Practice Questions for Metabolic Biochemsitry

Question 1Needs Marking

Degradation of amino acids yields compounds that are common intermediates in the major metabolic pathways. Explain the distinction between glucogenic and ketogenic amino acids in terms of their metabolic fates.Answer

Selected Answer:Glucogenic amino acids are used in gluconeogenesis and the production of glucose.Ketogenic amino acids are used in the production of ketone bodies.

Correct Answer:The glucogenic amino acids are those that are catabolised to intermediates that can serve as substrates for gluconeogenesis. Thus they ultimately lead to the production of glucose. These are pyruvate and any of the four- or five-carbon intermediates of the citric acid cycle.

Ketogenic amino acids are catabolised to yield acetyl-CoA or acetoacetyl-CoA, the precursors for ketone body formation. They ultimately lead to the production of ketone bodies.

Response Feedback:[None Given]


Name one amino acid whose oxidation proceeds via the intermediate shown:(a) pyruvate(b) oxaloacetate(c) alpha-ketoglutarate(d) succinyl-CoA(e) fumarate.Answer

Selected Answer:(a) alanine(b) aspartate(c) glutamate(d) valine(e) Phenylalanine

Correct Answer:Possible answers are:

(a) alanine, tryptophan, glycine, serine, cysteine(b) aspartate, asparagine(c) glutamate, glutamine, arginine, histidine,proline(d) isoleucine, threonine, methionine, valine(e) phenylalanine, tyrosine.



What is the first "committed" step in the biosynthetic sequence that leads to IMP? How is this step regulated?Answer

Selected Answer:The first committed step is the production of 5-phosphoribosylamine from PRPP. This step is regulated by feedback inhibition.

Correct Answer:The pathway being referred to in this question is the de novo purine biosynthetic pathway.

The first committed step in the pathway to IMP is the formation of 5-phosphoribosylamine from PRPP (see Lehninger, Fig. 22-31, p. 851).

This step is regulated by feedback inhibition; accumulated IMP allosterically inhibits the first enzyme in the pathway, as shown in Lehninger, Fig. 22-33, p. 853.


Question 42 out of 2 points

Transport of fatty acids from the cytoplasm to the mitochondrial matrix requires:Answer

Selected Answer:(B) ATP, carnitine, and coenzyme A.

Correct Answer:(B) ATP, carnitine, and coenzyme A.

Response Feedback:CORRECT!

Fatty acids are activated and then transported across the inner mitochondrial membrane by carnitine.

The activation recation:Fatty acid + Coenzyme A + ATP -->Acyl-CoA (activated fatty acid) + AMP PPi

Then:

Acyl-CoA + Carnitine --> Acyl-carnitine

the acyl-carnitine is transported into the mitochondrial matrix for beta oxidation.


An amino acid that DOES NOT derive its carbon skeleton, at least in part, from oxaloacetate is:Answer

Selected Answer:(E) methionine.

Correct Answer:(C) proline.

Response Feedback:INCORRECT!

Check your textbook and try again.


Oxidation of an 18-carbon fatty acid produces six molecules of propionyl-CoA.Answer

Selected Answer:True

Correct Answer:False


Propionyl CoA is only produced from the degradation of fatty acid chains with an uneven number of carbon atoms.

Check your textbook.


What is the correct order of the following reactions of beta oxidation?

1. oxidation2. thiolysis3. hydration4. oxidationAnswer

Selected Answer:(C) 4, 3, 1, 2

Correct Answer:(C) 4, 3, 1, 2


The first step in the series of four reactions of fatty acid beta oxidation is OXIDATION by FAD.

This is followed by HYDRATION.

The third step is an OXIDATION by NAD+.

The last step in the series of four reactions of beta oxidation is THIOLYSIS (or thiolytic cleavage by CoA).


The conversion of glutamate to an alpha-ketoacid and NH4+ is catalysed by glutamate dehydrogenase.Answer

Selected Answer:True

Correct Answer:True


This is an example of an oxidative deamination.


Which of the following statements concerning the beta oxidation of fatty acids is true?Answer

Selected Answer:(E) None of the above statements is true.

Correct Answer:(C) The free fatty acid must be converted to a thioester before the process of beta oxidation commences.


Consult your textbook and try again.


If the 16-carbon saturated fatty acid palmitate is oxidised completely to carbon dioxide and water (via the beta-oxidation pathway and the citric acid cycle), and all of the energy-conserving products are used to drive ATP synthesis in the mitochondrion, the net yield of ATP per molecule of palmitate is:Answer

Selected Answer:(B) 10.

Correct Answer:(D) 108.


Check your textbook and try again.


Carnitine is:Answer

Selected Answer:(C) essential for intracellular transport of fatty acids.

Correct Answer:(C) essential for intracellular transport of fatty acids.


Carnitine is a carrier of long chain fatty acids into the mitochondrial matrix for beta oxidation.

Activated long chain fatty acids are transported across the inner mitochndrial membrane by conjugating them to carnitine.


InsulinAnswer

Selected Answer:(A) increases the activity of glycogen synthase and increases the rate of glycogen synthesis.

Correct Answer:(A) increases the activity of glycogen synthase and increases the rate of glycogen synthesis.

Answer Feedback:Insulin promotes glycogen synthesis (glycogenesis) and inhibits glycogen breakdown (glycogenolysis).

Response Feedback:CORRECT! You have reviewed the lecture material well!


The coenzyme involved in a transaminase reaction is:Answer

Selected Answer:(B) pyridoxal phosphate (PLP).

Correct Answer:(B) pyridoxal phosphate (PLP).


All transaminases (aminotransferase enzymes) have the same prosthetic group and the same reaction mechanism.

The prosthetic group is PYRIDOXAL PHOSPHATE (PLP), the coenzyme form of pyridoxine or vitamin B6.


Which of the following does NOT provide a carbon skeleton for the synthesis of amino acids?Answer

Selected Answer:(A) Succinate

Correct Answer:(A) Succinate

Answer Feedback:Succinate is NOT one of the 6 precursors for the synthesis of amino acids.

Response Feedback:CORRECT! You have reviewed your material well!


Erythrose 4-phosphate is a precursor of alanine.Answer

Selected Answer:False



This compound is a precursor to Phenylalanine.


The product of purine degradation in humans is:Answer

Selected Answer:(B) uric acid.

Correct Answer:(B) uric acid.


Uric acid is the excreted end-product of purine degradation in primates.


In nucleotide metabolism, all of the following are true EXCEPT:Answer

Selected Answer:(A) The committed step in purine biosynthesis is the transfer of an amino group to PRPP.

Correct Answer:(D) Balanced pools of deoxyribonucleotides are necessary in DNA synthesis, given the complementary base-pairing of nucleotides in double stranded DNA.

Answer Feedback:This is true.

Response Feedback:INCORRECT! Review your lecture notes and have another try.


Nonessential amino acids:Answer

Selected Answer:(D) are synthesised by plants and bacteria, but not by humans.

Correct Answer:(C) can be synthesised in humans as well as in bacteria.

Answer Feedback:This statement describes the essential amino acids.

Response Feedback:INCORRRECT!



The branch points of glycogen are broken down toAnswer

Selected Answer:(C) glucose-1-phosphate

Correct Answer:(A) glucose

Answer Feedback:This is the major product of glycogenolysis. It is derived from the breakdown of the alpha 1-4 glycosidic linkages of glycogen.

Response Feedback:INCORRECT! Review your lecture material and have another try.


Fatty acid biosynthesis uses NADPH exclusively, whereas beta oxidation uses NAD+ exclusively.Answer


Correct Answer:True


Consult your textbook.


Which of the following conversions occurs in one step?1. alanine to pyruvate2. glutamate to alpha-ketoglutarate3. aspartate to oxaloacetate4. histidine to glutamate5. methionine to succinyl CoAAnswer

Selected Answer:(C) 2, 4, and 5

Correct Answer:(A) 1, 2, and 3




The amino acids serine, alanine, and cysteine can be catabolized to yield:Answer

Selected Answer:(E) none of the above.

Correct Answer:(D) pyruvate.

Answer Feedback:Remember that alanine can be converted to pyruvate by transamination.


Check you textbook and try again.


In purine biosynthesis, the first intermediate to have a complete purine ring is:Answer

Selected Answer:inosinate (IMP)

Correct Answer:inosinate (IMP)


The first intermediate to have a complete purine ring is inosinate (IMP). Both andenylate (AMP) and guanylate (GMP) are formed from AMP.


Which of the following nucleotide bases is not found in RNA?Answer

Selected Answer:(A) Thymine

Correct Answer:(A) Thymine

Answer Feedback:This nucleotide is found in DNA only and does not occur in RNA.

Response Feedback:CORRECT! Thymine is found only in DNA. You have reviewed the lecture material well!


Glycogen is broken down primarily to glucose-1-phosphate instead of glucose becauseAnswer

Selected Answer:(B) it allows a more controlled release of glucose

Correct Answer:(B) it allows a more controlled release of glucose

Response Feedback:CORRECT! You have reviwed the lecture material well and have selected the correct answer.


The reaction sequence that leads to fatty acid synthesis includes a recurring sequence of the following 4 reactions:

1. first reduction2. condenstion3. dehydration4. second reduction

What is the order of these reactions?Answer

Selected Answer:(B) 2, 1, 3, 4

Correct Answer:(B) 2, 1, 3, 4


Step 1 is the condensationStep 2 is the first reductionStep 3 is dehydration andStep 4 is the second reduction.

The fatty acid chain is extended by 2 carbons in each cycle of elongation consisting of the above 4 reactions.


(a) Describe the steps in the metabolic pathway in which cells oxidise a four-carbon, straight-chain, saturated fatty acid (butyrate; 4:0) to the fragments that enter the citric acid cycle.

(b) In what way would you change or add to your answer if the starting fatty acid had been five carbons long (also straight-chain and saturated)?Answer

Selected Answer:[None Given]

Correct Answer:(a) Butyrate is first activated:Butyrate + ATP + CoA --> butyryl-CoA + AMP + PPi

Then, the butyryl group is transferred to carnitine and transported into the mitochondrial matrix, where it is reconverted to the butyryl-CoA derivative.

This passes through the four steps of beta-oxidation (see Lehninger, Fig. 17-8a, p. 605) to ultimately produce 2 molecules of acetyl CoA.

(b) A five-carbon chain would undergo activation and one cycle of beta oxidation, producing acetyl-CoA and propionyl-CoA.

The propionyl-CoA would be converted to succinyl-CoA by the reaction sequence in Lehninger Fig. 17-11, p. 609.



Conversion of ornithine to citrulline is a step in the synthesis of:Answer

Selected Answer:(D) urea

Correct Answer:(D) urea


The reaction referred to is:

Ornithine + carbamoyl phosphate --> citrulline

The above reaction occurs in the urea cycle which leads to the synthesis of urea which is then excreted.


The transferase used in glycogen breakdownAnswer

Selected Answer:(D) breaks alpha-1,6 linkages

Correct Answer:(A) shifts 3 of the last 4 glucose residues from a branch onto another branch

Response Feedback:INCORRECT! Review your lecture material and try again.


Which of the following statements concerning fatty acids is correct?Answer

Selected Answer:(D) They are a constituent of sterols.

Correct Answer:(C) Some are precursors of triacylglycerols.

Response Feedback:INCORRECT! Check your textbook and try again.


GlucagonAnswer

Selected Answer:(B) inhibits the formation of glycogen (glycogeneis) and stimulates the breakdown of glycogen (glycogenolysis)

Correct Answer:(B) inhibits the formation of glycogen (glycogeneis) and stimulates the breakdown of glycogen (glycogenolysis)

Response Feedback:CORRECT! You have reviewed your material well!


Enzymes that catalyse the synthesis of long chain fatty acids in vertebrate cells are localised in the mitochondrial matrix.Answer




The enzymes of fatty acid synthesis occur in the cell cytosol. The mitochondrial matrix is the site of fatty acid degradation.

Practice Questions for Metabolic Biochemsitry

Documents

Transcript of Practice Questions for Metabolic Biochemsitry