Lecture 20 & 21 Flashcards
Dietary proteins —> amino acids
○ Enzymatic hydrolysis:
○ Pepsin:
§ Cuts proteins into peptides in the stomach.
○ Trypsin and chymotrypsin:
§ Cut proteins and larger peptides into smaller peptides in the small intestine.
○ Aminopeptidase and carboxypeptidases A and B:
§ Degrade peptides into amino acids in the small intestine.
• Dietary protein in the digestive tract:
○ Stomach: ^HCl and v pH
§ Proteins denatured.
○ Denaturation is essential
§ Native proteins are inert to proteases.
○ Pepsin: active at pH 2, inactive at pH 6.5
§ Made by stomach chief cells
○ HCO3- from pancreas brings metabolized material in small intestine pH up to 7.
• Metabolic Circumstances of Amino Acid Oxidation:
○ Leftover amino acids from normal protein turnover can be broken down to:
1. Supply amino acids for energy when carbohydrates are scarce (starvation; diabetes mellitus). 2. Supply an essential amino acid that is lacking from the diet (leads to negative nitrogen balance).
• Amino acid catabolism: Process
- Remove a-NH3 group.
- a-amino collected by a-ketoglutarate to become glutamate.
- Then, deaminate (remove amine) glutamate to get NH3+
□ Amino acid + a-ketoglutarate —> a-keto acid + glutamate - Remaining carbon skeleton is metabolized.
• Fates of Nitrogen:
- Used to synthesize N-containing compounds or…
- Carried through blood —> liver by glutamine or alanine and enters excretory pathway as ammonia (NH4+). TOXIC!!
• Amino group catabolism:
○ Potential paths for NH3 group:
1. Transamination. 2. Oxidative deaminatiom 3. Transport by glutamine
• Transamination (NUMBER 1):
○ Transamination: transfer NH3 from one molecule to another.
§ Requires transaminase (aka aminotransferase)
§ Requires coenzyme PLP (pyridoxal phosphate, Vit. B6)
- ALT: alanine aminotransferase AND
* AST: aspartate aminotransferase
▪︎ alanine + a-ketoglutarate pyruvate + glutamate
▪︎ aspartate + a-ketoglutarate oxaloacetate + glutamate
• Excess ALT and AST may indicate liver problems, but not necessarily:
○ Viral hepatitis
○ Excess Alcohol
○ Drug Allergies
○ Celiac Disease
• Pyridoxine (Vitamin B6):
○ Prosthetic coenzyme:
§ carries amines: transfer –NH3 from donor to acceptor
§ Aldehyde form can react reversibly with amino groups.
§ Aminated form react reversibly with carbonyl groups.
• Pyridoxal Phosphate (PLP):
○ Bound via Schiff base:
§ internal aldimine
○ Nu: attack of amino (Lys) forms a Schiff base
• Roles of PLP:
○ Transamination: § L-amino acid ---> a-keto acid ○ Racemization: § L-amino acid ---> D-amino acid ○ Decarboxylation: § L-amino acid ---> amine ○ All three mechanisms invove a quinoid intermediate.
• Oxidative Deamination(NUMBER 2):
○ Oxidative deamination: remove–NH3 via oxidation.
§ Catalyzed by glutamate dehydrogenase.
○ Occurs within mitochondrial matrix (liver).
○ Can use either NAD+ or NADP+ as electron acceptor.
○ Ammonia is processed into urea for excretion.
• Transport by Glutamine(NUMBER 3):
○ L-glutamine acts as temporary storage of nitrogen.
○ 2 enzymes for glu—> gln—> glu:
§ Glutamine synthetase (tissues)
§ Glutaminase (liver)
§ L-glutamine can donate–NH3 when needed for amino acid biosynthesis.
§ Excess glutamine is processed in intestines, kidneys, and liver.
• Glucose-Alanine Cycle:
○ Vigorous working muscles can operate nearly anaerobically (Glycolysis)
○ Glycolysis yields pyruvate:
§ If not eliminated, lactic acid will build up.
○ Pyruvate can be converted to alanine for transport into liver:
§ Transamination.
○ Two benefits:
§ Eliminate pyruvate/lactate
§ Oxidation of amino acids gets rid of NH3.
• Urea Cycle: Overview
○ Purpose: detoxification and excretion of excess NH3/ NH4+
§ NH4+ comes from amino acid and pyrimidine breakdown.
§ Liver process (mainly cytosol, but begins in mitochondria)
○ Steps:
§ Pre cycle: Carbamoyl phosphate synthesis.
1. Citrulline synthesis.
2. Argininosuccinate synthesis.
3. Argininosuccinate cleavage.
4. Arginine cleavage and urea formation.
• Pre-Urea Cycle: Carbamoyl Phosphate
○ 1st nitrogen-acquiring reaction of urea cycle.
§ Not formally part of the ura cycle, but a pre step.
○ Carbamoyl phosphate synthetase I (CPS I)
○ Essentially, your combining C, N, and P into one molecule.
○ You need energy to add these elements together (2 ATPs worth).
• Urea Cycle Regulation:
○ Expression of urea cycle enzymes increases when needed:
§ High protein diet
§ Starvation
§ When protein is being broken down for energy, or to salvage an essential amino acid missing in the diet.
○ Carbamoyl phosphate synthetase I is activated by N-acetylglutamate.
○ Formed by N-acetylglutamate synthase:
§ When glutamate and acetyl-CoA concentrations are high.
§ Activated by arginine.
• Urea Cycle Defects:
○ Urea cycle occurs in the liver
§ Malfunctioning liver—> defect in urea cycle
§ Defect in urea cycle —> increased NH4+ in blood —> TOXIC!!
• Celiac Disease:
○ Consumption of gluten triggers inflammatory immune response
§ Small intestine billi damaged
§ Adsorption of nutrients compromised.
• Tetrahydrofolate (THF):
-cofactor involved in amino acid catabolism.
formed from folate
§ An essential vitamin (vitamin B9)
○ THF can transfer 1-carbon in different oxidation states:
§ CH3, CH2OH, and CHO.
§ Forms interconverted on THF before use.
○ Used in a wide variety of metabolic reactions
○ Carbon generally comes from serine.
S-Adenosylmethionine (SAM):
○ S-Adenosylmethionine: preferred cofactor for methyl transfer in biological reactions.
§ AdoMet methyl is 1000x more reactive than THF methyl.
○ Synthesized from ATP + methionine
○ Regeneration uses N5-methyl THF
• Phenylketonuria (PKU):
○ Defect in 1st step of Phe degradation
§ Missing or deficient Phe hydroxylase
§ Inability to break down Phe—>Tyr
○ Buildup of Phe and Phenylpyruvate:
§ Phenylpyruvate builds up in tissues, blood, urine
§ Phenylacetate and phenyllactate also in urine
○ Impairs neurological development:
§ Phe can competitively inhibit Tyr and Trp transport into brain.
§ May also inhibit brain glycolysis (block pyruvate kinase)
○ Controlled by limiting dietary intake of Phe
• Degradation of Glycine:
○ Pathway #1: Hydroxylation to serine —> pyruvate
○ Pathway #2: Glycine cleavge enzyme
§ Major pathway in mammals.
□ Releases CO2 and NH3
□ Methylene group (-CH2) transferred to THF.
○ Pathway #3: D-amino oxidase:
§ Minor pathway.
§ Ultimately oxidized to.oxaloacetate.
□ Major component of kidney stones.
• Degradation of Branched Amino Acids:
○ Not in liver:
§ Instead, in muscle, adipose tissue, kidney, brain
○ Branched -chain a-ketoacid dehydrogenase complex (BCAA DC)
§ Same cofactors as the PDC and a-KG DC.
• Severe Afflictions:
○ Kwashiorkor:
§ Have enough caloric intake, but not enough protein.
§ Distended belly due to osmotic imbalance in fluids (edema)
○ Marasmus:
§ Lacking in all nutrients, including proteins.
§ Child weight is below 60% of normal
○ Treatment: slow introduction of nourishment, especially proteins.
