chapter 4

You want to test pentapeptides (short peptides with only five amino acids) for their ability to bind to and inhibit a particular receptor. To do this, you set out to synthesize all possible pentapeptides and test each individually. Assuming you'll use just the 20 common amino acids, how many different pentapeptides will you have to test for receptor binding?

- 3200000 (20^5) over 3 million peptides

Which of the following is true about amyloid protein structures?

- They consist of stacked ß sheets. Amyloid protein structures are ß sheets that interlock with each other through their side chains and form stacks. Since they are made from many ß sheets, they form strong structures that have many roles in the cell, including formation of important cellular compartments. However, some amyloid structures, but not all, are abnormal and cause disease.

What are the two types of β sheets?

- parallel and antiparallel The two types of β sheets are parallel and antiparallel. In a β sheet, several segments (strands) of an individual polypeptide chain are held together by hydrogen bonding between peptide bonds in adjacent strands. The amino acid side chains in each strand project alternately above and below the plane of the sheet.

What does the primary structure of a protein refer to?

- the linear amino acid sequence of the protein The primary structure of a protein refers to the linear amino acid sequence of the protein.The chain of linear polymers of amino acids that compose proteins is termed a polypeptide. The primary structure determines the secondary and tertiary structures.

For a given protein, hydrogen bonds can form between which of the following?

-(all of the above) -atoms in the polypeptide backbone -atoms of two peptide bonds -atoms in two side chains -a side chain and water For a given protein, hydrogen bonds can form between atoms in the polypeptide backbone, between atoms of two peptide bonds, between atoms in two side chains, and also between a side chain and water. The ability of a protein to bind selectively and with high affinity to a ligand is due to the formation of a set of weak, noncovalent interactions—hydrogen bonds.

For which reason are α helices and β sheets common folding patterns in polypeptides?

-The amino acid side chains are not directly involved in their formation. α helices and β sheets are common folding patterns in polypeptides because the amino acid side chains are not directly involved in their formation. Both of these folding patterns result from hydrogen bonds that form between N-H groups and C=O groups along the polypeptide backbone.

Predict what would happen to the secondary structure of a protein if an alcohol that disrupts hydrogen-bonding were added.

-The β sheets would unfold, disrupting protein structure. -The α helices would unfold, disrupting protein structure. Both α helices and β sheets result from hydrogen bonding of backbone atoms within the protein. Disrupting hydrogen bonding will cause both structures to unfold.

In a cytosolic folded protein, what orientation and/or interaction do the hydrophobic amino acids tend to have?

-They are interacting with other nonpolar amino acids. -They are tucked away inside the protein. In a folded protein, the hydrophobic amino acids tend to be tucked away deep inside the protein and interacting with other nonpolar amino acids in that area. This way, their disruptive effect on the hydrogen-bonded network of the surrounding water molecules is minimized.

Which is true of prion proteins?

-They are misfolded forms of normal proteins. -They can spread from one organism to another. Prions are misfolded forms of normal proteins and can spread from one organism to another. The aggregates formed by prion proteins are insoluble and very stable, thanks in part to the tight stacking of their β sheets. Therefore, prion proteins form very stable protein aggregates.

Generally speaking, what determines the biological activity of a protein?

-amino acid sequence In general, the biological activity of a protein is determined by its amino acid sequence. Each type of protein has a particular three-dimensional structure, which is determined by the order of the amino acids in its polypeptide chain.

Actin filaments, microtubules, and the spherical shells of certain virus particles are all structures built from which of the following?

-assemblages of identical proteins Actin filaments, microtubules, and the spherical shells of certain virus particles are all structures built from assemblages of identical proteins.

Disulfide bonds stabilize protein shape outside the cell by

-covalent bonds between cysteines. Using mechanisms such as noncovalent bonds between charged side chains, proteins fold into their final conformation based on their amino acid sequence inside the cell. However, in the harsh environment outside the cell, this structure needs to be stabilized to keep its final form and function. Disulfide bonds are covalent cross-linkages between cysteine groups juxtaposed in the three-dimensional structure, and they act to hold the shape of the protein.

Which amino acid is involved in the formation of disulfide bonds?

-cysteine Two cysteine side chains can be involved in the formation of a disulfide bond. Disulfide bonds help stabilize a favored protein conformation.

In an α helix, hydrogen bonds form between which of the following?

-every fourth amino acid In an α helix, hydrogen bonds form between every fourth amino acid. An α helix is generated when a single polypeptide chain turns around itself to form a structurally rigid cylinder.

What are protein families?

-evolutionarily related proteins that are similar in amino acid sequence and three-dimensional conformation Protein families are evolutionarily related proteins that are similar in amino acid sequence and three-dimensional conformation. Serine proteases are great examples of proteins that are classified in the same protein family.

In a globular protein, where would the amino acid arginine most likely be found?

-exposed at the protein's surface In a globular protein, the amino acid arginine would most likely be found exposed at the protein's surface. Polar, charged amino acids, such as arginine, tend to be found near a protein's surface, where they can form hydrogen bonds with water and other polar molecules.

Which important intramolecular force for polypeptide folding is demonstrated by the clustering of green side chains?

-hydrophobic interaction Hydrophobic forces help proteins fold into compact conformations. In a folded protein, nonpolar amino acid side chains are buried on the inside to form a tightly packed hydrophobic core of atoms hidden from water. In this figure, the hydrophobic nonpolar amino acids are shown in green.

Chaperone proteins can aid in protein folding by doing which of the following?

-isolating a polypeptide chain within the crowded cytoplasm to prevent aggregation of proteins -steering partially folded polypeptide chains along the most energetically favorable folding pathway Chaperone proteins can aid in protein folding by isolating a polypeptide chain within the crowded cytoplasm to prevent aggregation of proteins and by also steering partially folded polypeptide chains along the most energetically favorable folding pathway. In either case, the final three-dimensional shape of the protein is still specified by its amino acid sequence; chaperones merely make the folding process more efficient and reliable.

Mutations in the nucleic acid sequence of a gene can sometimes direct the substitution of one amino acid for another in the encoded protein. Which amino acid substitution would be most likely to severely disrupt the normal structure of a protein?

-methionine to arginine -Arginine and methionine have different chemical properties. Methionine has a nonpolar side chain that would likely be buried in the protein's interior; arginine, on the other hand, is a positively charged amino acid that would likely be facing the protein's exterior. Replacing methionine with arginine would likely disrupt a protein's structure.

Within the image, identify the oxidation and the reduction direction that determines the disulfide bond creation or resolution. Drag the label to the target that is associated with the correct arrow.

-polypeptide 1: Oxidation -Polypeptide 2: reduction -Disulfide bonds form by oxidation and resolve by reduction. Disulfide bonds generally do not form in the cell cytosol, where a high concentration of reducing agents converts such bonds back to cysteine -SH groups. The oxidation of the thiol group (loss of electrons) causes the new bond formation between both sulfur atoms.

Protein molecules that have a quaternary structure must have two or more of which of the following?

-polypeptide chains Protein molecules that have a quaternary structure must have two or more polypeptide chains. If a protein molecule exists as a complex of more than one polypeptide chain, then these interacting polypeptides form its quaternary structure.

What provides the information necessary to specify the three-dimensional shape of a protein? Choose the best answer in the context of which generally applies to most proteins.

-protein's amino acid sequence The protein's amino acid sequence provides the information necessary to specify the three-dimensional shape of a protein. The ordering of amino acids is what allows some amino acids to interact noncovalently with other amino acids when the protein begins to fold.

Which part of an amino acid gives it its unique properties?

-side chain The side chain of an amino acid is what gives the amino acid its unique chemical properties. All 20 naturally occurring amino acids are different from one another in the atoms that compose the side chain, which gives individual amino acids their unique chemical properties.

How many identical antigen-binding sites exist on a typical antibody monomer?

2 An antibody is Y-shaped and has two identical antigen-binding sites, one on each arm of the Y. Each of these antigen-binding sites is complementary to a small portion of the surface of the antigen molecule.

The figure shows a depiction of an antibody. Which label correctly identifies the region(s) of the antibody that contains variable amino acids for binding of a specific antigen?

A

Choose the statement that best describes the distribution of amino acid sequence variability in antibody molecules that accounts for the broad antigen-binding repertoire of an antibody, even while the overall structure of the antibody experiences little change.

Amino acid sequence variability is concentrated in the hypervariable loops of both the VH and VL domains, as it is the hypervariable loops that primarily make direct contact with the antigen.

Sort the following techniques into those that can help determine protein tertiary structure and quaternary structure (where applicable) and those that cannot.

Can determine 3-D: nuclear magnetic resonance spectroscopy (NMR); x-ray crystallography; cryoelectron microscopy Cannot determine 3-D: two-dimensional polyacrylamide-gel electrophoresis; mass spectometry NMR spectroscopy, X-ray crystallography, and cryo-EM are all used to determine the three-dimensional structure of proteins. NMR spectroscopy can be used with small proteins (those less than about 50,000 daltons). X-ray crystallography requires crystals of the protein of interest. Cryo-EM, the newest technique of the three, allows atomic resolution of biological macromolecules in a near-native state.

Which of the following is INCORRECT regarding how enzymes lower the activation energy of a reactin?

Enzymes reduce the free energy of the products of the reaction.

Which statement concerning feedback inhibition is false?

Feedback inhibition is difficult to reverse. Feedback inhibition is not difficult to reverse. Rather, it is very easy to do so. In feedback inhibition, for example, an enzyme acting early in a reaction pathway is inhibited by a molecule produced later in that pathway. Thus, whenever large quantities of the final product begin to accumulate, the product binds to an earlier enzyme and slows down its catalytic action, limiting further entry of substrates into that reaction pathway. Feedback inhibition can work almost instantaneously and is rapidly reversed when product levels fall.

Phosphofructokinase catalyzes a key control point in the glycolytic pathway, which ultimately leads to ATP production. The activity of phosphofructokinase as a function of ATP concentration was studied in vitro. Purified enzyme was added to test tubes. Each tube contained the same amount of fructose 6-phosphate and a different ATP concentration. As ATP levels increased, enzyme activity initially rose, then plateaued, and finally decreased at higher ATP levels, as shown here. As seen in the graph of the data, high ATP levels inhibit enzyme activity. What is a reasonable explanation for the inhibition of enzyme activity at high ATP levels?

High levels of ATP allosterically inhibit the enzyme. Phosphofructokinase has two binding sites for ATP: the active site, which binds ATP with high affinity, and an allosteric site, which binds ATP with low affinity (and hence is only bound when ATP levels are high). This results in negative feedback, inhibiting further ATP production when ATP levels are high.

How does an allosteric inhibitor work?

It binds to a site other than the active site, causing a conformational change in the enzyme that makes the active site less accommodating to the substrate. To regulate enzyme activity, an allosteric inhibitor binds to a second site, causing a conformational change in the enzyme that makes the active site less accommodating to the substrate. Unlike competitive inhibition, allosteric inhibition cannot be overcome by experimentally elevating the concentration of the substrate.

Which of the following correctly describes phosphorylation of a protein?

It can increase or decrease the protein's activity. The phosphorylation of a protein can either increase or decrease the protein's activity, depending on the site of phosphorylation and the structure of the protein. Binding sites can either be exposed to or hidden by these conformational changes.

How does the GTP-bound form of a GTP-binding protein switch to a GDP-bound form?

It hydrolyzes GTP, releasing a phosphate. The GTP-bound form of a GTP-binding protein switches to a GDP-bound form by hydrolyzing GTP, releasing a phosphate. When this happens, the protein retains the GDP and changes to the inactive conformation.

Which of the following statements is true regarding protein structure determination?

Mass spectrometry can be used to determine the amino acid sequences of a complex mixture of different proteins. To achieve the resolution needed to distinguish between peptides from different proteins, such mixtures are often subjected to tandem mass spectrometry. In this case, peptides that pass through the first spectrometer are digested into even smaller fragments and then analyzed by a second mass spectrometer. Mass spectrometry is used to determine the amino acid sequence—the primary structure of a protein, not its three-dimensional structure.

The epidermal growth factor receptor, HER2, contributes to the development of certain forms of breast cancer when over-expressed in breast tissue because it drives increased cell survival and division. HER2-positive breast cancers can sometimes be treated with monoclonal antibody drugs, which bind to HER2 and block its function. One such drug, pertuzumab (Perjeta), was FDA approved in 2012 for the treatment of advanced HER2-positive cancers. The mechanism of action of pertuzumab has been studied extensively and is dependent on its direct binding to HER2, which requires the interaction of the side chain of tyrosine 99 of the pertuzumab heavy chain with the side chain of histidine 296 of HER2. In rodents, the homolog of human HER2 is called Neu. Even though HER2 and Neu are closely related, pertuzumab does not block the function of Neu in rodents. Which of the following is the most likely hypothesis for why pertuzumab does not inhibit the function of Neu?

Neu has an amino acid substitution at the position equivalent to histidine 296 in HER2, and this amino acid in Neu does not have a side chain capable of forming the hydrogen bond with tyrosine 99 in pertuzumab required for binding.

How many different amino acids are commonly used in making proteins?

Proteins are made from a set of 20 common amino acids. Therefore, a protein that is n amino acids long has 20n different possible sequences.

The graph below shows the rate of reaction plotted against substrate concentration, with KM and Vmax indicated. Imagine that this experiment uses a mutant version that increases the enzyme's affinity, or ability, to bind its substrate. Using the graph as a comparison, what effect would the mutant version of the enzyme likely have on the KM?

The KM would move to the left (decrease). If a mutant version of the enzyme used to generate these data increased its ability to bind substrate, then we would expect the new KM to move to the left of this graph, meaning that it decreases. In general, a small KM indicates that a substrate binds very tightly to the enzyme (due to a large number of noncovalent interactions).

Within the representation of an antibody, identify the variable and constant regions by dragging the labels to the correct targets. Both targets are on the right side of the antibody representation. constant variable

The antibody is composed of four polypeptide chains (two identical heavy chains and two identical, smaller light chains). Each chain is made of several similar domains, here shaded in blue for the variable domains or gray for the constant domains.

Consider this image depicting aspartate transcarbamoylase regulation and then answer the question. Enzymes can have both active and regulatory sites. What is the purpose of these sites?

The binding of CTP at a regulatory site on the protein causes decreased production of carbamoyl aspartate. The binding of CTP at a regulatory site on the protein causes decreased production of carbamoyl aspartate. One of the final products of the pathway that this enzyme is involved in, cytosine triphosphate (CTP), binds to the enzyme to turn it off whenever CTP is plentiful. This is known as feedback inhibition and this enzyme is a very common example of the allosteric regulation of an enzyme.

A disulfide bond is a(n) _________ interaction within the protein.

The disulfide bond is a covalent interaction within the protein. Disulfide bonds help stabilize a favored protein conformation. Covalent disulfide bonds form between adjacent cysteine side chains by a reaction between their polar -SH groups.

Consider the following image of a protein-ligand interaction and then answer the question. Which of the following best describes the stable protein-ligand interaction that is represented in the image? Be sure to use the image as a guide but apply your knowledge regarding how protein and ligand commonly interact with each other. A note of caution though: the red lines in the figure are merely representing interactions and are not meant to be quantified.

The formation of a set of many weak, noncovalent interactions maintains the interaction between protein and ligand.

How does phosphorylation control protein activity?

The phosphate group induces a change in the protein's conformation. Proteins are commonly controlled by phosphorylation and dephosphorylation. When added to the protein, the phosphate group induces a change in the protein's conformation. Regulation of protein activity in this manner involves attaching a phosphate group covalently to one or more of the protein's amino acid side chains.

Which of the following is true of an enzyme that is operating at its maximum rate?

The substrate-binding sites on the enzyme molecules are fully occupied.

All of the following are true concerning enzymes except which statement?

They require an input of energy from ATP for activation. Enzymes do not require an input of energy from ATP for activation. Enzymes do not alter the overall thermodynamics of a reaction; they merely alter the kinetics, meaning they speed up the rate of a biochemical reaction. It is meaningful to note, however, that ATP, while not required for all enzymatic reactions, is used by certain enzymes (i.e., kinases).

Consider this image depicting an amino acid biosynthetic pathway and then answer the question. Which of the following is supported by the data in the figure?

Threonine inhibits the production of homoserine. Threonine inhibits the production of homoserine. This is supported by the data in the figure because there is an inhibitory blunt-ended arrow extending from threonine to homoserine.

Antibodies are large proteins with quaternary structure and multiple domains, but only a specific portion of the antibody directly binds to antigen. Which of the following antibody domains directly determine antigen binding?

VH domains VL domains

Which of these amino acids could be represented by the green side chains that are clustered in the center of the folded protein?

Valine, leucine, and phenylalanine are all nonpolar amino acids that would be expected to cluster in the center of a folded protein. In an aqueous environment, the nonpolar side chains of these amino acids tend to be forced together to minimize their disruptive effect on the polar interaction network formed by the surrounding water molecules.

Sort the following secondary structure characteristics into the correct categories.

alpha helix only: -cylindrical structure -one full turn every 3.6 amino acids Beta sheet only: -Consists of antiparallel or parallel strands -Side chains alternating above and below the structure Both alpha helix and Beta sheet: -Can be formed by many sequences -Formed by hydrogen-bonding between backbone atoms Cylindrical α helices and planar β sheets can be formed by hydrogen-bonding of many different sequences.

How does binding of GTP to a GTP-binding protein affect its activity?

always activates the protein Proteins in the GTP-binding protein family are always in their active conformation when GTP is bound, in contrast to phosphorylation of a protein, which can activate or inactivate a protein. The hydrolysis of GTP to GDP returns the protein to the inactive state.

Which of the following chemical group interactions may be represented by the red lines in the image?

an -OH of the ligand interacting with an -SH of the protein

Phosphofructokinase catalyzes a key control point in the glycolytic pathway, which ultimately leads to ATP production. The activity of phosphofructokinase as a function of ATP concentration was studied in vitro. Purified enzyme was added to test tubes. Each tube contained the same amount of fructose 6-phosphate and a different ATP concentration. As ATP levels increased, enzyme activity initially rose, then plateaued, and finally decreased at higher ATP levels, as shown here. How could enzyme activity levels have been determined?

by measuring the accumulation of fructose 1,6-bisphosphate Measuring the appearance of product is a way to determine the rate of catalysis.

Which of the following are methods for isolating a protein of interest?

chromatography and electrophoresis Both chromatography and electrophoresis are methods for isolating a protein of interest. The most efficient forms of protein chromatography separate polypeptides on the basis of their ability to bind to a particular molecule—a process called affinity chromatography. Proteins can also be separated by electrophoresis. In this technique, a mixture of proteins is loaded onto a polymer gel and subjected to an electric field; the polypeptides then migrate through the gel at different speeds depending on their size and net charge.

Which method is most suitable for determining the three-dimensional structure of an extremely large integral membrane protein complex?

cryoelectron microscopy Large proteins produce confounding signals on NMR spectra, and large integral membrane complexes are hard to crystallize for x-ray crystallography. Structural studies of large integral membrane protein complex could be performed with cryoelectron microscopy. In this method, the purified protein complex is embedded in ice and a beam of electrons is used to collect projected images in multiple orientations.

To purify a protein from cells or tissues, the protein's amino acid sequence must be known.

false This statement is false. A protein's amino acid sequence does not need to be known in order to purify a protein from cells or tissues. In fact, before the advent of mass spectroscopy, a protein had to be obtained in pure form to determine its amino acid sequence.

1) Is the following statement true, false, or impossible to determine? Phosphorylation is the only form of covalent modification that can affect a protein's activity or location.

false This statement is false. Phosphorylation is not the only form of covalent modification that can affect a protein's activity or location. Many proteins are modified by the addition of an acetyl group to a lysine side chain, including the histones. The addition of the fatty acid palmitate to a cysteine side chain drives a protein to associate with cell membranes. Also, attachment of ubiquitin, a 76-amino-acid polypeptide, can target a protein for degradation.

2) Is the following statement true, false, or impossible to determine? Phosphorylation can only take place in the cell nucleus.

false This statement is false. Phosphorylation regularly occurs in the cytoplasm. For example, when extracellular signals stimulate a class of cell-surface, transmembrane proteins called receptor tyrosine kinases, they cause the receptor proteins to phosphorylate themselves on certain tyrosines.

When a ligand binds to an allosteric enzyme's regulatory site, it changes the activity of that enzyme by

inducing a conformational change. Once a regulatory molecule or ligand binds to a regulatory site, an allosteric protein undergoes a conformational change that is transmitted to the active site. Inducing the conformational change can change the activity of the enzyme, but it does NOT directly block the active site.

What kind of enzyme adds a phosphate group to another protein?

kinase Protein phosphorylation involves the enzyme-catalyzed transfer of the terminal phosphate group of ATP to the hydroxyl group on a serine, threonine, or tyrosine side chain of the protein. This reaction is catalyzed by a protein kinase. The reverse reaction—removal of the phosphate group, or dephosphorylation—is catalyzed by a protein phosphatase.

A binding site on the surface of a protein interacts specifically with another protein through

many weak noncovalent interactions.

What kind of enzyme removes a phosphate group from a protein?

phosphatase Protein phosphorylation involves the enzyme-catalyzed transfer of the terminal phosphate group of ATP to the hydroxyl group on a serine, threonine, or tyrosine side chain of the protein. This reaction is catalyzed by a protein kinase. However, the reverse reaction—removal of the phosphate group, or dephosphorylation—is catalyzed by a protein phosphatase.

What determines the specificity an antibody has for its antigen?

polypeptide loops in its variable domains The polypeptide loops in its variable domains determine the specificity an antibody has for its antigen. A detailed examination of antibody structure reveals that the antigen-binding sites are formed from several loops of polypeptide chains that protrude from the ends of a pair of closely juxtaposed protein domains. The amino acid sequence in these loops can vary greatly without altering the basic structure of the antibody.

Consider the thermodynamic properties of chemical reactions. Even though enzymes do not affect the overall energy of the reactants or the products (i.e., the thermodynamics), they alter the speed of the reaction. Enzymes accomplish this by doing which of the following?

reducing the activation energy of a reaction Enzymes reduce the activation energy of a reaction. The activation energy is an energy barrier to reactions. For a colliding water molecule to break a bond linking two sugars, the polysaccharide molecule has to be distorted into a particular shape—the transition state—in which the atoms around the bond have an altered geometry and electron distribution. Conditions are thereby created in the microenvironment of the enzyme-active site that greatly reduce the activation energy necessary for the hydrolysis to take place.

The majority of proteins belong to "protein families" that share which attribute?

sequence patterns and therefore structural domains The majority of proteins belong to "protein families" that share sequence patterns and therefore structural domains. The fact that so many proteins fall into families that share similar structural domains means that investigators have some information about the three-dimensional shape of almost three-quarters of the proteins currently archived in the public databases.

Chemical modifications like phosphorylation and acetylation of proteins occur on ___________ of amino acids and can affect interaction of proteins with other cell components or structures.

side chains The chemical modification of phosphorylation and acetylation occurs on the side chain of the amino acid and can affect how proteins interact with other proteins or components of the cell. This effect can occur through the direct alteration of binding sites if the phosphorylation and acetylation occur on an amino acid in a binding site, or through a conformational change that indirectly alters the binding site.

Many proteins are regulated by the binding of GTP or GDP. Which form is the active state of the protein?

the GTP-bound form A GTP-binding protein requires the presence of a tightly bound GTP molecule to be active. The active protein can shut itself off by hydrolyzing its bound GTP to GDP and inorganic phosphate (Pi), which converts the protein to an inactive conformation.

The Michaelis constant (KM) of an enzyme is a measure of

the binding strength of enzyme to substrate.

1) Is the following statement true, false, or impossible to determine? Many protein molecules (not just enzymes) are allosteric.

true It is true that many protein molecules (not just enzymes) are allosteric. Proteins regularly interact with other proteins and small molecules within the cell. Any interaction between a protein and another chemical (whether protein or nonprotein) causes a shape change in the protein. This shape change can be dramatic or subtle.

2) Is the following statement true, false, or impossible to determine? The binding of a regulatory ligand can change the equilibrium between two protein conformations.

true It is true that the binding of a regulatory ligand can change the equilibrium between two protein conformations. Each ligand will stabilize the conformation that it binds to most strongly.

In an enzyme-catalyzed reaction, a small value of KM indicates that a substrate binds to the enzyme in which manner?

very tightly

Inhibitor molecules can block an enzyme's activity. Some inhibitors, called competitive inhibitors, compete directly with the substrate for the enzyme's active site. What effect will such inhibitors have on the reaction?

They will have no effect on the Vmax of the reaction.

Ras is a GTP-binding protein involved in cell proliferation (division). In its active form, with GTP bound, Ras activates cell signaling pathways that promote cell division. Mutations in the gene that encodes Ras can lead to cancer. How might mutations in the gene encoding Ras lead to the uncontrolled proliferation characteristic of cancer cells?

They decrease the rate at which Ras hydrolyzes GTP. A mutation that decreases the rate at which Ras hydrolyzes GTP would be a logical mutation in the gene encoding Ras that could lead to the uncontrolled proliferation characteristic of cancer cells. Ras is a G protein that is activated upon GTP binding. If a mutation happened that caused a decrease in the ability of Ras to hydrolyze that GTP, then cell-proliferative Ras signaling would lead to cancer.