All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources
Example Question #1 : Help With Enzyme Types
Which of the following types of enzymes is responsible for joining molecules by forming new chemical bonds?
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 #2 : Help With Enzyme Types
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?
Since chymotrypsin uses a water molecule in order to cleave the polymer, it is considered a hydrolase enzyme.
Example Question #3 : 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?
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 #4 : Help With Enzyme Types
Which of the following is not a class of enzymes that alter epigenetic states?
None of these
Pioneer transcription factors
None of these
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 #5 : Help With Enzyme Types
Which of the following are not enzymes that act on DNA?
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 #6 : 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)?
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.