Amino Acids:

1

• Amino Acids
• Disposal of Nitrogen

2
Overview

• Amino acids are not stored in the body
• So, Amino Acids must be obtained from
• 1-Diet
• 2-Synthesized de novo
• 3-produced from normal protein degradation
  (turnover)
• Amino Acids in excess of biosynthetic needs of
  the cell
• Not stored
• But rapidly degraded
  
• first step removal of the a-amino group
  a-ketoacid
• (of
  a.a.)
• 
  ammonia
• 
  exc. in urine
  UREA

3
Synthesis
of proteins

other
compoundsAmino Acid
Diet Degrad. of Proteins De novo synthesis

• Degradation
• a-Ketoacid
  Ammonia
• (carbon skeleton) (nitrogen of
  a.a.)
• Glucose Fatty acids other
  exc. In urine UREA
• ketone Bodies compounds
• Glycogen
  exc. in
  urine
• ENERGY

4
(No Transcript)
5
Amino Acid Pool

• 100 grams of a.a.
• Collected from
• Dietary Proteins (by hydrolysis)
• Degradation of Tissue Proteins
• De novo synthesis of a.a.
• Fate of amino acids obtained by tissue protein
  hydrolysis
• 75 of amino acids used to
  synthesize new proteins
• 25 of amino acids metabolized
• 
  precursor for other compounds
• compensated by dietary proteins
  ---- amino acids

6
Protein Turnover

• Protein turnover results from the simultaneous
  synthesis degradation of tissue proteins
• The total amounts of protein in the body is
  constant because the rate of protein synthesis
  is just sufficient to replace the degraded
  protein
• Protein turnover leads to hydrolysis synthesis
  of 300-400 grams of body protein each day

7
Rate of Protein Turnover

• Short-lived proteins minutes hours half-life
• (as many regulatory proteins misfolded
  proteins)
• Long-lived proteins days weeks half-life
• (majority of proteins in the cell)
• Structural proteins months years half-live
• (as collagen)

8
Protein Degradation

• By Two Major Enzyme Systems
• 1- Ubiquitin-proteasome mechanism
• energy-dependent
• mainly for endogenous proteins
• (proteins synthesized within the cell)
• 2- Lysosomes
• non-energy-dependen
• primarily for extracellular proteins as
• - plasma proteins that are taken into cells
  by endocytosis
• - cell surface membrane proteins for
  receptor-mediated endocytosis

9
Ubiquitin-proteasome pathway
10
Mechanism of action of ubiquitin-proteasome
system

• Protein is covalently attached to ubiquitin
  (small globular protein)
• More ubiquitin is added to form polyubiquitin
  chain (protein is tagged with ubiquitin)
• Ubiquitin-tagged protein is recognized by the
  proteasome (proteolytic molecule)
• The proteasome cuts the target protein into
  fragments
• (requires ATP)
• Fragments are cut by non-specific proteases to
  amino acids

11
Chemical Signals for Protein Degradation

• Protein degradation is influenced by some
  structural aspect of the protein
• Examples
• The half-life of a protein is influenced by the
  nature of the N-terminal
• residue
• with serine long-lived proteins (half-life
  is more than 20 hours)
• with aspartate short-lived (half-life is
  about 3 minutes)
• Proteins rich sequence containing PEST
• rapidly degraded (i.e. with short
  half-life)
• PEST sequence proline, glutamate, serine
  threonine

12
Digestion of Dietary Proteins

• Most of the nitrogen in diet is consumed in the
  form of protein
• Dietary protein/day 70 100 grams
• Dietary proteins must be hydrolyzed to amino
  acids by proteolytic
• enzymes
• Proteolytic enzymes are produced by three
  different organs
• stomach
• pancreas
• small intestine

13
(No Transcript)
14
1- Digestion of Proteins By Gastric Secretion

• Gastric secretions contains
• 1- hydrochloric Acid - pH 2 3 (for activation
  pepsinogen)
• - kills
  some bacteria
• -
  denature proteins
• 2- Pepsin secreted as peopsinogen (inactive
  zymogen)
• activated to pepsin by Hcl or
  autocatalytically by pepsin
• product of hydrolysis of
  proteins peptides few free amino acids
• 
  (oligo- poly-)

15
2- Digestion of Proteins by Pancreatic Enzymes

• On entering the small intestine
• Pancreatic Proteases
• Large polypeptides are further cleaved
  to oligopeptides free amino acids
• PROTEASES
• Secreted as inactive zymogen from pancreatic
  cells
• Secretion of zymogens
• is mediated by the secretion of
  cholecystokinin secretin (polypeptide hormones
  of GIT)
• Activation of zymogen
• by enteropeptidase (enterokinase) enzyme
  present on the luminal surface of intestinal
  mucosal cells
• converts trypsinogen to trypsin (by
  removal of hexapeptide from trypsinogen)
• Trypsin converts other trypsinogen
  molecules to trypsin
• Trypsin activates all other pancreatic
  zymogens (chymotrypsinogen, proelastase
• procarboxypeptidases)
• Specificity

16
(No Transcript)
17

• Abnormalities in Protein Digestion
• Deficiency of pancreatic secretion
• occurs due to chronic pancreatitis, cystic
  fibrosis or
• surgical removal of the pancreas
• Digestion absorption of fat protein is
  incomplete
• Abnormal appearance of lipids (steatorrohea)
  undigested protein in feces

18
3- Digestion of oligopeptides by enzymes of the
Small Intestine

• Aminopeptidases
• - on the luminal surface of the intestine
• - is an exopeptidase
• - cleaves the N-terminal residue from
  oligopeptides
• to produce free amino acids smaller
  peptides

19
Transport of Amino acids into cells

• Movement of a.a. to cells is performed by active
  transport (requires ATP)
• Seven different transport systems
• with overlapping specificity for different
  amino acids
• for example cystine, ornithine, argenine
  lysine are transported in kidney tubules
• by one transporter
• In cystinuria Inherited disease , one of the
  most common inherited dis.
• 
  1 7000 individuals
• defective carrier
  system for these 4 amino acids
• appearance of all 4
  amino acids in the urine
• precipitation of
  cystine to form kidney stones (may block urinary
  tract)

20
(No Transcript)
21
Removal of Nitrogen from Amino Acids

• Removing the a-amino group
• Essential for producing energy from any amino
  acid
• An obligatory step for the catabolism of all
  amino acids

22
Amino Acids
23
Amino Acid
24
Amino Acid (deamination)removal of amino
group (nitrogen)

• Amino group carbon
• (nitrogen)
  skeleton
• 
  (a-ketoacid)
• incorporated
• into
• other excreted
  catabolised synthesis
• Compounds
  of other compounds
• (e.g. urea)
• 
  energy
  
• 
  

25
Deamination Pathways

• Amino group (nitrogen) is removed from an
  amino acid by either
• 1- Transamination (BY
  TRANSAMINASES)
• 2- Oxidative Deamination (BYGLUTAMATE
  DEHYDROGENASE)

26
1- Transamination
Funneling of amino groups to glutamate

• a-ketoglutarate accepts the amino group from
  amino acids to become glutamate
• By aminotransferases (transaminases)
• Glutamate
• Oxidat. Deam ammonia urea cycle
• Or
• gives amino group to oxalacetate to
• produce aspartate--- urea cycle
  
• Or
• gives amino group to carbon skeleton to
• produce new amino acid
• All amino acids (with the exception of
• lysine threonine) participate in transamination

27
Substrate Specificity of Aminotransferases

• Each aminotransferase is specific for one or few
  group of donors
• Aminotransferase is named after the specific
  amino group donor (amino acid that donates its
  amino group)
• The 2 most important aminotransferases
• 1- alanine aminotransferase (ALT)
• 2- aspartate aminotranspeptidase (AST)

28
ALanine AminoTransferase (ALT) ASpartate
AminoTransferase (AST)
amino acid Carbon skeleton of
alanine UREA CYCLE
29
Mechanism of Action of Aminotransferase
30
Equlibrium of Transamination Reactions

• Equilibrium constant 1
• Allowing the reaction to function in both
  directions
• after protein-rich meal
• amino acid degradation
• (removal of amino group from amino acid
  a-keto acid)
• supply of amino acids from diet is not adequate
• amino acid biosynthesis
• (addition of amino group to a-keto acid
  amino acid)

31
Diagnostic Value of Plasma Aminotransferases

• Aminotransferases are normally intracellular
  enzymes
• Plasma contains low levels of aminotransferases
• representing release of cellular contents
  during normal cell turnover
• Elevated plasma levels of aminotransferases
• indicate damage to cells rich in these
  enzymes
• (as physical trauma or disease to tissue)
• Plasma AST ALT are of particular diagnostic
  value

32
Diagnostic Value of Plasma Aminotransferases

• 1- liver disease
• Plasma ALT AST are elevated in nearly all
  liver diseases
• but, particularly high in conditions that
  cause cell necrosis as
• viral hepatitis
• toxic injury
• prolonged circulatory collapse
• ALT is more specific for liver disease than
  AST
• AST is more sensitive (as liver contains a
  large amount of AST)
• 2- Nonhepatic disease
• as myocardial infarction
• muscle disorders
• These disorders can be distinguished clinically
  from liver disease

33
2- Oxidative deamination of amino acids By
Glutamate Dehydrogenase

• Oxidative Deamination of glutamate by glutamate
  dehydrogenase results in liberation of the amino
  group as free ammonia
• (i.e. no transfer of amino group)
• primarily in the liver kidney (but can occur in
  other cells of the body)
• Oxi Deamin. Of Glutamate provides
• 1- a-ketoglutarate (can be reused for
  transamination of a.a.)
• 2- free ammonia urea cycle
  urea

34

• GLUTAMATE (from
  transamination)
• Glutamate oxi.
  deamin. (liver kidney (mainly) others)
• Dehydrogenase
• ammonia a-ketoglutarate
• Urea Cycle
• Urea used for tranasmination
• of a.a.

35
OXIDATIVE DEAMINATIONbyGLUTAMATE DEHYDROGENASE
36
Glutamate Dehydrogenase

• Amino group of most amino acids are funneled to
  GLUTAMATE
• (by transamination with a-ketoglutarate)
• GLUTAMATE is the only amino acid that undergoes
  oxidative deamination (by glutamate
  dehydrogenase) to give a-ketoglutarate ammonia
  
• Amino Acids donate their amino group to
• a-ketoglutarate to produce glutamate
  (Transamination)
• Glutamate is oxi deamin. to a-ketoglutarate
  ammonia
• (by Glutamate dehydrogenase)

37

• Amino acid a-ketoglutarate
  ammonia
• TRANS AMINASE
  GLUTA MATE UREA
  
• 
  
  CYCLE
• 
  
  DEHYDROGENASE
• a-keto acid Glutamate
  
• (carbon Skeleton)
• metabolized
• 
  UREA
• Energy other compounds
• (catab.)

38
Removal of Amino Group from an Amino Acid
UREA CYCLE
Amino acid
Carbon skelton of an amino acid
Coenzyme for glutamate dehydrogenase NAD
39
Synthesis of an Amino Acid from its carbon
skeleton(reductive amination)
Amino acid
Carbon skelton of an amino acid
Coenzyme for glutamate dehydrogenase NADPH
40
Combined Actions of Transaminases Glutamate
Dehydrogense reactions
41
Direction of Reactions

• After protein ingestion
• Glutamate level in liver is elevated
• Reactions proceeds in direction of amino
• acid degradation formation of ammonia

42
Allosteric Regulation of GlutamateDehydrogenase

• ATP GTP are allosteric inhibitors of glutamate
  dehydrogenase
• ADP GDP are allosteric activators of glutamate
  dehydrogenase
• Energy in cells
• glutamate degradation by glutamate dehydrogenase
• Energy production
• from the carbon skeleton of amino acids

43
D-amino acid oxidase

• D-amino acids are found in plants cell walls of
  microorganisms
• Not used in synthesis of mammalian proteins
• D-amino acids is available in diet (from plants)
• Metabolized by D-amino acid oxidase (in liver)
  FAD-dependent
• Oxidative Deamination of D-amino
  acids
• a-keto acids
• Energy Reaminated
  ammonia
• 
  L-amino acids UREA

44
Transport of Ammonia to Liver
in most tissues ammonia glutamate

Glutamine
synthase Glutamine By blood
to liver
Glutaminase Glutamate
ammonia
45

• In skeletal muscles
• Transamination of pyruvate to form alanine
• Alanine is transported in blood to liver
• In liver, alanine is converted to pyruvate
• ammonia (by transamination)
• Pyruvate can be converted to glucose
• (by gluconeogenesis)
• Glucose can enter the blood and used by
• sk. Muscles
• (GLUCOSE - ALANINE PATHWAY)