Nitrogen Metabolism Flashcards
protein turnover
Proteins are constantly synthesized and degraded and this process is known as turnover. The turnover of proteins occurs at different rates. Some proteins such as collagen have a very long half life some have turnover in few minutes. &5% of amino acids are reutilized for protein synthesis. 25% are used for gluconeogenesis especially during starving
nitrogen balance
regardless of the nutritional or metabolic status, humans constantly excrete nitrogen. Nitrogen balance is comparison between intake of nitrogen and excretion. Negative nitrogen balace results when dietary protein intake is inadequate. Positive nitrogen balacen is when there is net increase in body proteins
what causes positive nitrogen balance
growth, pregnancy lactation
recovery from metabolic stress or injury
what causes negative nitrogen balance?
inadequate dietary protein
metabolic stress, sepsis, trauma
deficiency of an essential amino acid
preotein requirement of an individual is determined by what?
age pregnancy lactation convalescence after illness catabolic states
what factors affect protein utilization
quality of protein
digestibility
caloric level of diet
what are some of the effects of protein excess?
loss of calcium in the urine -> may lead to osteoporosis in women
increase in workload of kidney -> hyperfiltration.
What are some of the effects of protein deficiency
lack of growth, negative nitrogen balance, reduced serum albumin, edema, increased susceptibility to infection
Marasmic -> ematicated
Kwashikor -> edema and hepatomegaly
relative protein deficiency caused by trauma and sepsis which require the use of proteins for recovery.
what are endopeptidases and exopeptidases
cleave protein by hydrolyzing peptide bonds within polypeptide chain. Exopeptidases cleave amino acids from either the n or c terminal ends of peptides and proteins
what are the ssecretions that facilitate digestion?
aqueous - varying pH to provide the optimal environment for the enzymes
enzyme precursors- proteolytic enzymes - inactive precursors, activated after secretion into lumen by limited proteolysis
mucus - lubricant
what are the concents of gastric juice
HCl (parietal cells)
Gastrin
Pepsinogen
Functions of gastrin
in response to vagal stimulation
stimulates HCl secretion in parietal cells
pepsinogen secretion stimulation in chief cells
purpose of hcl
decreases pH
denatures dietary protein
proper pH for pepsin
initiates limited proteolysis of pepsinogen to pepsin
activation of pepsinogen
process initiated by H+ and becomes autocatalytic. cleave peptide bond. at pH <2, peptide dissociates to give active form of pepsin. in parietal cell deficiency, dissociation does not occur because HCl is insufficient
specificity of pepsin
broad specificitycleaves to the c terminal side of aromativ and bulky aliphatic amino acid residues to produce large peptide fragments
Hormones of duodenum and functions
secretin binds to pancreatic cells and stimulates release of pancreatic juice which is slightly alkaline (enriched in HCO3-) to neutralize acidic stomach contents.
CCK two sites of action - binds to exocrine cells of hte pancreas and stimulates the relase of pancreatic zymogens into the lumen of hte small intestine. inactive precursor forms of trypsin, chymotrypsin, elastase, carboxypeptidases A and B enter the intestine through the pancreatic cuct CCK-PZ also acts on gallbladder to initiate contraction and release bile into the lumen of intestines
activation of pancreatic zymogens
enteropeptidase - brush border (intestinal cells)
catalyzes conversion of trypsinogen to trypsin, initiating cascade of proteolytic events which result in activation of all the pancreatic zymogens
trypsin catalyzes limited proteolysis of all of the remaining zymogens to produce active forms of chymotrypsin, elastase, CPA and CPB
Specificity of proteases
- each protease has a different specificity and the products of one can be used as substrates for another. Specificities complement one another in such a way that collectively they can disassemble a protein into a mizture that contains about 35% neutral and basic amino acids and 65% oligopeptides
What are the endopeptidases
Trypsin - cleaves to the C-side of basic amino acids (lys, arg)
Chymotrypsin - cleaves to the C-side of aromatic amino acids (phe, tyr, trp)
Elastase - cleaves to the C-side of small aliphatic amino acids (gly, ala, ser)
What are the exopeptidases
CPA - cleaves neutral amino acids from the carboxyl end.
CPB - cleaves basic amino acids from the carboxyl end
Brush border hydrolysis of oligopeptides
The oligopeptides produced by
hydrolysis in the lumen are further hydrolyzed by a family of oligopeptidases that
are localized in the brush border membrane. These enzymes are all glycoproteins
and the carbohydrate moiety ensures that the peptidase is oriented so that the
active site is accessible. The final products of hydrolysis by the brush border
peptidases are a mixture of free amino acids, dipeptides, and tripeptides.
Describe transport of amino acids
A. Luminal Membrane. Several specific transport proteins exist in the brush border
membrane for transporting amino acids into the intestinal cell. (Note: Many of these
transporters are also expressed in the brush border membranes of renal tubules.) These
systems co-transport Na
+
(provides driving force for uptake, similar to intestinal glucose
uptake) and include the following transporter classes:
1. Neutral Amino Acid Transporter
2. Aromatic/Hydrophobic Amino Acid Transporter
3. Imino Acid Transporter
4. Acidic Amino Acid Transporter
5. Basic Amino Acid Transporter
B. Basolateral Membrane. There is a different set of transport proteins in the
basolateral membrane. Most of these are Na
+
independent.
Describe abnormalities in protein digestion and absorption
A. Disorders of Digestion (usually pancreatic or intestinal in origin)
1. Primary: Parietal cell deficiency
Secondary: Long-term use of proton pump inhibitors (PPIs)*
2. Zollinger-Ellison Syndrome (gastrin-secreting tumors in pancreas)
3. Pancreatic Insufficiency (cystic fibrosis; pancreatitis)
4. Congenital enterokinase deficiency
5. Trypsinogen-trypsin deficiency
B. Disorders of Absorption
1. Hartnup’s Disease (Neutral Amino Acid Transporter)
2. Cystinuria (Basic Amino Acid Transporter). This is the most common
disorder in amino acid metabolism. It is characterized by the loss of cystine,
ornithine, arginine, and lysine in the urine. The low solubility of cystine at acidic
pH values also results in kidney stones.
3. Prolinuria (Imino Acid Transporter)
4. Acquired disorders (Crohn’s disease, celiac spru, radiation enteritis,
infection)
Describe the effects of longterm PPI use
*NOTE: Because the parietal cells targeted by PPIs also produce the Intrinsic Factor required for
absorption of dietary Vitamin B12, long-term PPI usage can result in B12 deficiency, which can
result in anemia and peripheral sensory and motor neural deficiencies. This can easily be
treated by B12 injections.
OVERVIEW OF GENERAL PATHWAY for catabolism of amino acids
- removal of the α-amino group.
- nitrogen can be incorporated into other compounds or excreted.
The carbon skeletons that result from the removal of the amino group are major
metabolic intermediates of carbohydrate metabolism and the citric acid cycle.
Types of Reactions for Removal of α -Amino Groups
- Deamination Reactions produce free ammonia
- Transamination Reactions transfer amino groups to a common
acceptor, usually α -ketoglutarate
Fate of Carbon Skeletons in amino acid catabolism
form major metabolic intermediates that can be converted into glucose or ketones. The
carbon skeletons of the 20 common amino acids funnel into seven metabolites.
Some of the amino acids (e.g., leucine, tryptophan, isoleucine, phenylalanine) are
fragmented in such a way that they produce more than one of these seven metabolites.
a. Pyruvate is produced by the degradation of alanine, serine, glycine, cysteine, and threonine. b. Oxaloacetate is produced by the catabolism of aspartate and asparagine. Alanine Serine Cysteine Threonine Pyruvate (3 C) Asparagine Aspa rtate Oxaloacetate
What is the fate of glucogenic amino acids?
Glucogenic Amino Acids have carbon skeletons that are converted to either pyruvate, oxaloacetate, α -ketoglutarate, succinyl-CoA or fumarate. The most important of these amino acids are alanine,
aspartate, and glutamate.
What amino acids produce pyruvate?
alanine, serine, glycine, cystein, and threonine
What amino acids produce oxaloacetate?
catabolism of aspartate and asparagine
asparagine –> aspartate –> OAA
What amino acids produce alpha-ketoglutarate?
glutamine, proline, arginine and histidine converted to glutamate and then transaminated to alpha-ketoglutarate
What amino acids produce succinyl coA?
degradation by methionine, isoleucine, and valine, which form propionyl CoA then methyl malonyl CoA and finally succinyl CoA
What amino acids produce fumerate?
phenylalanine forms tyrosine which forms homogentisate to fumarylacetoacetate which breaks down to acetoacetate and fumerat
What are ketogenic amino acids
amino acids that are degrade to acetyl CoA or acetoacetyl CoA. These products can be used for the synthesis of ketones (and fatty acids)
Leucine and lysine are purely ketogenic
isoleucine, phenylalanine, tyrosine and tryptophane are both ketogenic and glucogenic
Deamination
These reactions generate free ammonia which is toxic
and must be “fixed” or detoxified. There are two general classes of deamination
reactions: nonoxidative and oxidative.
Non-oxidative deamination
Dehydration of Serine and Threonine eliminates ammonia in a
reaction involving the side chain hydroxyl group and the hydrogen
attached to the α -carbon atom (these enzymes require pyridoxalP).
Serine → pyruvate + NH4+ + H2O Threonine → α -ketobutyrate + NH4+
+ H2O
b. Hydrolytic deamination of side chain amide groups on Asparagine
(via asparaginase) and Glutamine (via glutaminase). (These enzymes do
not require pyridoxal phosphate)
Asparagine + H2O → Aspartic Acid + NH4+
Glutamine + H2O → Glutamic Acid + NH4+
c. Direct deamination of Histidine and Glycine
(no requirement for pyridoxal phosphate)
Histidine → Urocanate + NH4+
Glycine → CO2 + NH4+
Oxidative Deamination
Oxidative Deamination of glutamic acid to produce α -ketoglutarate and
ammonia. This reaction requires NAD+
or NADP+ as a coenzyme. The enzyme glutamate dehydrogenase is present in the mitochondrial matrix in very high concentrations. glutamate + NAD+
→ α -ketoglutarate + NADH + H+ + NH4+
Transamination Reaction
These reactions do not directly release free ammonia, but rather transfer the α -amino group from amino acids to some α -keto acid acceptor.
Transaminases require pyridoxal-P as a coenzyme. This is the major pathway
for removal of nitrogen from amino acids. There are transaminases for most of
the amino acids. With the exception of the branched chain amino acids (val, ile,
leu), the catabolism of most dietary amino acids starts in the liver. There is no
branched chain amino acid transaminase in the liver. The concentration of
branched chain amino acids in the blood leaving the liver is as high as it was in
the portal blood bringing dietary amino acids to the liver.
Strategy of Transamination Reactions
Strategy of Transamination Reactions is to transfer the amino nitrogen from a diverse group of donor amino acids to a smaller number of -keto acid acceptors so that there can be a central pathway for disposal. Most transaminases use α -ketoglutarate as the amino acceptor. Transaminases are usually named by the amino acid which is the amino donor. These reactions are reversible, so the direction of the reaction can be altered by changes in concentrations of substrates and products.
Substrate Specificity of Transaminases
Each transaminase is specific for
one or a few amino acid nitrogen donors. Although the nitrogen acceptor for
most transaminases is α -ketoglutarate, oxaloacetate and pyruvate are also two
very important amino group acceptors. Quantitatively, the two most important
transaminases are AST and ALT
ALT
Alanine transaminase (ALT) also known as
glutamate:pyruvate transaminase (GPT, or SGPT when referring to
its presence in the serum). Note that these reactions are reversible.
In skeletal muscle pyruvate is the major acceptor for amino groups
from glutamate, thus producing large quantities of alanine (which
is transported to the liver). In the liver, alanine donates this amino
group back to α -KG to form glutamate.
AST
Aspartate transaminase (AST) also known as
glutamate:oxaloacetate transaminase (GOT or SGOT). This
reaction is especially important in the liver where oxaloacetate acts
as an acceptor for some of the amino groups that have been
funneled into glutamate. The product of the reaction in liver is
aspartate, which is a nitrogen donor for urea synthesis.
ROLE OF PYRIDOXAL PHOSPHATE IN AMINO ACID METABOLISM
A. Vitamin B6 is the Precursor for Pyridoxal Phosphate - Vit B6 exists in a
number of forms, including pyridoxine and pyridoxal. The coenzyme forms of B6
have phosphate esterified to the methoxy side chain.
B. Function. Pyridoxal phosphate acts as a coenzyme for many enzymes that
catalyze reactions involving transformations around the α-carbon atom of amino acids. For example transaminases, decarboxylases, deaminases, racemases and aldolases (dehydratases).
C. Mechanism. Pyridoxal phosphate activates the α-carbon atom and makes
one of the three bonds attached to the α -carbon atom labile.
D. Role of Pyridoxal-P in Transaminase Reactions. It acts as a carrier of
amino groups in a two-step reaction. Pyridoxal-P first accepts the amino group
from a donor amino acid to form pyridoxamine-P. In the second half of the
reaction, the amino group is donated to an α-keto acid acceptor (usually α -
ketoglutarate), thus regenerating pyridoxal-P and forming a new amino acid
(usually glutamate).
Drug-Induced Vitamin B6 Deficiency
Isoniazid is a drug that is used for the treatment of tuberculosis. It reacts
with pyridoxal and thus makes it unavailable for phosphorylation by
pyridoxal kinase. Transaminase activities may be lower than normal in
patients receiving isoniazid.
Isoniazid + Pyridoxal → Inactive compound
- Penicillamine is used to treat Wilson’s disease, a Cu
+2
storage disease.
Penicillamine inactivates pyridoxal and thus can affect transaminase
activity. Such patients require extra pyridoxine to normalize transaminase
levels.
Penicillamine + Pyridoxal → Inactive compound
Conditions Associated with Increased or Decreased Serum Levels of
Transaminases
- Increased serum levels: myocardial infarct (AST); hepatitis (AST, ALT); alcoholic liver damage (AST, ALT); liver cancer (AST, ALT); muscular dystrophy (ALT)
- Decreased serum levels: nutritional pyridoxine deficiency; patients on hemodialysis.
