Biochemistry : Catabolic Pathways and Metabolism
Study concepts, example questions & explanations for Biochemistry
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Example Questions
Example Question #2 : Other Protein Catabolism Concepts
During the urea cycle, which enzyme catalyzes the rate-limiting step in the mitochondria?
Phosphofructokinase (PFK)
Triose phosphate isomerase
Citrate synthase
Glucose-6-phosphate dehydrogenase
Carbamoyl phosphate synthase
Carbamoyl phosphate synthase
Triose phosphate isomerase catalyzes the isomerization between dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. Glucose-6-phosphate dehydrogenase is a regulatory enzyme for the pentose phosphate pathway. Citrate synthase is a regulatory enzyme for the Krebs cycle, catalyzing the synthesis of citrate from acetyl-CoA and oxaloacetate. PFK catalyzes the rate-limiting step in glycolysis.
Example Question #3 : Other Protein Catabolism Concepts
What is the difference between ketogenic and glucogenic amino acids?
I. Ketogenic amino acids are degraded to acetyl-CoA and ketones; glucogenic amino acids can be converted to glucose
II. Ketogenic amino acids are alanine and glutamine
III. The ketogenic amino acids are leucine and lysine
IV. Amino acids that are glucogenic and ketogenic are: phenylalanine, tyrosine, tryptophan, isoleucine and threonine
I and IV
II and III
III and IV
I, III, and IV
I, II, III, and IV
I, III, and IV
Ketogenic amino acids are degraded to Acetyl Coenzyme A (CoA) and ketones; glucogenic amino acids can be converted to glucose. Amino acids that are both ketogenic and glucogenic can be metabolized to both glucose and ketone bodies. Purely ketogenic aminoacids are leucine and lysine. Amino acids that are glucogenic and ketogenic are: phenylalanine, tyrosine, tryptophan, isoleucine and threonine. All the other amino acids are glucogenic.
Example Question #4 : Other Protein Catabolism Concepts
All of the following are intermediate molecules in the urea cycle except __________.
ornithine
citrulline
arginosuccinate
citrate
arginine
citrate
In the urea cycle, carbomyl phosphate first combines with the molecule ornithine. This forms citrulline. Citrulline then reacts with aspartate to form arginosuccinate. Fumarate dissociates from arginosuccinate forming arginine, and then the addition of water forms urea and ornithine once again to complete the cycle. Citrate is not involved in this cycle, it is however in the Krebs cycle.
Example Question #749 : Biochemistry
Carbomyl phosphate is a molecule that can enter into the urea cycle. It is formed from the coupling of what two molecules?
Free 
Example Question #741 : Biochemistry
What cofactor is required for the oxidation of beta-hydroxyacyl-CoA to beta-Ketoacyl-CoA by hydroxyacyl-CoA dehydrogenase?
NADH
FAD
FADH2
NAD+
CoASH
NAD+
NAD is required for the oxidation of beta-hydroxyacyl-CoA to beta-Ketoacyl-CoA by hydroxyacyl-CoA dehydrogenase.
Example Question #2 : Lipid Catabolism
What enzyme performs the shown step in beta oxidation?
Acyl-CoA oxidase
Acyl-CoA dehydrogenase
CoA dehydroxlase
Thiolase
Acetyl-CoA dehydrogenase
Acyl-CoA dehydrogenase
This first step of beta oxidation is catalyzed by acyl-CoA dehydrogenase. One way to remember is that the enzyme is named for exactly what it does: remove a hydrogen (dehydrogenate) from acyl-CoA, which is the reactant. In order to be acetyl-CoA the R-group must specifically be a methyl group.
Example Question #3 : Lipid Catabolism
What enzyme performs the shown step in beta oxidation?
Acyl-CoA dehydrogenase
Acyl-CoA oxidase
Enoyl-CoA hydratase
Thiolase
Acyl-CoA hydratase
Enoyl-CoA hydratase
This second step of beta oxidation is catalyzed by enoyl-CoA hydratase. One way to remember is that the enzyme is named for exactly what it does: adding water (hydrate) across the double bond of enoyl-CoA, which is the reactant.
Example Question #4 : Lipid Catabolism
What enzyme catalyses the shown step in beta oxidation?
L-hydroxacyl-CoA dehydrogenase
Acyl-CoA dehydrogenase
Enoyl-CoA hydratase
L-hydroxacyl-CoA dehydrogenase
Acyl-CoA dehydroxylase
L-hydroxacyl-CoA dehydrogenase
This third step in beta oxidation is catalyzed by L-hydroxacyl-CoA dehydrogenase. One way to remember is that the enzyme is named for exactly what it does: remove a hydrogen (dehydrogenate) L-hydroxyacyl-CoA (the reactant).
Example Question #5 : Lipid Catabolism
What enzyme catalyses the below step in beta oxidation?
Enoyl-CoA hydratase
L-hydroxacyl-CoA dehydrogenase
Acyl-CoA dehydrogenase
Thiolase
Acetyl-CoA synthase
Thiolase
This fourth step in beta oxidation is catalyzed by thiolase. The reduced form of coenzyme-A is used as a cofactor to cleave the bond between the alpha and the beta carbon.
Example Question #6 : Lipid Catabolism
Which cofactor is required in the conversion of fatty acyl-CoA to trans enoyl-CoA by acyl-CoA dehydrogenase?
Water
Biotin
Pyridoxal
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