GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Biochemistry

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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Example Questions

Example Question #4 : Enzymes

The class of enzymes that break bonds by forming a new double bond or ring structure (rather than by hydrolysis or oxidation) is best characterized as which of the following?

Possible Answers:

Transferases

Isomerases

Kinases

Lyases

Ligases

Correct answer:

Lyases

Explanation:

The correct answer is lyases. This class of enzymes only requires one substrate for the forward reaction.

Ligases catalyze the formation of a bond between two molecules, isomerases rearrange the atoms of a molecule, kinases phosphorylate molecules, and transferases transfer or join functional groups from one molecule to another. 

Example Question #1 : Enzymes

The third step of glycolysis converts fructose-6-phosphate to fructose-1,6-bisphosphate. What type of enzyme mediates this?

Possible Answers:

An isomerase

A reductase

A phosphatase

A polymerase

A kinase

Correct answer:

A kinase

Explanation:

Kinases are enzyme that catalyze the transfer of a phosphate group from ATP to a substrate molecule. The phosphorylation of fructose-6-phosphate to fructose-1,6-phosphate is mediated by a kinase phosphofructokinase.

Example Question #11 : Enzymes

Which of the following types of enzymes is responsible for joining molecules by forming new chemical bonds?

Possible Answers:

Isomerases

Ligases

Lyases

Transferases

Correct answer:

Ligases

Explanation:

Ligases are enzymes that catalyze the formation of new bonds between molecules. A classic example is DNA ligase, an enzyme that synthesizes phosphodiester bonds in the DNA backbone.

Transferases move small molecules from one molecule to another, sometimes altering the functional groups of a compound. Isomerases convert molecules from one isomer to another. Lyases are enzymes that break bonds through a means other than hydrolysis (typically by formation of a double bond). 

Example Question #21 : Macromolecules And Enzymes

Chymostrypsin cleaves a polypeptide into two smaller subunits by using water in order to make the new amino and carboxyl termini. Based on this mechanism, what type of enzyme is chymostrypsin?

Possible Answers:

Oxidoreductase

Ligase

Lyase

Hydrolase

Correct answer:

Hydrolase

Explanation:

Since chymotrypsin uses a water molecule in order to cleave the polymer, it is considered a hydrolase enzyme.

Example Question #2 : Help With Enzyme Types

During glycolysis, glucose-6-phospate is rearranged in order to form fructose-6 phosphate. The enzyme that accomplishes this does not change the intermediate's chemical formula in any way, but simply alters the shape of the molecule.

Based on this action, what type of enzyme is involved in this step in glycolysis?

Possible Answers:

Lyase

Hydrolase

Isomerase

Oxidoreductase

Correct answer:

Isomerase

Explanation:

Since the enzyme has changed the shape of the molecule without altering its chemical formula, the enzyme has simply made a new isomer of the molecule. This action is accomplished by isomerase enzymes.

Example Question #11 : Enzymes

Which of the following is not a class of enzymes that alter epigenetic states?

Possible Answers:

Pioneer transcription factors

Histone methyltransferases

DNA methyltransferases

None of these

Histone acetyltransferases 

Correct answer:

None of these

Explanation:

All answer choices fit the description. Epigenetics (above the gene) are heritable modifications of chromatin and DNA that affect gene expression. Pioneer transcription factors are able to bind DNA in heterochromatin and recruit enzymes that promote euchromatin formation which allows other transcription factors to bind and effect gene expression. Histone methyltransferases and acetyltransferases methylate and acetylate histones, respectively, to alter gene expression. DNA methyltransferases are also enzymes that confer epigenetic changes to DNA by methylation, which usually represses gene expression. 

Example Question #3 : Help With Enzyme Types

Which of the following are not enzymes that act on DNA?

Possible Answers:

Polymerases 

Topoisomerases 

Methylases 

Ligases 

Acetylases

Correct answer:

Acetylases

Explanation:

The correct answer is acetylases. DNA can be directly methylated by methylases, mended during DNA repair by ligases, uncoiled by topoisomerases, and replicated by polymerases. However, DNA cannot be acetylated. Epigenetic associated-acetylation occurs only on histones to determine the chromatin state of a specific region. 

Example Question #4 : Help With Enzyme Types

What is the name of the class of enzymes that permit a phospholipid in the cellular membrane to move from facing the exoplasm (outside of the cell) to the cytosol (cellular interior)?

Possible Answers:

Phospholipases

Kinases

Flippases

Migratases

Floppases

Correct answer:

Flippases

Explanation:

Flippases use ATP to permit membrane lipids to reorient themselves in the cellular membrane, specifically in the direction from extracellular to intracellular facing. Floppases catalyze the reverse movement: intracellular to extracellular. Migratases are not a class of enzyme. Phospholipases and kinases catalyze other types of reactions and certainly can act on lipids, but not this particular lipid movement. 

Example Question #11 : Enzymes

Which of the following is an example of allosteric regulation of enzymes?

Possible Answers:

Phosphorylation of an amino acid in the active site

The non-covalent binding of cAMP somewhere other than the active site

The non-covalent binding of cAMP to the active site

Phosphorylation of an amino acid somewhere other than the active site

Correct answer:

The non-covalent binding of cAMP somewhere other than the active site

Explanation:

The difference between the binding of cAMP and phosphorylation is that the latter is a covalent modification. Covalent modifications are a different way to regulate proteins, and do not fall under the category of allosteric regulation. Allosteric regulation only occurs outside of the active site, often simply called an allosteric site. The non-covalent binding of cAMP to a region of an enzyme outside of the active site thus qualifies as allosteric regulation.

Example Question #1 : Help With Inhibitors

A researcher has designed a new type of inhibitor that binds at the active site of an enzyme. What type of inhibition does this molecule display?

Possible Answers:

Competitive inhibition

Uncompetitive inhibition

Suicide inhibition

Noncompetitive inhibition

Correct answer:

Competitive inhibition

Explanation:

Because the inhibitor binds at the active site, it is actively competing with the ligand for access to the enzyme. This type of inhibitor displays competitive inhibition. Competitive inhibition can be overcome by adding excessive amounts of substrate. If the amount of substrate greatly out-measures the amount of inhibitor, then the substrate will still bind the enzyme very frequently and allow the reaction to proceed.

Noncompetitive inhibitors bind an enzyme at a spot that is not the active site. Uncompetitive inhibitors bind the enzyme-substrate complex, once the substrate has already entered the active site. Suicide inhibitors "kill" enzymes, typically by making permanent modifications to amino acids in the active site.

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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