Non-Enzymatic Protein Function

Question 1

True or false:

All proteins are enzymes.

Answer 1

False

A wide variety of non-enzymatic proteins exist! You can generally distinguish enzymes from non-enzymatic proteins because enzymes often end in “-ase.”

Question 2

Name at least three types of non-enzymatic proteins.

Answer 2

1. binding proteins
2. motor proteins
3. immunoglobulins

While these are the only non-enzymatic proteins listed explicitly on the MCAT outline, others exist, such as cytoskeletal proteins and cell adhesion proteins (which may also be tested in their own contexts).

Question 3

Which category of non-enzymatic proteins ensures that cellular cargo (such as nutrients and proteins) do not rely on diffusion to travel within the cell?

Answer 3

motor proteins

Without motor proteins, cellular cargo would travel through the cell very slowly and would be highly unlikely to reach its proper destination (such as a specific organelle or the cell membrane).

Question 4

Which category of non-enzymatic proteins are produced by B lymphocytes?

Answer 4

Immunoglobulins

Immunoglobulins, or antibodies, are immune system proteins secreted by B lymphocytes.

Question 5

Kinesins and dyneins both fall under the larger category of:

Answer 5

motor proteins

Motor proteins are proteins that help the cell move or that help move certain components (proteins, waste, nutrients, etc.) within the cell.

Question 6

Motor proteins power their movement by hydrolyzing which high-energy molecule?

Answer 6

ATP

Specifically, motor proteins have ATPase functionality, which means they are capable of enzymatically cleaving ATP to release energy.

Question 7

What is the difference between anterograde and retrograde transport?

Answer 7

• Anterograde transport is movement from the inside of the cell toward the periphery.
• Retrograde transport is movement in the opposite direction (hence the “retro-“ in its name)—from the periphery of the cell toward the center.

Question 8

Microtubules have polarity, meaning that each microtubule has a positive and a negative end. Which end is closer to the cell membrane?

Answer 8

The positive end

The positive end of a microtubule faces the cell periphery, while the negative end faces the center of the cell.

Question 9

Retrograde transport along a microtubule involves movement of cargo toward which end?

Choose from either the positive or the negative end of the microtubule.

Answer 9

The negative end

Retrograde transport involves movement toward the center of the cell. The negative end of a microtubule similarly faces the cell’s center.

Question 10

When traveling along a microtubule, in which direction do dyneins move?

Choose from “toward the positive end” or “toward the negative end.”

Answer 10

Toward the negative end

Dyneins move toward the negative end of a microtubule, which means they are involved in retrograde transport.

Question 11

When traveling along a microtubule, in which direction do kinesins move?

Choose from “toward the positive end” or “toward the negative end.”

Answer 11

Toward the positive end

Kinesins move toward the positive end of a microtubule, which means they are involved in anterograde transport.

Question 12

Which motor protein(s) are involved in mitosis, specifically in the polymerization and depolymerization of spindle fibers?

Answer 12

Both kinesins and dyneins

Research has actually shown that both kinesins and dyneins play different roles in the polymerization (lengthening) and depolymerization (shortening) of spindle fibers during mitosis, which is crucial for the proper positioning of chromosomes.

Question 13

Which motor protein(s) are involved in the motion of cilia and flagella?

Answer 13

Dyneins

Dyneins (specifically, a subset of dyneins termed axonemal dyneins) are responsible for the sliding of microtubules that occurs in eukaryotic flagella and cilia.

Question 14

Fill in the blank.

The three main classes of motor proteins are kinesins, dyneins, and ________.

Answer 14

myosins

These are the three types of motor proteins you should know for the MCAT. Unlike kinesins and dyneins, myosins generally function in cell movement (as in skeletal muscle) more than they do in the transport of cargo.

Question 15

In a neuron, a motor protein is observed carrying a neurotransmitter-filled vesicle toward the synaptic terminal. What type of motor protein was this?

Answer 15

A kinesin

Since this motor protein is carrying cargo away from the center of the cell (and toward the synaptic terminal, which is at the periphery of the neuron), it must be a kinesin.

Question 16

In a neuron, a motor protein is observed carrying a vesicle full of old, damaged organelles toward the cell soma. What type of motor protein was this?

Answer 16

A dynein

The soma is another name for the cell body of the neuron. Therefore, this motor protein is carrying cargo from the periphery of the cell toward the center, and it must be a dynein.

Question 17

A motor protein is identified as composing the thick filament of a sarcomere in a skeletal muscle cell. Which motor protein was this?

Answer 17

Myosin

Myosins (often simply termed “myosin,” especially in the context of muscle) are motor proteins that are best-known for their composition of the thick filament in myofibrils (muscle fibrils).

Question 18

Cadherins, selectins, and integrins all fall under the larger category of:

Answer 18

cell adhesion molecules

(CAMs)

CAMs assist cells in binding to other cells or to the extracellular matrix.

Question 19

This type of transmembrane cell adhesion molecule (CAM) binds calcium and plays a crucial role in the attachment of cells via adherens junctions.

Answer 19

Cadherins

Cadherins are actually named for the “calcium-dependent adhesion” in which they engage. These transmembrane proteins are the main protein component of adherens junctions.

Question 20

Describe the main function of selectins.

Answer 20

They are expressed on the surfaces of endothelial cells in response to an infection.

These selectins then adhere to leukocytes traveling through the bloodstream, causing those leukocytes to stop and aggregate at the site of infection or injury.

Question 21

Name at least two types of cells on which selectins are expressed.

Answer 21

1. Endothelial cells
2. Platelets
3. Immune cells (such as neutrophils, leukocytes, and monocytes)

Question 22

Describe the main function of integrins.

Answer 22

They help a wide variety of cells adhere to the extracellular matrix (assisting in processes such as cell migration and platelet aggregation).

Integrins also function in signal transduction.

Question 23

Name the three types of cell junctions, which are large-scale, somewhat complex structures that hold cells together.

Answer 23

1. Tight junctions
2. Gap junctions
3. Anchoring junctions

Question 24

Which type of cell junction holds together cells to form an impermeable barrier, such as the intestinal epithelia or the blood-brain barrier?

Answer 24

Tight junctions

As their name indicates, tight junctions involve formation of an extremely tight seal between adjacent cells. This helps ensure that solutes travel through the cells’ membranes (through regulated transport) as opposed to just slipping through gaps between the cells.

Question 25

Which type of cell junction allows numerous cardiac muscle cells to function as one connected unit?

Answer 25

Gap junctions

Gap junctions, which consist of connexin proteins, form direct connections between the cytoplasms of adjacent cells. In the case of the heart, these junctions allow solutes to diffuse between cells, facilitating their electrical contraction as a cohesive unit.

Question 26

Describe the function of anchoring junctions.

Answer 26

They firmly attach cells to each other as well as to the extracellular matrix.

This is what gives the tissues of the body much of their structural cohesion.

Question 27

Name the two main types of anchoring junctions.

Answer 27

1. adherens junctions
2. desmosomes

Question 28

Which type of anchoring junction is largely composed of cadherin proteins that interact with the cytoskeletons of adjacent cells via actin filaments?

Answer 28

Adherens junctions

Of the two types of anchoring junctions that you should be familiar with for the MCAT (adherens junctions and desmosomes), adherens junctions are the only type that interact with actin.

Question 29

This type of anchoring junction, which interacts via intermediate filaments, is commonly found in tissues that experience a large amount of mechanical stress.

Answer 29

Desmosomes

Unlike adherens junctions, desmosomes interact with the cytoskeletons of adjacent cells via intermediate filaments. They are found in the epithelia, the heart, and other tissues that are required to withstand significant stress.

Question 30

A researcher examining a tissue sample with an electron microscope notices direct connections between the interiors of adjacent cells. These connections are most likely what type of cell junction?

Answer 30

Gap junctions

Gap junctions are the only MCAT-relevant cell junctions that involve direct connections between the cytoplasms of cells.

Question 31

Predict what would occur if the epithelia of the duodenum lacked tight junctions.

Answer 31

Molecules that are not supposed to be absorbed would make it into the body.

We rarely think about it, but the interior (or lumen) of the intestine is kept entirely separate from the rest of the body and its cells. This separation is accomplished by tight junctions. Without them, all sorts of random, undesired molecules may slip through the gaps between adjacent duodenal cells, “cheating” their way into the body.

Question 32

Antibodies (or immunoglobulins) are what type of protein?

Answer 32

glycoproteins

In other words, antibodies are proteins that are covalently attached to oligosaccharide (sugar) chains.

Question 33

Name the structures that are attacked and bound to by antibodies that are highly specific to them.

Answer 33

Antigens

Antibodies bind antigens in a highly specific manner. Interestingly, antigens vary widely in their composition and structure; put simply, anything that an antibody specifically binds to is termed an antigen.

Question 34

Fill in the blank.

In humans, the structure of an antibody consists of ________ chains.

Answer 34

four

All human antibodies contain two heavy chains (which are larger) and two light chains (which are smaller), for four chains in total.

Question 35

Is the constant region of an antibody found on the heavy chains, the light chains, or both?

Answer 35

heavy chains

In a given species, the constant region is always the same across immunoglobulins (antibodies) of a given class (IgM, IgG, etc.).

Question 36

What is the function of the hypervariable region of an antibody?

Answer 36

It binds the antigen for which the antibody is specific.

This region is located at the very ends (tips) of the Y-shaped antibody structure. As its name indicates, it varies with incredible diversity among antibody molecules.

Question 37

True or false:

Antibodies in the human bloodstream are always present as monomers.

Answer 37

False

Like many other proteins, antibodies can be present as polymers, or structures consisting of multiple subunits. In particular, antibodies are often secreted as dimers or pentamers.

Question 38

Microfilaments, thin fibers that play a critical role in cellular structure, are composed of what protein?

Answer 38

Actin

Actin also serves as the thin filament in the sarcomere, which overlaps with the thick filament to facilitate muscle contraction.

Question 39

What is the difference between G-actin and F-actin?

Answer 39

• G-actin refers to globular actin monomers.
• F-actin refers to actin polymers, which are complexes consisting of many G-actin subunits. F-actin is string-shaped and is also known as “filamentous actin.”

Question 40

The proteins keratin, vimentin, and desmin are found in which of the three key types of cytoskeletal filament?

Answer 40

Intermediate filaments

Even if you aren’t familiar with these three proteins, you should know that intermediate filaments are the only one of the three key cytoskeletal filaments to be composed of a variety of proteins.

(For the MCAT, we recommend knowing that keratin is an intermediate filament protein. Vimentin and desmin are much less likely to be tested.)

Question 41

True or false:

Both microfilaments and microtubules can polymerize (lengthen) in a process that requires energy input.

Answer 41

True

This statement is absolutely true. Specifically, microfilament polymerization requires the hydrolysis of ATP, while microtubule polymerization involves GTP hydrolysis.

Question 42

The spindle apparatus that functions in cell division is composed of which type of cytoskeletal filament?

Answer 42

Microtubules

Microtubules are the thickest of the three key types of cytoskeletal filaments. In cell division, they extend from centrosomes to form the spindle apparatus that positions and pulls apart chromosomes in the dividing cell.

Question 43

Which non-enzymatic protein is the main component of microtubules?

Answer 43

Tubulin

This contrasts from microfilaments (which are composed of actin) and intermediate filaments (which can consist of a variety of proteins).

Question 44

Microtubules form via the polymerization of tubulin [monomers/dimers/trimers].

Choose one term from the box above to accurately complete the sentence.

Answer 44

dimers

Specifically, the fundamental unit of a microtubule is a dimer consisting of one unit of alpha-tubulin and one unit of beta-tubulin.

Question 45

True or false:

As the main protein comprising eukaryotic flagella, flagellin exemplifies a non-enzymatic protein.

Answer 45

False

While this statement may sound logical, it contains a critical flaw: it is prokaryotic, not eukaryotic, flagella that are composed of flagellin.

Eukaryotic flagella are composed of microtubules.