Question

11. In the Pyruvate Dehydrogenase complex, the acetyl unit is passed to the coA directly from __________________.
    A.   biotin
    B.   TPP
    C.   FAD
    D.   pyruvate
    E.   lipoyllysine

13. A common feature of molecules that signal through steroid and metabolite hormone receptors is:
    A.   they are hydrophobic
    B.   they are polar
    C.   their receptors are mostly found in the cell surface membrane
    D.   their receptors signal through second messengers.
    E.   their receptors signal rapidly

15. Which of the following reactions requires a Vitamin B12 cofactor?
    A.   beta-oxidation of 18:0
    B.   beta-oxidation of (18:1 delta 9)
    C.   beta-oxidation of (16:2 delta 9 delta 12)
    D.   complete oxidation of (19:0)
    E.   all of these require Vitamin B12

Answer 1

11.

Answer:     E. lipoyllysine

Explanation:

In pyruvate dehydrogenase complex, the acetyl group from acetyl-dihydrolipoamide from the lipoamide (lipoyllysine) is transferred to the Coenzyme A.

TPP is used to form the Hydroxyethyl-TPP from pyruvate by the pyruvate dehydrogenase enzyme.

Biotin is not associated with PDH complex. It is involved with pyruvate carboxylase enzyme.

FAD acts as the coenzyme of dihydrolipoyl-transacetylse and is tightly bound to the enzyme.

Pyruvate is the substrate from which the acetyl skeleton is obtained.

2.

Answer: A. they are hydrophobic

Explanation:

The steroid hormones, owing to their sterol nucleus, has high degree of hydrophobicity due to the non-polar skeleton of it.

They are not predominantly polar.

The receptors of steroid hormones are found in the nucleus. Owing to the hydrophobic nature of steroid hormones, they easily pass through the membranes.

The receptors of steroid hormones act directly after binding to the ligand. They do not involve secondary messengers.

The receptors cannot signal rapidly because they mainly alter the rate of gene expression. This requires synthesis of new proteins (eg. transcription factors) and thus are relatively slower than other pathways.

3.

Answer: D. complete oxidation of (19:0)

Explanation:

19:0 is has odd number of fatty acids and thus after 8 rounds of beta-oxidation, a propionyl-CoA is left. It is then carboxylated with a bicarbonate into D-methylmalonyl-CoA with the enzyme propionyl-CoA carboxylase. The D form is converted to L form by methyl-malonyl-CoA epimerase. Now this L form undergoes rearrangement to form succinyl-CoA with the help of a mutase enzyme that requires Vitamin B12 as a coenzyme.

All other options have fatty acids with even number of carbons. Thus, they do not require vitamin B12.

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