Lecture 74

Question 1

How many amino acids to humans use? How many can we synthesize? How many are essential?

Answer 1

20 total
11 synthesizable
9 essential
*2 that we synthesize can only be made from essential AA precursors, so we can only make 9 starting from scratch

Question 2

Which amino acids can be made from transamination reactions?

Answer 2

Alanine, aspartate, glutamate.

alanine pyruvate
aspartate oxaloacetate
glutamate a keto glutarate

Question 3

which AAs are involved in one-carbon metabolism?

Answer 3

glycine, serine and methionine

Question 4

What is the other name for B12?

Answer 4

Cobalamin

Question 5

Describe the structure of cobalamin.

Answer 5

Cobalamin contains a cobalt ion complexed in a ring structure (a corrin ring).

Question 6

Which reactions use cobalamin as a cofactor?

Answer 6

• synthesis of succinyl-CoA (replenishes succinyl-CoA for the TCA cycle)
• methionine synthase (catalyzes conversion of homocysteine into methionine)

Question 7

Draw the cycle of methylation.

Answer 7

Well..?

Question 8

Name 5 reactions in which SAM functions as a methyl donor.

Answer 8

Norepinephrine --> Epinephrine
Guanidinoacetate --> Creatine
Nucleotides --> Methylated Nucleotides
Phosphatidylethanolamine --> Phosphatdylcholine
Acetylserotonin --> Melatonin

Question 9

Describe the Absorption of Cobalamin

Answer 9

1. Low pH releases B12 from other proteins.
2. R proteins (salivary and gastric glands) sequester B12
3. Intrinsic Factor is released (parietal cells) *Highly Glycosylated
4. In the lumen, pancreatic proteases digest R proteins
5. Intrinsic factor binds to B12 and is taken up by a specific receptor in the ileum (Intrinsic factor not absorbed)

Question 10

How is B12 (Cobalamin) transported in the blood?

Answer 10

Transcobalamin

Question 11

What happens to circulating Cobalamin (B12)

Answer 11

half is taken up by the liver
half is taken up by peripheral tissues
via specific transcobalamin receptor protein

Question 12

What is the most common form of B12 deficiency?

Answer 12

Pernicious anemia (caused by failure of parietal cells to secrete Intrinsic Factor or loss of parietal cells)

Question 13

What are the two components of pernicious anemia?

Answer 13

Megaloblastic Anemia (defect in DNA synthesis disrupts hematopoietic precursor cells)

Neurologic Abnormalities (due to demyelination of nerves. Thought to be due to the accumulation of methylmalonyl-CoA)

Question 14

What enzyme is involved in transferring hydrogens to folate? What electron carrier is used?

Answer 14

dihydrofolate reductase (2X)

NADPH–>NADP+

Question 15

What form of THF is used to form purines?

Answer 15

N10-formyl-THF

Question 16

What form of THF is used to form TMP?

Answer 16

N5, N10-methylene-THF

Question 17

What form of THF is used to form methionine?

Answer 17

N5-methyl-THF

Question 18

What happens when serine reacts with tetrahydrofolate?

Answer 18

serine + tetrahydrofolate glycin + N5, N10 methylene Tetrahydrofolate

Question 19

Describ the characteristics of megaloblastic anemia

Answer 19

• Large, abnormally nucleated erythrocytes that accumulate in bone marrow
• Decreased number of WBCs and platelets that are abnormally large

Question 20

What is the methyl trap hypothesis?

Answer 20

In the absence of vitamin B12, folate gets trapped in its fully reduced 5-methyl tetrahydrofolate form. Consequently, insufficient THF is available to support DNA synthesis. Thus, vitamin B12 deficiency creates a metabolic folate deficiency!