# 2 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 glutamine3. Discuss the roles of transamination4. Discuss the glutamate dehydrogenase

(GDH) & regulation5. Discuss ammonia disposal

Amino Acid DegradationAmino 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

.

TransaminationTransamination

Transamination15 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 NEAADegradation most AA Redistribution

Transamination ReactionTransamination Reaction1. There are many transaminases2. Coenzyme is pyridoxal PO4 (PPal) formed from

vitamin B63. All AA except THR &v LYS can undergo

transamination with α ketoglutarate4. 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 UseTransaminases – Clinical Use

Transaminases – Clinical UseTransaminases – Clinical Use1. ASP & ALA transaminases are the most abundant

2. Several are present in both cytosol and mitochondria (isoenzymes)

3. 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

4. Nomenclature of transaminases is confusing: same enzyme =

aspartate – glutamate transaminase

aspartate transaminase

glutamate – oxaloacetate transaminase

SGOT (clinical literature)

Role of TransaminationRole 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 poolMost 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 CouplingGlutamate 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 RequirementsGDH Requirements1. 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 DehydrogenaseGlutamate 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 GDHRegulation 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 DisposalAmmonia DisposalNH4

+ must be disposed of due to toxicitysymptoms = 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/energyNH3 (diffuses across tubular membranes) + H+

NH4+

(urine)

This lowers [H+] and increases H+/Na+ exchangeAccounts for 1/2 to 2/3 of daily acid load

Ammonia Disposal via KidneyAmmonia Disposal via Kidney

Chronic Metabolic AcidosisChronic 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 NHImportance of NH44+ + in acidosisin 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 DietsHigh 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: Case #2 Amino Acid Degradation: Starvation  Case DiscussionStarvation  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 amino-butyrate 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 → glucoseMuscle + 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 alanine-glucose cycle? Why does alanine drop so much?

Pyruvate ALA transport from muscle liver for glucose synthesis. When P breakdown is maxed, then ALA decreases