Amino acid metabolism

1
Metabolism of Amino Acids

• M.Prasad Naidu
• MSc Medical Biochemistry, Ph.D,.

2
introduction

• Proteins ? most abundant org.compound
• Major part of the body dry wt (10-12Kg)
• Perform wide variety of functions. Viz
• 1. Static functions ( Structural functions)
• 2. Dynamic functions( Enzy, hor, receptors)
• Half of the body protein is (Collagen) is present
  in supportive tissue (skeletan connective)
  while the other half is intracellur.

3
introduction

• Proteins are the N- containing macro molecules
• Consists of L- AAs as repeating units
• Of the 20 AAs half can be synthesized
• Essential and non-essential AAs
• Proteins on degradation release AAs
• Each AA undergoes its own metabolism
• Proteins metabolism is more appropriately learnt
  as metabolism of amino acids.

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Amino Acid Pool

• An adult has about 100 gm of Free AA which
  represent the AA pool of the body.
• Glutamate and Glutamine together constitute about
  50 and EAA 10 of the body pool.
• The conc of intracellular AA is always higher
  than the Extracellular AA
• AAs enter the cells againt Active transport
• The AA pool is maintained by the sources that
  contribute ( input) and the metabolic pathways
  that utilize (out put) the amino acids.

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Sources of AA Pool

• 1. Turnover of body protein
• 2. intake of dietary protein
• 3. synthesis of non- EAAs

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Protein turnover

• The protein present in the body is in a dynamic
  state.
• About 300-400 gm of protein per day is constantly
  degraded and synthesized which represent the body
  protein turnover.
• There is wide variation the turnover of
  individual proteins.
• Eg plasma proteins digestive enzymes are
  rapidly degraded ( half life is hrs/days)
• Structural proteins have long half lives often
  months and years.

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Control of protein turnover

• many factors
• 1. Ubiquitin small PP 8,500 tags with the
  proteins and facilitates degradation.
• 2. PEST Sequences - Certain proteins with Pro,
  Gln, Ser, Thr sequence are rapidly degraded.

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Dietary Protein

• Regular loss of protein due to degradation of
  AAs.
• About 30-50 gm protein is lost every day from the
  body.
• This amount must be supplied daily in the diet to
  maintain N Balance.
• There is no storage form of AAs unlike the
  Carbohydrates and lipids (TG)
• The excess AAs metabolised oxidized Energy
  or glucose or fat.
• The daily protein intake by adults is 40-100gm

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Synthesis of AAs

• 10 out of 20 naturally occurring AAs can be
  synthesized by the body which contributes to AA
  pool.

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Utilization of AAs from body pool

• 1. most of the body proteins (300-400g/D)
  degraded are synthesized from the AA pool. (
  enzymes, hormones, immuno proteins, contractile
  proteins)
• Many imp N compounds ( porphyrins, purines
  pyrimidines) are produced from AA . About 30g of
  protein is daily utilized for this purpose.
• Generally, about 10-15 of body energy
  requirements are met from the AAs
• The AAs are converted to Car, fats. This becomes
  predominant when the protein consumption is in
  excess of the body requirements.

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Metabolism of amino acidsgeneral aspects

• AAs undergo common reactions
• Transamination followed by
• Deamination for the liberation of NH3
• The NH2 group of AAs is utilized for the
  formation of urea (excretory end product of
  protein metabolism)
• The C-skeleton of the AAs is first converted to
  keto acids (by transamination) which meet one or
  more of the following fates

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Fate of keto acids

• Utilized to generate energy
• Used for the synthesis of glucose
• Derived for the formation of fat / ketone bodies
• Involved in the production of non-EAAs

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Transamination

• Transfer of an amino group from an AA to a keto
  acid
• This process involves the interconversion of a
  pair of AAs and a pair of keto acids
• Transaminases / aminotransferases

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Salient features of Transamination

• All transaminases require PALP
• Specific transaminases exist for each pair of
  amino and keto acids
• However, only two namely Asp. transaminase Ala.
  transaminase make a significant contribution for
  transamination
• There is no free NH3 liberated, only the transfer
  of NH3 group occurs
• Reversible
• Production of non-EAAs as per the requirement of
  the cell
• Diverts the excess of AAs towards Energy
  generation

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Salient features of Transamination

• AAs undergo TAN to finally concentrate N in
  glutamate
• Glutamate is the only AA that undergoes OD to
  liberate free NH3 for urea synthesis
• All AAs except Lys, Thr, Pro Hy.pro participate
  in TAN
• TAN is not restricted to a-group only. (eg
  d-amino group of Ornithine is transaminated.
• Serum transaminases are important for diagnostic
  and prognostic purposes
• SGPT or ALT is elevated in all liver diseases
• SGOT or AST is increased in myocardial infarction

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Mechanism of Transamination

• Occurs in 2 stages.
• 1. Transfer of the NH2 group to the coenzyme PLP
  ( bound to the coenzyme) to form Pyridoxamine
  Phosphate.
• 2. The NH2 group of Pyridoxamine PO4 is then
  transferred to a keto acid to produce a new AA
  and the enzyme with PLP is regenerated.

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Mechanism of Transamination

• All the transaminases require PLP , a derivative
  of Vit B6
• The CHO group of PLP is linked with ?-NH2 group
  of Lys, at the active site of the enzyme forming
  a Schiffs base (imine linkage)
• When an AA comes in contact with the enzyme, it
  displaces lys and a new Schiff base linkage is
  formed.
• The AA-PLP-Schiff base tightly binds with the
  enzyme by non covalent forces.
• Snell Braustein proposed Ping-Pong Bi Bi
  mechanism involving a series of intermediates (
  aldimines ketimines) in transamination reaction.

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Deamination

• The removal of amino group from the AAs as NH3
• Transamination involves only shuffling of NH3
  groups among the AAs
• Deamination results in the liberation of NH3 for
  urea synthesis
• Simultaneously, the C-skeleton of AAs is
  converted to keto acids
• 2 types (Oxidative Non oxidative)
• Transamination Deamination occurs
  simultaneously, often involving glutamate as the
  central molecule (Transdeamination)

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Oxidative deamination

• Liberation of free NH3 from the AAs coupled with
  oxidation
• Liver kidney
• Purpose of OD to provide NH3 for urea synthesis
  a-ketoacids for a variety of reactions,
  including Energy generation

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Role of GDH

• In the process of Transamination, the NH3 groups
  of most of the AAs are transferred to a-KG to
  produce glutamate
• Thus , glutamate serves as a collection centre
  for amino groups in the biological system
• Glutamate rapidly undergoes oxi.deamination by
  GDH to liberate NH3
• GDH is unique in that it can use utilize either
  NAD or NADP
• Conversion of glutamate to a-KG occurs through
  the formation of a-iminoglutarate
• GDH catalyzed reaction is imp as it reversibly
  links up glutamate metabolism with TCA cycle
  through a-KG
• GDH is involved in both catabolic anabolic
  reactions.

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Regulation of GDH activity

• Zn containing mitochondrial enzyme
• Complex enzyme containing 6 identical units with
  a mol.wt of 56000 each.
• GDH is controlled by allosteric regulation
• GTP , ATP, steroid Thyroid hormones are
  inhibitors of GDH
• GDP and ADP are activators
• After ingestion of protein meal, liver glutamate
  level is ?.
• It is converted to a-KG with liberation of NH3
• Further , when cellular E levels are ?low, the
  degradation of glutamate is ? to provide a-KG
  which enters TCA cycle to liberate Energy

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Oxidative deamination by AAoxidases

• L- AAoxidase D-AAoxidase are flavo proteins,
  possessing FMN and FAD respectively.
• They act on corresponding AAs to produce
  a-Ketoacids NH3
• In this reaction, O2 is reduced to H2O2, which is
  later decomposed by catalase
• The activity of L-AAoxidase is much low while
  D-AAoxidase is high in tissues (liver kidneys)
• L-AAoxidase does nt act on Gly
  dicarboxylicacids

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Fate of D-aminoacids

• D-AAs are found in plants mos
• Absent in mammalian proteins
• But D-AAs are regularly taken in diet and are
  metabolized
• D-AAoxidase converts them into a-ketoacids by od.
• The a-ketoacids so produced undergo TAN to be
  converted to L-AAs
• Ketoacids may be oxidized to generate energy or
  serve as precursor for glucose fat synthesis
• Thus D-AAoxidase is imp as it initiates the first
  step for the conversion of unnatural D-AAs to
  L-AAs in the body.

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Non oxidative deamination

• Some of the AAs can be deaminated to liberate NH3
  without undergoing oxidation
• A) Aminoacid dehydrases
• Ser,Thr,Homoserine? a-ketoacids
• Catalyzed by PLP dependent dehydrases
  (dehydratases)
• B)Aminoacid desulfhydrases
• Cys, homocysteine ? pyruvate
• Deamination coupled with desulfhydration
• C) Deamination of histidine
• Histidine ? urocanate
• histidase