Lecture 16

• Oxidative degradation of amino acids (AA)


  1. AA released during protein turnover aren't needed for new protein synthesis

  2. Ingested AA exceed the body's needs for protein synthesis

  3. Cellular proteins are used as field because carbohydrates are either unavailable or not properly utilized

  4. NADH is produced in the mitochondria

  
• Free ammonia is TOXIC
• Dietary protein degradation


  1. Enzymatic degradation to amino acids in the gastrointestinal tract

  2. Gastrin = hormone secreted when dietary protein enters the stomach

  3. Pepsinogen = zymogen that is converted to active pepsin by autocatalytic cleavage at low pH

  
• Pepsin action


  Cleaves long polypeptide chains into a mixture of smaller peptides
• Secretin action


  1. Secreted into the blood in response to low pH in the small intestine

  2. Stimulates pancreas to secrete BICARBONATE into the SMALL Intestine

  
• Zymogens secreted by the Pancreas


  • Trypsinogen
  • Chymotrypsinogen
  • Procarboxypeptidase A
  • Procarboxypeptidase B

  
• Cholecystokinin action


  1. Secreted into the blood in response to the arrival of peptides in the first part of the small intestine

  2. Stimulates secretion of several pancreatic zymogens

  3. Trypsinogen = zymogen of trypsin (activates additional trypsinogen, chymotrypsinogen, the procarboxypeptidases, and proelastase)

  4. Enteropeptidase = a proteolytic enzyme secreted by the intestinal cells that converts trypsinogen to trypsin

  5. Chymotrypsinogen is the zymogen of chymotrypsin

  6. Procarboxypeptidases A and B are the zymogens of carboxypeptidases A and B

  
• The intestinal mucosa absorbs amino acids
• Paths for AA catabolism


  • Involves the amino groups
  • Involves the carbon skeletons
• Glutamate releases its amino group as ammonia in the liver


  1. L-glutamate dehydrogenase: Catalyzes the oxidative deamination of glutamate to produce NH4+ and α-ketoglutarate

  2. Uses either NAD+ or NADP+

  3. Present in the mitochondrial matrix

  4. α-ketoglutarate: can either enter the citric acid cycle or be used for glucose synthesis

  5. Positively modulated by ADP (signals low glucose levels)

  6. Negatively modulated by GTP (signals high levels of α-ketoglutarate)

  
• Glutamate transports ammonia in the bloodstream


  1. Glutamine synthetase = catalyzes the combination of free ammonia with glutamate to yield glutamine

  2. Requires ATP

  3. Critical to transport toxic ammonia to the liver

  4. Glutaminase = catalyzes the conversion of glutamine to glutamate and NH4+

  
• Alanine transports ammonia from skeletal muscles to the liver


  1. Skeletal muscles produce pyruvate, lactate, and ammonia

  2. Alanine aminotransferase = interconverts pyruvate and alanine via transamination with glutamate

  
• The Glucose-Alanine Cycle


  1. Pathway by which alanine carries ammonia and the carbon skeleton from pyruvate to the liver

  2. Ammonia is excreted

  3. Pyruvate is used to produce glucose, which is returned to the muscle

  
• Nitrogen Excretion and the Urea Cycle


  1. Pathway by which ammonia deposited in the mitochondria of hepatocytes is converted to urea

  2. Urea enters bloodstream and is excreted into the urine

  
• Urea production from ammonia in 5 enzymatic steps


  1. Carbamoyl phosphate synthetase I: Catalyzes formation of carbamoyl phosphate from NH4+ and CO2 (as HCO3-), Requires 2 ATP, In mitochondrial agents

  2. Formation of Citrulline: Ornithine transcarbamoylase = catalyzes this formation

  3. Formation of Argininosuccinate: Citrulline enters the cytosol, Argininosuccinate synthetase catalyzes the condensation of the amino group of aspartate and the ureido group of citrulline to form argininosuccinate

  4. Formation of Arginine: Argininosuccinase catalyzes the reversible cleavage of argininosuccinate to form arginine and fumarate (which is converted to malate + joins citric acid cycle intermediates)

  5. Formation of Urea: Arginase = catalyzes the cleavage of arginine to form urea and ornithine (ornithine is transported into mitochondria to initiate another round)

  
• Regulation of urea cycle activity


  • At the level of enzyme synthesis for the four urea cycle enzymes or at carbamoyl phosphate synthetaseI
  • By allosteric regulation of carbamoyl phosphate synthetase I

  
• Essential amino acids


  Amino acids that cannot be synthesized by humans and must be obtained in the diet

  
• Catabolic pathways converge to form 6 major products


  • Pyruvate
  • acetyl-CoA
  • α-ketoglutarate
  • succinyl-CoA
  • Fumarate
  • Oxaloacetate

  
• Ketogenic amino acids


  Can yield ketone bodies in the liver
• Glucogenic amino acids


  Can be converted to glucose and glycogen

  
• Tetrahydrofolate (H4 folate)


  Consists of substituted pterin (6-methyl pterin), p-aminobenzoate and glutamate moieties

  
• Folate


  The oxidized form of tetrahydrofolate
• 1. Adenosylmethionine (adoMet)


  Preferred cofactor for biological methyl group transfers
• Methionine adenosyl transferase


  Catalyzes the synthesis of S-adenosylmethionine from ATP and methionine

  
• 1. adenosylhomocysteine


  Formed when the methyl group from S-adenosylmethionine is transferred to an acceptor

  
• Megaloblastic Anemia


  • Observed in vitamin B12 deficiency
  • Decline in the production of mature erythrocytes
  • Appearance of megaloblasts in bone marrow
  • Involved the replacements of erythrocytes with macrocytes
• Phenylketonuria (PKU)


  • A disease caused by a genetic defect in phenylalanine hydroxylase
  • Can lead to elevated levels of phenylalanine in the blood

  
• Gastrin


  Secreted by G cells in the stomach in response to food intake. Stimulates the secretion of hydrochloric acid by parietal cells in the stomach. Also stimulates the motility of the stomach and relaxation of the pyloric sphincter.
• Pyridoxal Phosphate participates in . . . 


  transfer of alpa-Amino groups to alpha-Ketoglutarate
• Aminotransferases (transaminases)


  catalyze removal of the alpha-amino groups;

  1. contain different types that differ in their specificity of the L-amino acid
  2. many are specific for alpha-ketoglutarate as the amino group acceptor
  3. Freely reversible
• Pyridoxamine Phosphate function and structure


  Function: donates amino group to an alpha-keto acid

  Structure: aminated form

  
• Pyridoxal Phosphate (PLP) function and structure

  1. Accepts an amino group
  2. aldehyde form

  
• Urea Cycle Reaction

  2NH4(+) + HCO3(-) + 3АТР4(-) + H2O ->urea + 2ADP3(-)+ 4Pi2(-)+ AMP2(-) + 2H+
• Synthesis of N-Acetylglutamate

  • N-acety|glutamate synthase = catalyzes N-acetylglutamate from (acetyl-CoA +glutamate)
  • If partial activaty CPSI, arginine can positively regulate
  • N-acetylglutamate = allosterically activates carbamoyl phosphate synthetase I
• Essential Amino Acids include

  • Histidine
  • Isoleucine
  • Leucine
  • Lysine
  • Tryptophan
  • Threonine
  • Phenylalanine
  • Valine
  • Methionine
• Nonessential Amino Acids

  • Alanine
  • Asparagine
  • Aspartate
  • Glutamate
  • Serine
• Conditionally Essential Amino Acids include

  • Arginine
  • Cysteine
  • Glutamine
  • Glycine
  • Proline
  • Tyrosine
• Examples of Ketogenic Amino Acids

  • Phenylalanine
  • Tyrosine
  • Isoleucine
  • Leucine
  • Tryptophan
  • Threonine
  • Lysine
• Glucogenic Amino Acid Examples are all AAs except:

  • Lysine
  • Leucine
• One-carbon transfers usually involve 1 of these cofactors:

  1. Biotin (adds Co2)
  2. H4 (transfers intermediate oxidation states)
  3. S-adenosylmethionine (transfers methyl groups)