Transcript Degradation of Amino Acids

# 2 Degradation of Amino Acids
1. Discuss in general terms the use of
amino acids for the synthesis of
nitrogen-containing compounds
2. Discuss the various functions of
glutamine
3. Discuss the roles of transamination
4. Discuss the glutamate dehydrogenase
(GDH) & regulation
5. Discuss ammonia disposal
Amino Acid Degradation
Degradation : → removal & disposal of amino group
→ utilization of the carbon skeleton for
energy and gluconeogenesis
ALA & GLN → non-toxic vehicles for transport of NH4+
from the periphery to the liver for AA catabolism
Most nitrogenous waste is disposed of as → urea. N is
also disposed of as NH4+, uric acid & creatinine
.
Transamination
Transamination
15 N AA  free exchange
among AA (except THR & LYS)
(not true of the carbon portion)
Enzyme = transaminase or
aminotransferase
Quantitatively most important
reaction of AA metabolism
Involved in:
Synthesis NEAA
Degradation most AA
Redistribution
Transamination Reaction
1. There are many transaminases
2. Coenzyme is pyridoxal PO4 (PPal) formed
from vitamin B6
3. All AA except THR &v LYS can undergo
transamination with α ketoglutarate
4. Equilibrium of reaction is close to 1
therefore reaction direction depends on the
[reactants] which are directed by other
cellular processes
5. Directionality  removal/addition of
products of AA pool
6. Urea Synthesis provides direction by
withdrawing amino groups from the AA pool
 increase deamination and AA catabolism
Transaminases – Clinical Use
Transaminases – Clinical Use
1.
2.
3.
4.
ASP & ALA transaminases are the most abundant
Several are present in both cytosol and mitochondria
(isoenzymes)
ASP aminotransferase is one of the most frequently
assayed enzymes in the clinical laboratory. Its
determination in serum  diagnostic acid especially for
assessing liver disorders
Nomenclature of transaminases is confusing: same enzyme =
aspartate – glutamate transaminase
aspartate transaminase
glutamate – oxaloacetate transaminase
SGOT (clinical literature)
Role of Transamination
(i) Redistridution of  amino groups to balance AA pool
-- dietary proteins provide a mixture of AA whose
proportions differ from AA pool required by body
 correct imbalance
(ii) AA synthesis / degradation performed in conjunction with
glutamate dehydrogenase (GDH)
GDH can remove or add amino groups to the AA pool
Most  amino groups  glutamate due to the action of
transaminases.
When there is a surplus of AAs in the pool, the  amino
groups can be funneled through glutamate and released
as NH4+
Glutamate Dehydrogenase Coupling
The release of  amino groups as NH4+ is catalysed by glutamate
dehydrogenase through oxidative deamination. Since the
reaction is reversible it can also synthesize amino groups.
GDH Requirements
1. The enzyme is the principal source of NH4+ in the body. GDH is a
mitochondrial enzyme located in matrix, present in liver cells and
most tissues.
2. Important for three reasons:
(i) Link between TCA cycle & metabolism of AA ( keto acids are TCA
cycle intermediates).
(ii) In mammals, only reaction in which an inorganic molecule (NH4+)
can be fixed to a C skeleton. Therefore essential AA could be
provided in the diet as  keto acids and the amino groups as NH4+
because NH4+  glutamate  other AA by transamination.
(iii) GDH is the major AA oxidative pathway and the major source of
NH4+
Also provides directionality to transamination/GDH. In vivo,  [GLU] ,
NAD+ & removal of NH4+ drive deamination of glutamate. With
excess NH4+ (bacterial metabolism in intestine), glutamate can be
formed.
Glutamate Dehydrogenase
1. Driving Force: necessity to maintain low levels of ammonia
which is toxic. Therefore Transaminase + GDH mediates
α amine  NH3  urea
2. Glutamate: link between transamination and Urea
synthesis
Transamination  funnels amino groups through
glutamate & a single dehydogenase suffices therefore
activity of GDH is key
Regulation of GDH
Regulation of GDH: allosteric control through diverse substances.
Major:
(i) energy  is there enough? If not oxidize AA
(ii) AA load  surplus? Therefore degrade (even when energy is
high)
Energy:
a) GTP (&  ATP) inhibit GDH. When GTP (TCA cycle) & ATP
(glycolysis / oxid. phosphorylation) are , energy index cell 
therefore GDH 
b) Conversely ADP and GDP , energy  therefore GDH active in
order to produce Keto acids  TCA cycle to produce ATP/GTP
c)  NAD H inhibits GDH
AA LOAD:
Excess AA: override inhibition caused with  energy therefore AA
themselves can  GDH activity.
Ammonia Disposal
NH4+ must be disposed of due to toxicity
symptoms = feeding intolerance, vomiting, lethargy.
Irritability, respiratory distress, seizures and coma
Most is disposed of through urea (to be discussed)
Smaller amounts can be disposed of through the kidney via
GLN
GLN + H2O  GLU + NH3 + H+ (glutaminase)
GLU  glucose/energy
NH3 (diffuses across tubular membranes) + H+  NH4+
(urine)
This lowers [H+] and increases H+/Na+ exchange
Accounts for 1/2 to 2/3 of daily acid load
Ammonia Disposal via Kidney
Chronic Metabolic Acidosis
Activation of NH4+ excretion system takes
2-3 days and is maximal at 5-6 days.
 glutaminase, GDH, mitochondrial
glutamine transport
 NH4+ urinary excretion
 renal gluconeogenesis from AAs
 urea synthesis by liver (therefore more
GLN available to kidney)
Importance of NH4+ in acidosis
(i) pKa of NH4 = 9.3
(therefore does not lower pH of urine)
pH must be  4.4 since Na+ / H+ exchange cannot
function if the difference is > 1000 fold (pH 7.4/4.4 =
log3 = 1000)
(ii) large amounts of acids can be excreted as NH4+ (NH3
readily available from AA)
(iii) spares body stores Na+ and K + which are excreted
with titratable acids (H2PO4) to balance +/- (maintain
electrical neutrality). Na+ used first, then K+ therefore
NH4+ availability important, since it can fulfill this
function.
High Protein Diets
• PRO
• Reduce fat intake
• Less calorie dense
than fat, may
reduce total caloric
intake
• Important for
athletes who have 
P breakdown
(exercise)
• But  P diet
= animal P
=  fat as well
• ANTI
• Excess over caloric
needs will be
converted to
glucose/glycogen
• AA metabolized, fat
stores untouched
• Increased liver &
renal stress due to 
NH4+ disposal
Case #2 Amino Acid Degradation:
Starvation Case Discussion
An obese patient volunteered to go on a starvation diet
as part of a study of amino acid metabolism. Blood samples
were taken and analyzed for plasma amino acids for as long as
5 to 6 weeks following the fast. Blood ketone bodies increased
at the end of the first week.
Valine, leucine, isoleucine, methionine, and α aminobutyrate concentrations were transiently increased during the
first week, but dropped below initial levels later. Glycine,
threonine, and serine levels decreased more slowly.
13 other amino acids eventually decreased. The
decrease was largest for alanine, which dropped 70% in the
first week. Total plasma amino nitrogen concentration
decreased only 12%.
1. What changes in carbohydrate, lipid & protein
metabolism occur at the beginning of a fast?
Body’s major energy source: fat & glucose (not protein)
Beginning fast: muscle → switches FA oxidation
brain → needs glucose
liver → breakdown glycogen → glucose
blood → maintain glucose
Long term fast: brain → ketone bodies use
liver → ketone bodies from FA
In between… liver must maintain glucose therefore uses
AA → glucose
Muscle + Liver → Protein breakdown → AA
2. Explain the ketosis and acidosis observed in
starvation.
KETOSIS: FA oxidation  acetyl CoA  ketone
(Brain)
ACIDOSIS: ketones  pH blood, normally buffered
with bicarbonate which , when maxed
hyperventilation then  CO2.
3. What might cause an increase in plasma
branched-chain amino acids after 5 days of
starvation?
BCAA , from MUSCLE & LIVER protein breakdown,
associated with starvation, diabetes (imbalances)
4. Why do the other amino acids eventually
decrease?
Because of depletion of protein « reserves », below this
level, use would compromise cellular function
5. Why does total plasma amino nitrogen only
decrase by 12%?
As catabolism (use) of AA → glucose ↑, protein
breakdown ↑ to compensate, maintaining AA constant
6. Is the decreased plasma alanine concentration
related to gluconeogenesis? What is the alanineglucose cycle? Why does alanine drop so much?
Pyruvate  ALA transport from muscle  liver for
glucose synthesis. When P breakdown is maxed, then
ALA decreases