Nonenzymatic Protein Function for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about nonenzymatic protein function, plus practice questions and answers
(Note: This guide is part of our MCAT Biochemistry series.)
Part 1: Introduction
Part 2: Structural proteins
a) Actin and myosin
b) Cytoskeletal elements
c) Kinesins and dyneins
Part 3: Transmembrane proteins
a) GPCRs
b) Pores, ion channels, and active transporters
Part 4: Signaling proteins
a) Peptide hormones
b) Antibodies
Part 5: High-yield terms
Part 6: Passage-based questions and answers
Part 7: Standalone questions and answers
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Part 1: Introduction
Pop quiz! What is the most abundant protein in our cells? Given how vital enzymatic proteins are to speeding up reactions in our bodies to sustain life, you might guess that the most abundant protein is some sort of enzyme. Hexokinase? PFK? The answer is actually actin.
Actin is a structural protein that helps give cells their shape. In fact, actin is a nonenzymatic protein. Proteins are hugely diverse, and not all proteins have enzymatic properties! Nonenzymatic proteins play an equally important role in our cells as enzymatic proteins. Without them, we wouldn’t survive.
Many unique nonenzymatic proteins co-exist to provide structure and signaling capabilities. While studying nonenzymatic proteins, it’s easy to get them confused. As you work through this guide, it may be helpful to create a chart organizing the different nonenzymatic proteins based on their area of action in a cell and function.
Let’s begin!
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Part 2: Structural proteins
a) Actin and myosin
How do our bodies and cells maintain their shape? Surely, they’re not just sacs of cytoplasm surrounded by a plasma membrane. There must be something to give our cells and our bodies some structure.
The cytoskeleton and extracellular matrix are compositions of proteins and macromolecules that serve as a scaffold for the cell and tissue, respectively. You can think of the cytoskeleton and extracellular matrix as the framework and foundation of a house; both structures provide support. The cytoskeleton and extracellular matrix are composed of five primary proteins: actin, tubulin, collagen, elastin, and keratin. Actin and tubulin are primarily found within the cytoskeleton of the cell while collagen, elastin, and keratin make up the extracellular matrix.
As noted earlier, actin is the most abundant protein in eukaryotic cells. Inside the cell, actin assembles into long polymers known as microfilaments (shown below). These actin microfilaments possess polarity or a distinction between both ends of the polymer. Just like a magnet has a north and south pole or a battery has a positive and negative terminal, microfilaments have two unique ends.
Figure: An actin microfilament has polarity.
Microfilaments have both a plus (+) end and minus (-) end, which may also be referred to as barbed and pointed ends, respectively. The plus end of microfilaments is where new actin monomers attach to the polymer, while the minus end is where actin monomers dissociate. The association and dissociation of actin monomers is regulated by adenosine triphosphate (ATP). When ATP is bound to actin, the actin monomer will attach to the polymer. As the ATP is hydrolyzed into adenosine diphosphate (ADP), the actin monomer will detach. This constant cycle of actin monomers attaching and detaching is known as treadmilling. This results in an apparent motion of actin strands across a cell’s cytoplasm, which is often useful in cell movement.
Actin also plays an important role in muscle movement. Along with a motor protein called myosin, actin creates movement to contract individual muscle cells. (Recall that a motor protein is a protein that creates movement via the hydrolysis of ATP.) The “heads” of myosin interact with actin filaments in a crossbridge cycle to pull actin close together and shorten the sarcomere, the functional unit of muscle tissue, causing a muscle contraction. Many myosin heads must work together at once to produce enough force to contract a muscle.
Figure: A depiction of the actin-myosin crossbridge cycle.
The first step of the crossbridge cycle involves ATP binding to myosin. When ATP binds to myosin, myosin relaxes its grip to actin and dissociates from the microfilament. Myosin then hydrolyzes ATP into ADP and inorganic phosphate, or Pi. Hydrolyzing ATP causes the myosin head to swing backward and bind to the microfilament. Once myosin releases Pi, myosin starts its power stroke. The power stroke refers to the re-cocking of the myosin head to its original position while attached to the microfilament. Once myosin finishes its power stroke, ADP is released and the cycle can start all over again.
Although each power stroke covers a tiny amount of distance, the distance is amplified by the sheer number of myosin heads and actin filaments in our muscles! You can find additional information on the actin-myosin crossbridge cycle in our guide on the musculoskeletal system.
b) Cytoskeletal elements
In addition to actin, tubulin, collagen, elastin, and keratin work together to form the structure of cells. Inside the cell, alpha and beta forms of tubulin monomers assemble into long polymers called microtubules.
Figure: Microtubules are composed of tubulin.
Microtubule structure is quite different from microfilament structure. The insides of microtubules are hollow. Tubulin monomers associate into heterodimers of alpha-tubulin and beta-tubulin. Due to these heterodimers, microtubules, like microfilaments, have a polar structure. There are both positive and negative ends to microtubules. Alpha-tubulin is exposed at the negative ends of microtubules and tends to be located at the centrosome in the interior of the cell. Beta-tubulin is exposed at the positive ends of microtubules and tends to be located at the edges of the cell, near the plasma membrane. Microtubule polarity is an important feature that dictates the role microtubules play in the cell. We’ll talk more about that role in the next section.
Collagen, elastin, and keratin are largely found in the extracellular matrix. Collagen is a helical fiber made of three interwoven strands and composes a large portion of the extracellular matrix in connective tissue. Collagen provides structure to our tissues, bones, ligaments, and tendons.
Elastin also provides structure to the rest of our body, although it is not quite as abundant as collagen. In their natural state, elastin fibers are long and coiled. When stretched, elastin fibers become more linear in shape while preserving the cross-linked structure of the extracellular matrix. Elastin allows our tissues to stretch and snap back into shape without permanent structural damage.
Keratin is not directly localized in the extracellular matrix but is concentrated in epithelial cells. Keratin provides cells with the necessary structure and stability to protect our bodies and acts as a hard barrier from the outside world. Keratin is also largely found in our fingernails and hair.
c) Kinesins and dyneins
What exactly do microtubules do? Many of the functions microtubules serve are structural. Microtubules provide support so that cells don’t collapse on themselves. Microtubules are vital in mitosis and meiosis to ensure chromosomes are separated.
Recall that microtubules, composed of tubulin, have polarity. This polarity is important because motor proteins—in particular, kinesins and dyneins—use this polarity to walk along microtubules.
Kinesins and dyneins are classified as motor proteins. This means that these proteins move in the cell by hydrolyzing ATP. Scientists have recorded the way kinesins and dyneins move and have theorized that these proteins move in a stepwise manner, similar to how humans walk on two feet. How cool is that?! In fact, kinesins and dyneins move along microtubules. You can think of microtubules as a highway for motor proteins. On a highway, traffic goes in two directions. Similarly, the polarity of microtubules allows kinesin and dynein to travel in opposite directions. Kinesins travel towards the positive end of microtubules (towards the periphery of the cell), while dyneins travel towards the negative end of microtubules (towards the nucleus of the cell).
However, kinesin and dynein aren’t just moving without a purpose. Attached to the heads of kinesin and dynein are usually vesicles or organelles that need to be transported towards the periphery or center of the cell. Kinesin and dynein are like 18-wheeler trucks carrying large cargo from one location to another. These motor proteins are responsible for the bulk of active molecular transport within cells.
Figure: Kinesins and dyneins carry cargo out of or into the cell.
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