CONVERSION OF AMINO ACIDS TO SPECIALIZED PRODUCTS Flashcards
The entire glycine molecule becomes atoms
4, 5, and 7 of purines
The nitrogen and a-carbon of glycine are incorporated into the
pyrrole rings and the methylene bridge carbons of heme
constitutes a major fraction of the free amino acids in plasma
Alpha alanine
Present in coenzyme A
Beta alanine
Mammalian tissues form b-alanine from
cytosine, carnosine, and anserine
b-Alanine Also present as
b-alanyl dipeptides, principally carnosine
activate myosin ATPase, chelate copper, and enhance copper uptake
carnosine and anserine (Nmethylcarnosine
Participates in the biosynthesis of sphingosine, a component of sphingomyelin
Serine
the principal source of methyl groups in the body
S-Adenosylmethionine
Methionine It also contributes its carbon skeleton for the biosynthesis of the
3-diaminopropane portions of the polyamines spermine and spermidine
precursor of the thioethanolamine portion of coenzyme A and of the taurine
L. Cysteine
L-Cysteine is a precursor of the thioethanolamine portion of coenzyme A and of the taurine that conjugates with bile acids such as
Taurocholic acid
Decarboxylation of histidine to histamine is catalyzed by a broad-specificity aromatic
L-amino acid decarboxylase
catalyzes the decarboxylation of dopa, 5hydroxytryptophan, phenylalanine, tyrosine, and tryptophan
L-amino acid decarboxylase
Histidine to histamine
Histidine decarboxylase
formamidine donor for creatine synthesis and via ornithine to putrescine, spermine, and spermidine
Arginine
The 24-hour urinary excretion of creatinine is proportionate to muscle mass and is a useful indicator of kidney function
Creatinine
Glycine, arginine, and methionine participate in
Creatinine synthesis
Arginine is also the precursor of the intercellular signaling molecule
Nitric oxide
serves as a neurotransmitter, smooth muscle relaxant, and vasodilator
Nitric oxide
function in cell proliferation and growth
polyamines spermidine and spermine
Following hydroxylation of tryptophan to 5hydroxytryptophan by
liver tyrosine hydroxylase
a potent vasoconstrictor and stimulator of smooth muscle contraction
5-hydroxytryptamine or 5-HT
Kidney tissue, liver tissue, and fecal bacteria all convert tryptophan to ____________, then to indole ____________
Trytamine; 3 acetate
Neural cells convert tyrosine to
Epineprine
precursor of triiodothyronine and thyroxine
Tyrosine
functions in brain tissue as an inhibitory neurotransmitter by altering transmembrane potential differences
g-Aminobutyrate (GABA
It is formed by decarboxylation of Lglutamate, a reaction catalyzed by Lglutamate decarboxylase
g-Aminobutyrate
an excess of ingested over excreted nitrogen, accompanies growth and pregnancy
Positive nitrogen balance
where output exceeds intake, may follow surgery, advanced cancer, and kwashiorkor or marasmus
Negative nitrogen balance
ammonia, derived mainly from the
aamino nitrogen of amino acids
Subsequent deamination of glutamine in the liver releases ammonia, which is then converted to
Non toxic urea
Each day, humans turn over ______ of their total body protein, principally muscle protein
1-2%
High rates of protein degradation occur in tissues undergoing structural rearrangement such as
uterine tissue during pregnancy, tadpole tail tissue during metamorphosis, or skeletal muscle in starvation
Of the liberated amino acids, approximately _____are reutilized
75%
The excess nitrogen forms
Urea
The susceptibility of a protein to degradation is expressed as its half-life
1/2
Half-lives of liver proteins range from under
30 minutes - 150 hours
Typical “housekeeping” enzymes have
t1/2 values of over 100 hours
By contrast, many key regulatory enzymes have a
t1/2 of 0.5–2 hours
target some proteins for rapid degradation
PEST proline, glutamate, serine and treonine
The resulting peptides are then degraded to amino acids by
Endopeptidases
remove amino acids sequentially from the amino and carboxyl terminals, respectively
Aminopeptidases and carboxypeptidases
Extracellular, membrane-associated, and long-lived intracellular proteins are degraded in lysosomes by
ATP independent process
By contrast, degradation of abnormal and other short-lived proteins occurs in the cytosol and requires
ATP and ubiquitin
protein that targets many intracellular proteins for degradation
Ubiquitin
Only ___of 76 residues differ between yeast and human ubiquitin
3
A thioester exchange reaction transfers activated ubiquitin to
E2
catalyzes transfer of ubiquitin to e -amino groups of lysyl residues of target proteins
E3
Degradation occurs in a multicatalytic complex of proteases known as the
Proteasome
responsible for degrading proteins are in the stomach, pancreas, and small intestine
Proteolytic enzyme
Pancreatic proteases further cleave ___________initially digested by the stomach
Polypeptides
Enteropeptidase from the intestine activates pancreatic enzymes such as ___________ to its active form
Trypsinogen
SMALL INTESTINE •Contains ____________that cleaves oligopeptides to produce free amino acids
Aminopeptidase
further hydrolyzed in the cytosol before being released to the portal system
Dipeptides
ammonotelic
Excrete ammnonia fish
which must conserve water and maintain low weight, are uricotelic and excrete uric acid as semisolid guano
Birds
Many land animals, including humans, are ________ and excrete nontoxic, water-soluble urea
Ureotelic
Urea biosynthesis occurs in 4 stages
transamination (2) oxidative deamination of glutamate (3) ammonia transport (4) reactions of the urea cycle
Transamination interconverts pairs of
a-amino acids and a-keto acids
participate in transamination except
lysine, threonine, proline, and hydroxyproline
present at the catalytic site of aminotransferases and of many other enzymes that act on amino acids
Pyridoxal phosphate
During transamination, bound PLP serves as a
Carrier of amino acids
removal of a-amino nitrogen by transamination, the remaining carbon “skeleton” is
Degraded
These 2 enzymes catalyze the transfer of amino groups to pyruvate (forming alanine) or to aketoglutarate (forming glutamate
Alanine amino transferase ang glutamate amino transferase
Since alanine is also a substrate for glutamate aminotransferase, all the amino nitrogen from amino acids that undergo transamination can be concentrated in
Glutamate
the only amino acid 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 the
a -amino nitrogen of L-glutamate
Transfer of amino nitrogen to a-ketoglutarate forms
L glutamate
Release of this nitrogen as ammonia is then catalyzed by hepatic L-glutamate dehydrogenase (GDH), which can use either NAD+ or NADP+
L glutamate dehyrogenase GH
Conversion of a-amino nitrogen to ammonia by the concerted action of glutamate aminotransferase and GDH is often termed
Transdeamination
allosterically inhibited by ATP, GTP, and NADH and activated by ADP
Liver GDH activity
The reaction catalyzed by GDH is freely reversible and functions also in
Amino acid synthesis
Ammonia may be toxic to the brain in part because it reacts with
a-ketoglutarate to form glutamate
The urea cycle consists of
five reactions - two mitochondrial and three cytosolic
the carrier of these carbon and nitrogen atoms
Ornithine
functions as an enzyme activator
N-acetylglutamate
This is the rate-limiting enzyme of the urea cycle
Carbamoyl Phosphate Synthase I
rate-limiting enzyme of the urea cycle •This enzyme is active only in the presence of its allosteric activator
N-acetylglutamate
Formation of carbamoyl phosphate requires
2 mol of atp
catalyzes transfer of the carbamoyl group of carbamoyl phosphate to ornithine, forming citrulline and orthophosphate
L-Ornithine transcarbamoylase
autosomal recessive defect in the gene for arginase
Hyperargininemia
