs in the
Cytosol of liver cells
In addition to α-ketoglutarate, there are other intermediates in the formation of urea that are linked to the TCA cycle. one example is
Oxaloacetate
Transaminated to aspartate by aspartate aminotransferase
Oxaloacetate
Arginino-succinate is cleaved to yield
Arginine and fumarate
The conversion of arginine to ornithine requires the enzyme
-expressed only in the liver
Arginase
Transamination and oxidative deamination reactions are
Reversible
The incorporation of ammonia into CO2 by carbamoyl phosphate synthase I requires
2 ATPs
The conversion of aspartate to argininosuccinate requires
1 ATP
The addition of NH3 to glutamate/aspartate by glutamine/asparagine synthase requires
1 ATP
Digestive proteases are synthesized in the stomach, pancreas and intestine, but are active only in an
Extracellular location
The α-NH3 groups are generated in all tissues, but are assimilated into urea only in the
Liver
Only the liver expresses which three enzymes?
Arginase, glutaminase, and asparaginase
The two nitrogen’s in urea are obtained from
Ammonia and aspartate
Both nitrogens were originally present in
Glutamate
The addition of nitrogen occurs in which two distinct intracellular regions of the liver cell?
Mitochondria and cytosol
The carbon is obtained from CO2, which is transported as soluble
Bicarbonate
Two key metabolites in the urea cycle are
Ornithine and citrulline
Converts arginine to ornithine + urea, and is present only in the liver
Arginase
What are the three key steps in nitrogen regulation?
Glutamine synthase, Alanine aminotransferase, and glutaminase
When there is a sudden increase in the levels of amino acids, the cellular capacity to assimilate nitrogen into urea can become
Overwhelmed
Since ammonia is highly toxic (due to CNS toxicity), a temporary mechanism functions to sequester excess nitrogen. I.e. NH3 is added to
-serves as a temporary storehouse for excess nitrogen
Glutamate (generates glutamine)
This process requires
ATP
Urea is synthesized only in the liver, and several enzymes can release NH3 from compounds that are transported to this organ. What releaes NH3 from glutamine
Glutaminase
Glutaminase specifically releases NH3 from glutamine, while oxidative deamination of glutamate also yields free
NH3 + α-ketoglutarate
Glutaminase (unlike glutamate dehydrogenase), is only expressed in the
Liver
Pyruvate that is released from alanine can be used for the synthesis of glucose, and returned to muscle (and other) tissues for energy production. This is termed the
Glucose-Alanine cycle
Rapid increase in intracellular ammonia can be temporarily suppressed by the conversion of
Glutamate into glutamine
Bacteria in the gut, and gastrointestinal bleeding can increase the blood levels of
Ammonia
Activated to convert α-ketoglutarate to glutamate when ammonia levels rise rapidly
Glutamate dehydrogenase
However, this results in a proportionate reduction in the level of a key TCA intermediate, which leads to reduced
Energy production
Reduced function of the kidney can result in elevated levels of urea in the blood. An increased level of urea can then cross the intestinal cell wall and become cleaved by
Bacterial urease
Bacterial urease releases
H2O and ammonia
Ammonia can rapidly diffuse back into the blood and cause
Hyperammonemia
The deamination of glutamine in the liver, by glutaminase, generates glutamate and NH3. NH3 is condensed with CO2 to form
Carbonoyl Phosphate
NH3 is condensed with CO2 to form carbamoyl phosphate by carbamoyl phosphate synthase I in the
Mitochondrial matrix
NH3 is condensed with CO2 to form carbamoyl phosphate by carbamoyl phosphate synthase I in the mitochondrial matrix. This represents the first assimilation of
Nitrogen into urea
The addition of NH3 to oxaloacetate (via glutamate), provides a source for the
-occurs in cytosol of liver cells
2nd nitrogen into urea
The essential co-factor in all aminotransferases, and participates directly in the removal of alpha amino groups from amino acids
Pyridoxil Phosphate
Derived from vitamin B6, and forms a Schiff base with an epsilon amino group in a lysine residue in the aminotransferases
Pyridoxil
Pyridoxil is derived from vitamin B6, and forms a Schiff base with an epsilon amino group in a lysine residue in the
Aminotransferase
However, when an aminotransferase binds an amino acid, the alpha-amino group in the amino acid forms a transient Schiff base with
Pyridoxal phosphate
The key enzyme, and the rate-limiting step, in the Urea Cycle
Carbamoyl phosphate synthase I
This rate-limiting step is positively regulated by
N-acetylglutamate
The synthesis of N-acetylglutamate is stimulated by the presence of
Arginine
The initial step in urea synthesis occurs in the mitochondria, and requires
2 ATP’s per urea
