Biochemistry : Other Protein Functions

Study concepts, example questions & explanations for Biochemistry

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Example Question #1 : Other Protein Functions

Hemoglobin is an important component of red blood cells that allows for efficient delivery of oxygen from the lungs to tissues throughout the body. Without hemoglobin, oxygen would not be able to dissolve in the blood well enough to meet the metabolic needs of tissues. Hemoglobin is well-suited to this function because its affinity for oxygen varies depending on its chemical environment. Which of the following situations would cause a rightward shift on the oxygen dissociation curve for hemoglobin?

Possible Answers:

Increased partial pressure of oxygen

Decreased pH

Decreased carbon dioxide levels

Decreased 2,3-bisphosphoglycerate concentration

Hemoglobin's oxygen dissociation curve remains constant in the face of changing environmental parameters, thus it cannot move to the right

Correct answer:

Decreased pH

Explanation:

An oxygen dissociation curve for hemoglobin plots the percent saturation of oxygen on the -axis vs. the partial pressure of oxygen on the -axis. A rightward shift of the curve means that for a given oxygen saturation level, there needs to be a higher partial pressure of oxygen. Thus, a rightward shift is indicative of a decreased affinity of hemoglobin for oxygen.

There are several factors that can influence hemoglobin's affinity for oxygen. One such factor is pH. At lower pH levels, hemoglobin has a more difficult time holding onto oxygen. Physiologically this makes sense, because the blood is likely to be slightly more acidic in regions where tissues are metabolically active, hence they are going to need more oxygen to sustain their metabolism. Likewise, carbon dioxide is also capable of lowering hemoglobin's affinity for oxygen. And again, this makes sense physiologically, because tissues with a high metabolism are going to be generating more carbon dioxide, which serves as a signal to allow hemoglobin to drop off more oxygen for these active tissues. And finally, an additional regulatory factor is a glycolytic intermediate called 2,3-bisphosphoglycerate (2,3-BPG). Binding of this compound to hemoglobin lowers oxygen affinity, thus higher concentrations of 2,3-BPG also cause a rightward shift of the curve.

Also note that oxygen binds hemoglobin in a cooperative fashion. This means that when one molecule of oxygen binds to hemoglobin, the other three oxygen binding sites on hemoglobin gain subsequently increased affinity for oxygen. And when a second oxygen molecule binds, the other binding sites gain more affinity, and so on. Thus, when the partial pressure of oxygen increases, hemoglobin's affinity for oxygen becomes greater. 

Example Question #31 : Protein Structure And Functions

Which of the following statements about ribosomes is false?

Possible Answers:

None of the other answers are false

tRNA has an ACA sequence at its 3’ end

Amino acids are attached to the corresponding tRNA 3’ end

Ribosomes are ribozymes

Parts of tRNA sequences are complementary to ribosomal RNA sequences

Correct answer:

tRNA has an ACA sequence at its 3’ end

Explanation:

Ribozymes are RNA molecules that catalyze specific biochemical reactions, so ribosomes (which catalyze the linking of amino acids) are indeed ribozymes. tRNA is complementary to ribosomal RNA in the sites where the two bind. Aminoacylation produces a tRNA with its 3’ end covalently linked to an amino acid. The sequence at the 3’ end is not ACA, however’ it is CCA.

 

 

Example Question #2 : Other Protein Functions

What term is used to describe enzymes that have different chemical structures but which catalyze the same reactions?

Possible Answers:

Isozymes

Coenzymes

Apoenzymes

Holoenzymes

None of these

Correct answer:

Isozymes

Explanation:

The correct answer choice is isozymes, also called isoenzymes. Even though these enzymes can catalyze the same reaction, they often have differences in their kinetic parameters or in the way they're regulated. Coenzymes are a type of cofactor. They are generally complex organic molecules that are usually derived from vitamins, and they serve the purpose of assisting the enzyme to which they are bound. Examples include pyridoxal phosphate, biotin, coenzyme A, etc. Apoenzymes are enzymes that normally require a cofactor, but are in a state in which they lack that cofactor. Holoenzymes are apoenzymes that have their cofactor bound.

Example Question #33 : Protein Structure And Functions

Which of the following is true about chromoproteins?

Possible Answers:

Hemoglobin has a higher affinity for oxygen than myoglobin

Hemoglobin F has a higher oxygen affinity than hemoglobin A

None of these

Myoglobin is mostly concentrated in the blood

Correct answer:

Hemoglobin F has a higher oxygen affinity than hemoglobin A

Explanation:

Heme normally binds to iron. Myoglobin is mostly concentrated in muscles, and after a muscle injury can be present in blood. Myoglobin has a higher affinity for oxygen than hemoglobin; myoglobin’s oxygen saturation curve is hyperbolic, whereas hemoglobin’s is sigmoidal. Hemoglobin F (fetal hemoglobin) has a higher oxygen affinity than hemoglobin A (adult hemoglobin). This improves the transfer of oxygen from the circulation of the mother to that of the fetus.

Example Question #34 : Protein Structure And Functions

Which of the following is not a characteristic of chymotrypsin?

Possible Answers:

A catalytic triad of serine, histidine, and aspartate

Performs proteolysis

A tetrahedral intermediate

A thiol nucleophile

Correct answer:

A thiol nucleophile

Explanation:

Chymotrypsin is a digestive enzyme that breaks down proteins (proteolysis). It has a catalytic triad of serine, histidine, and aspartate. The hydroxyl group on serine acts as a nucleophile and attacks the carbonyl group on the amino acid, forming a tetrahedral intermediate. Histidine acts as a base, which cleaves the peptide bond. Aspartate acts as an acid, which restores the active site. Since this catalytic triad has a defined nucleophile, base, and acid, we know that there will not be an additional thiol nucleophile. Thiol nucleophiles are found in cysteine proteases.

Example Question #35 : Protein Structure And Functions

Generally speaking, proteins have many important functions in living organisms. Which of the following is not a potential function of proteins?

Possible Answers:

Serves as the primary structural component of cell membranes

Transport substances across cell membranes

Serve to protect an organism from harmful pathogens

Serve as chemical messengers

Speed up chemical reactions

Correct answer:

Serves as the primary structural component of cell membranes

Explanation:

As stated in the question stem, proteins are super important because they perform an enormous variety of functions. Proteins can sometimes serve a structural role, such as the protein collagen which helps give some connective tissues their unique properties. They can also act as chemical messengers, as some protein hormones do such as growth hormone. Some proteins, such as antibodies, can even protect against infection by neutralizing invading pathogens such as bacteria. In addition, proteins can also exist as enzymes, which act to greatly increase the rate of specific reactions. Other proteins can act as carriers or transporters that help to facilitate the movement of some chemicals across an otherwise impermeable cell membrane. While it's true that proteins play a very important structural and functional role in the cell membrane, they are not the primary structural component. Rather, phospholipids serve as the most abundant component of cell membranes and helps to give them their unique role as "cellular barriers."

Example Question #36 : Protein Structure And Functions

Which of these describes the protein myoglobin?

Possible Answers:

Myoglobin does not contain a heme group.

It is the primary oxygen-carrying protein of muscle tissues.

Myoglobin's highest level of protein structure is quaternary.

Myoglobin can bind four oxygen atoms.

Correct answer:

It is the primary oxygen-carrying protein of muscle tissues.

Explanation:

Myoglobin and hemoglobin are not the same molecule; their functions are similar but different in several ways. Myoglobin is an oxygen-binding protein of muscle tissues. In contrast, hemoglobin is the oxygen-transport protein found in blood. Hemoglobin can bind four oxygen atoms, while myoglobin can only bind one. Both hemoglobin and myoglobin contain heme groups, with hemoglobin containing four and myoglobin containing one. These iron-containing groups are responsible for binding the oxygen atom(s). 

Example Question #37 : Protein Structure And Functions

Which of these is not a function of membrane proteins?

Possible Answers:

Transport of substances into and out of the cell

Catalysis

Acting as a receptor

Energy storage

Correct answer:

Energy storage

Explanation:

Membrane proteins have several functions. They can act as catalysts, receptor proteins for different molecules attempting to enter/exit the cell, and also function in transport as channels or transporters. However, energy storage is a function that is carried out by carbohydrates and lipids.

Example Question #38 : Protein Structure And Functions

Which of the following correctly describes an allosteric enzyme?

Possible Answers:

It is versatile, in that it can act as either an anabolic enzyme or a catabolic enzyme

It operates most optimally in an acidic environment

It operates most optimally in a basic environment

It is regulated by binding to small molecules other than its substrate

It operates most optimally in a neutral environment

Correct answer:

It is regulated by binding to small molecules other than its substrate

Explanation:

Allosteric enzymes are enzymes that can be regulated by binding of a small molecule to an "allosteric site" on the enzyme. This is a location on the enzyme that is distinct from the active site, which is where the enzyme binds to its substrate.

Allosteric compounds can be either activators or repressors. Activators cause the enzyme's activity to increase, whereas repressors cause the enzyme's activity to decrease. This happens because binding of an allosteric molecule acts to change the enzyme's structural conformation, even if just slightly, which thus makes it either more active or less active.

For example, phosphofructokinase is an enzyme found in glycolysis, the metabolic pathway that breaks glucose down in cells for energy. This enzyme is a critical component of the pathway, because it is largely responsible for regulating the flux of glucose through this pathway. As such, there are many cellular metabolites that act to either increase or decrease this enzyme's activity. As an example, molecules like ATP and other intermediates of glycolysis and, subsequently, the citric acid cycle are able to decrease this enzyme's activity. Some of these metabolites include phosphoenolpyruvate and citric acid, in addition to ATP, all of which indicate that the cell has an abundant amount of energy available. AMP, on the other hand, acts to allosterically activate phosphofructokinase because it signals that the cell is low in energy. Thus, allosteric enzymes are an important component of how biochemical processes can be regulated.

Example Question #3 : Other Protein Functions

Where does steroid synthesis and detoxification of drugs and poisons occur in a cell? 

Possible Answers:

Nucleolus

Rough endoplasmic reticulum (RER)

Golgi apparatus

Smooth endoplasmic reticulum (SER)

Correct answer:

Smooth endoplasmic reticulum (SER)

Explanation:

Liver hepatocytes and steroid hormone producing cells of the adrenal cortex are rich in the SER. RER is the site of synthesis of secretory (exported) proteins. The golgi apparatus does many things, but in general, think of it as the distribution center and vesicular trafficking. It organizes and directs where everything should go. The nucleolus is unrelated to this topic, but it does produce ribosomes. 

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