Lecture 20 & 21

Question 1

Dietary proteins —> amino acids

Answer 1

○ 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.

Question 2

• Dietary protein in the digestive tract:

○ Stomach: ^HCl and v pH

Answer 2

§ 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.

Question 3

• Metabolic Circumstances of Amino Acid Oxidation:

Answer 3

○ 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).

Question 4

• Amino acid catabolism: Process

Answer 4

1. 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
2. Remaining carbon skeleton is metabolized.

Question 5

• Fates of Nitrogen:

Answer 5

• Used to synthesize N-containing compounds or…

- Carried through blood —> liver by glutamine or alanine and enters excretory pathway as ammonia (NH4+). TOXIC!!

Question 6

• Amino group catabolism:

Answer 6

○ Potential paths for NH3 group:

		1. Transamination.
		2. Oxidative deaminatiom 
		3. Transport by glutamine

Question 7

• Transamination (NUMBER 1):

Answer 7

○ Transamination: transfer NH3 from one molecule to another.
§ Requires transaminase (aka aminotransferase)
§ Requires coenzyme PLP (pyridoxal phosphate, Vit. B6)

Question 8

• ALT: alanine aminotransferase AND

* AST: aspartate aminotransferase

Answer 8

▪︎ alanine + a-ketoglutarate pyruvate + glutamate

▪︎ aspartate + a-ketoglutarate oxaloacetate + glutamate

Question 9

• Excess ALT and AST may indicate liver problems, but not necessarily:

Answer 9

○ Viral hepatitis
○ Excess Alcohol
○ Drug Allergies
○ Celiac Disease

Question 10

• Pyridoxine (Vitamin B6):

Answer 10

○ 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.

Question 11

• Pyridoxal Phosphate (PLP):

Answer 11

○ Bound via Schiff base:
§ internal aldimine
○ Nu: attack of amino (Lys) forms a Schiff base

Question 12

• Roles of PLP:

Answer 12

○ 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.

Question 13

• Oxidative Deamination(NUMBER 2):

Answer 13

○ 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.

Question 14

• Transport by Glutamine(NUMBER 3):

Answer 14

○ 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.

Question 15

• Glucose-Alanine Cycle:

Answer 15

○ 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.

Question 16

• Urea Cycle: Overview

Answer 16

○ 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.

Question 17

• Pre-Urea Cycle: Carbamoyl Phosphate

Answer 17

○ 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).

Question 18

• Urea Cycle Regulation:

Answer 18

○ 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.

Question 19

• Urea Cycle Defects:

Answer 19

○ Urea cycle occurs in the liver
§ Malfunctioning liver—> defect in urea cycle
§ Defect in urea cycle —> increased NH4+ in blood —> TOXIC!!

Question 20

• Celiac Disease:

Answer 20

○ Consumption of gluten triggers inflammatory immune response
§ Small intestine billi damaged
§ Adsorption of nutrients compromised.

Question 21

• Tetrahydrofolate (THF):

Answer 21

-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.

Question 22

S-Adenosylmethionine (SAM):

Answer 22

○ 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

Question 23

• Phenylketonuria (PKU):

Answer 23

○ 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

Question 24

• Degradation of Glycine:

Answer 24

○ 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.

Question 25

• Degradation of Branched Amino Acids:

Answer 25

○ 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.

Question 26

• Severe Afflictions:

Answer 26

○ 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.