Endo: Catabolism Of Proteins & AA Nitrogen Flashcards
An excess of ingested over excreted nitrogen.
Positive nitrogen balance
Protein and AA catabolism described how
Nitrogen is converted to urea
Positive nitrogen balance accompanies
Growth and pregnancy
Nitrogen Output exceeds intake
Negative nitrogen balance
Negative nitrogen balance accompanies
Surgery, Advanced cancer & Marasmus/Kwashiorkor “SAM K”
Ammonia, derived mainly from _______, is highly toxic, tissues covert ammonia to _______ of nontoxic glutamine.
a-amino nitrogen of AAs. Amide nitrogen.
Subsequent deamination of glutamine in the liver releases ammonia, which is then converted to
Nontoxic Urea
Liver function is compromised in this disease states, elevated ammonia levels generate clinical s/sx
Cirrhosis or Hepatitis
_______ turnover occurs in all forms of life.
Protein
Each day, humans turn over ____% of their total body protein. Principally __________.
1-2%. Muscle protein.
High rates of protein degradation occur in tissue undergoing structural rearrangement such as
Uterine tissue during pregnancy
Of the liberated AAs, approximately __% are reutilized
75%
Excess of nitrogen forms
Urea
Since excess AAs are not stored , those not immediately incorporated into new protein are
Rapidly degraded
Degrade protein to AAs
Proteases & Peptidases
The susceptibility of a protein to degradation is expressed as its
Half life
Half lives of liver proteins range from
Under 30mins- over 150hrs
Typical housekeeping enzymes have a half
Iife values of over
100hrs
Key regulatory enzymes have a half life of
0.5-2hrs
Target some proteins for rapid degradation
PEST sequence: Proline, Glutamate, Serine & Threonine
Resulting peptides are then degraded to AAs by ______ that cleave internal bonds.
Endopeptidases
Remove AAs sequentially from the amino terminals
Aminopeptidases
Remove AAs sequentially from the carboxy terminals
Carboxypeptidases
Extracellular, membrane associated and long lived intracellular proteins are degraded in _______ by __________.
Lysosomes. ATP-independent processes.
Degradation of abnormal and other short-lived proteins occur in ______ and requires ____ & _______.
Cytosol. ATP & Ubiquitin.
Is a small(8.5 kDa) proteinthat targets many intracellular proteins for degradation. The primary structure of this is highly conserved. Only _____ residues differ between yeast and human.
Ubiquitin. 3 of 76.
Several molecules of ubiquitin are attached by nonpetide bonds formed between the
Carboxy terminal of ubiquitin & amino groups of lysyl residues
The terminal of COOH of ubiquitin forms a ________ bond with an -SH of E1 in a reaction driven by conversion of ___ to ___ and ___. Subsequent hydrolysis of PPi by ________ ensures that reaction 1 will proceed readily.
Thioester. ATP. AMP & PPi. Pyrophosphate.
A thioester exchange reaction transfers activated ubiquitin to __.
E2
Catalyzes transfer of ubiquitin to amino group of lysyl residues of target proteins.
E3
Affects whether a protein is ubitiquinated
Residue present at its amino terminal
Amino terminal that retards ubiquitination
Met or Ser
Amino terminal that accelerates ubiquitination
Asp or Arg
Degradation occurs in a multicalytic complex of proteases known as
Proteasome
Proteins are hydrolyzed to this for absorption to make them smaller
AAs
Proteolytic enzymes responsible for degrading proteins are in the
Stomach, pancreas & small intestine
Contains gastric juices HCL and pepsin
Stomach
Kills bacteria
HCl
Is from pepsinogen which is activated by HCl
Pepsin
Further cleave polypeptides initially digested by the stomach
Pancreatic proteases
From the intestine activates pancreatic enzymes such as ________ to its active form.
Enteropeptidase. Trypsinogen.
Small intestine contains _______ that cleaves oligopeptides to produce ________.
Aminopeptidases. Free AAs.
Absorbed by the intestinal epithelium
Free amino acids and dipeptides
Dipeptides are further hydrolyzed in the _______ before being released to the portal system.
Cytosol
Excrete ammonia
Ammonotelic
Excrete uric acid
Uricotelic
Ammonotelic in nature, compels them to excrete water continuously, facilitating excretion of highly toxic ammonia.
Teleostean fish
Must conserve water and maintain low weight, uricotelic in nature and excrete uric acid as ___________.
Birds. Semisolid guano.
Many land animals, including humans, are _______ and excrete nontoxic, water-soluble urea
Ureotelic
A consequence- not a cause- of impaired renal function
High blood urea levels
4 stages in Urea biosynthesis
-Transamination
-Oxidative deamination of glutamate
-Ammonia transport
-Reactions of the urea cycle
“TOAR”
Transfers a-Amino Nitrogen to a-Ketoglutarate, forming Glutamate. Reaction is readily reversible.
Transamination
Transamination interconverts pairs of
a-amino acids & a-keto acids
Do not participate in transamination
Proline, Hydroxyproline, Lysine & Threonine
Also function in amino acid biosynthesis
Aminotransferases
A coenzyme present at the catalytic site of aminotransferases and of many other enzymes that act on AAs
Pyridoxal Phosphate
During transamination, bound ____ serves as a carrier of amino groups. Following removal of a-amino nitrogen by transamination, the remaining carbon skeleton is _________.
PLP. Degraded.
2 enzymes that catalyze the transfer of amino groups to pyruvate(forming ______) or to a-ketoglutarate(forming ____)
Alanine pyruvate aminotransferase(alanine aminotransferase) & glutamate-a-ketoglutarate aminotransferase(glutamate aminotransferase)
Alanine. Glutamate.
Each aminotransferase is specific for _____ of substrates but nonspecific for other pair.
One pair
Since alanine is also a substrate for ____________, all the amino nitrogen from amino acids that undergo transamination can be concentrated in __________.
Glutamate aminotransferase. Glutamate.
Is the only AA that undergoes oxidative deamination at an appreciable rate in mammalian tissues.
L-glutamate
The formation of ammonia from a-amino groups thus occurs mainly via
A-amino nitrogen of L-glutamate
Occupies a central position in nitrogen metabolism
L-glutamate dehydrogenase
Transfer of amino nitrogen to a-ketoglutarate forms
L-glutamate
Release of this nitrogen as ammonia is then catalyzed by hepatic ______________, which can either use ___ or ____.
L-glutamate dehydrogenase. NADP NADH
Conversion of a-amino nitrogen to ammonia by the concerted action of glutamate aminotransferase and GDH is often termed
Transdeamination
Liver GDH is allosterically inhibited by
ATP, GTP & NADH
Liver GDH is allosterically activated by
ADP
Reaction catalyzed by ____ is freely reversible and functions also in AA biosynthesis
GDH
Also remove nitrogen as ammonia
Amino acid oxidases
Convert AAs to an a-amino acid that decomposes to an a-ketoacid with release of ammonium ion.
L-amino acid oxidase
The reduced _____ is reoxidized by molecular oxygen , forming hydrogen peroxide, which then is split to ___ & ___ by ________.
Flavin. O2 & H2O. Catalase.
Is life threatening
Ammonia intoxication
The ammonia produced by enteric bacteria and absorbed into portal venous blood and the ammonia produced by tissues are rapidly removed from the circulation by the _____ and converted ____.
Liver. Urea.
Percent of ammonia always present in peripheral blood
10-20g/dL
Toxic to the CNS
Ammonia
Symptoms of ammonia intoxication
Tremors, slurred speech, blurred vision, coma & death “TSB CD”
Ammonia may be toxic to the brain in part because it reacts with _________ to form _______.
a-ketoglutarate. Glutamate.
Resulting depleted levels of a-ketoglutarate then impair function of the
Tricarboxylic acid(TCA) in neurons
Catalyzed the formation of glutamine and fixes ammonia as glutamine.
Glutamine synthase
Since amide bond synthesis is coupled to the hydrolysis of __ to ___ and ____, the reaction strongly favors glutamine synthesis.
ATP. ADP & Pi.
Sequester ammonia in a no toxic form.
Glutamine
Strongly favors glutamate formation. Catalyze the hydrolytic release of the amide nitrogen of glutamine as ammonia.
Glutaminase
Catalyzes the interconversion of free ammonium ion and glutamine. An analogous reaction is catalyzed by _________.
Glutamine synthase & Glutaminase. L-Asparaginase.
Formation & secretion of this maintains acid base balance
Ammonia
Facilitates cation conservation and regulation of acid base balance. Produced by _________.
Excretion into urine of ammonia. Renal tubular cells.
Ammonia production from intracellular renal AAs, especially glutamine, increases in
Metabolic acidosis
Ammonia production from intracellular renal AAs, especially glutamine, decreases in
Metabolic alkalosis
Series of reactions that produce urea from ammonia (NH3). In mammals this cycle occurs in the _____.
Urea/Ornithine cycle. Liver.
Urea cycle was first discovered by
Krebs & Kurt Henseleit, 1932
Organisms that cannot easily and quickly remove ammonia usually have to convert it to some other substance, like ___ or ____, which are much less toxic.
Urea. Uric acid.
Result of liver failure is the accumulation of nitrogenous waste , mainly ammonia, which leads to a neuropsychiatric abnormality
Hepatic encephalopathy
The urea cycle consists of five reactions.
2 mitochondrial, 3 cytosolic
Urea cycle: converts two amino groups, one from NH4 and one from Asp, and a carbon atom from HCO3, to relatively nontoxic excretion product, urea at the cost of four high energy phosphate bonds _____ hydrolyzed to ____ & ____.
3 ATP. 2 ADP. 1 AMP.
Is the carrier of the urea cycle carbon and nitrogen atoms
Ornithine
Major end product of nitrogen catabolism in humans
Urea
Synthesis of 1 mol of urea requires ______ plus ______ and _____________.
3mol of ATP. 1 mol of each ammonium ion. a-amino nitrogen of aspartate.
Initiates urea biosynthesis
Carbamoyl Phosphate Synthase
Rate limiting enzyme of the urea cycle. Condensation of CO2, ammonia and ATP to form carbamoyl phosphate is catalyzed by mitochondrial
Carbamoyl Phosphate Synthase I
An allosteric activator of carbamoyl phosphate synthase I. Enhances the affinity of the synthase for ATP.
N-acetylglutamate
Formation of carbamoyl phosphate requires ________.
2mol of ATP
Defect in the enzyme carbamoyl phosphate synthase
Hyperammonenia type I
Carbamoyl phosphate plus ornithine forms
Citrulline
Catalyze transfer of the carbamoyl group of carbamoyl phosphate to ornithine, forming _____ and ______. Reaction occurs in the _______.
L-Ornithine transcarbamoylase. Citrulline & orthophosphate. Mitochondrial matrix.
Results from mutation of the ORNT1 gene that encoded the mitochondrial membrane ornithine transporter.
Hyperornithinemia, hyperammonemia and homocitrullinuria “HHH syndrome”
Failure to import cytosolic ornithine into the mitochondrial matrix renders the ________ inoperable.
Urea cycle
Citrulline olus aspartate forms
Argininosuccinate
Links aspartate and citrulline via the amino group of aspartate and provides the second nitrogen of urea. The reaction requires ___.
Argininosuccinate synthase
Subsequent displacement of AMP by aspartate forms
Citrulline
Cleavage of Argininosuccinate forms
Arginine & Fumarate
Cleavage of argininosuccinate, catalyzed by ______, proceeds with retention of nitrogen in ______ and release of the aspartate as _______.
Argininosuccinase. Arginine. Fumarate.
Fumarate is recycled back to
Aspartate
Addition of water to fumarate forms ________ and subsequent NAD+dependent oxidation of malate forms ________. Analogous to reactions of the citric acid cycle but are catalyzed by __________ & ___________.
L-malate. Oxaloacetate. Cytosolic fumarase & malate dehydrogenase.
Transamination of oxaloacetate by ________ then re forms ______.
Glutamate aminotransferase. Aspartate.
Arginine is produced from
Argininosuccinate
ASL gene is located on chromosome
7
Deficiency in argininosuccinase which is accompanied by elevated levels of argininosuccinate in blood, CSF and urine.
Argininosucciniaciduria
Argininosucciniaciduria is associated with friable & tufted hair
Trichorrhexis nodosa
Cleavage of arginine releases ____ & Re forms ____.
Urea. Ornithine.
Hydrolytic cleavage of the guanidino group of arginine, catalyzed by liver ______, releases _____.
Arginase. Urea.
Re enters liver mitochondria for additional rounds of urea synthesis.
Ornithine
Are potent inhibitors of arginase, competitive with arginine.
Ornithine & Lysine
Arginine also serves as the precursor of the potent muscle relaxant __________ and catalyzed by ___________.
Nitric oxide. Nitric oxide synthase.
Is an autosomal recessive defect in the gene for arginase. Symptoms of this appear until age __ to __ years.
Hyperargininemia. 2-4
Is the pacemaker enzyme of the urea cycle
Carbamoyl Phosphate Synthase I
The activity of carbamoyl phosphate synthase I is determined an allosteric activator
N-acetylglutamate
N-acetylglutamate level is dictated by its rate of synthesis from _____ & ______ and its rate of hydrolysis to ______ & _______. These reactions are catalyzed by __________ & _________
Acetyl CoA & Glutamate. Acetate & Glutamate. N-acetylglutamate synthase & hydrolase.
Defects in NAGS gene result in
Hyperammonemia
Elevate enzyme levels to cope with the increased production of ammonia
Starvation
All defects in urea synthesis results in
Ammonia intoxication
The effects are more severe when the metabolic block occurs at reactions
1 or 2
Clinical symptoms common to all urea cycle disorders include
Vomiting, avoidance of high protein foods, intermittent ataxia, irritability, lethargy and severe mental retardation
Significant improvement and minimization of brain damage accompanies
Low CHON diet & Small frequent meals
Offers promise for correcting defects in urea biosynthesis
Gene therapy
2ATP+HCO3+NH4. Name product, Enzyme and Location
Carbamoyl phosphate. Carbamoyl phosphate synthase. Mitochondria.m
Carbomoyl phosphate+ornithine. Name product, Enzyme and Location
Citrulline. Ornithine transcarbamoylase. Mitochondria.
Citrulline+aspartate+ATP. Name product, Enzyme and Location
Argininosuccinate. Argininosuccinate synthase. Cytosol.
Argininosuccinate. Name product, Enzyme and Location
Arg+fumarate. Argininosuccinate lyase. Cytosol.
Arg+H2O. Name product, Enzyme and Location
Ornithine+urea. Arginase. Cytosol.
