MCAT Biology : Protein Structure

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #11 : Protein Structure

Hemoglobin is the principal oxygen-carrying protein in humans. It exists within erythrocytes, and binds up to four diatomic oxygen molecules simultaneously. Hemoglobin functions to maximize oxygen delivery to tissues, while simultaneously maximizing oxygen absorption in the lungs. Hemoglobin thus has a fundamentally contradictory set of goals. It must at once be opitimized to absorb oxygen, and to offload oxygen. Natural selection has overcome this apparent contradiction by making hemoglobin exquisitely sensitive to conditions in its microenvironment.

One way in which hemoglobin accomplishes its goals is through the phenomenon of cooperativity. Cooperativity refers to the ability of hemoglobin to change its oxygen binding behavior as a function of how many other oxygen atoms are bound to the molecule. 

Fetal hemoglobin shows a similar pattern of cooperativity, but has unique binding characteristics relative to adult hemoglobin. Fetal hemoglobin reaches higher saturation at lower oxygen partial pressure.

Because of cooperativity, adult and fetal oxygen-hemoglobin dissociation curves appear as follows.

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Beyond its ability to carry oxygen, hemoglobin is also effective as a blood buffer. The general reaction for the blood buffer system of hemoglobin is given below.

H+ + HbO2 ←→ H+Hb + O2

 

Because hemoglobin can act as a buffer in blood, it helps keep the pH constant. Which of the following portions of an amino acid can change with pH change?

Possible Answers:

Carboxy end

Side chain

Amino end and carboxy end

Amino end

Amino end, carboxy end, and side chain

Correct answer:

Amino end, carboxy end, and side chain

Explanation:

All three portions can change with pH. The amino end can take on an extra proton to become positively charged, the carboxy end can lose a proton and take on a negative charge, and the side chain can do either depending on its structure. An amino acid with both a positively charged amino end and a negatively charged carboxy end is called a zwitterion.

Example Question #201 : Organic Chemistry, Biochemistry, And Metabolism

Collagen, the most abundant protein in the body, is an example of what type of protein?

Possible Answers:

Structural

Globular

Integral

Peripheral

Correct answer:

Structural

Explanation:

Collagen is a structural protein that adds significant strength and resilience to the skin, tendons, and ligaments. Structural proteins, including collagen, also fall under the category of fibrous proteins. Globular proteins, in contrast, usually act as enzymes  in the body or transport channels in the membrane.

Peripheral proteins are a type of globular protein found adjacent to the membrane, while integral proteins are transmembrane globular proteins.

Example Question #11 : Protein Structure

Amino acids are joined together to form polypeptides. Each amino acid is attached to another by a peptide bond.

What functional group is created when amino acids are joined together?

Possible Answers:

Ketone

No new functional groups are created

Amide

Ester

Correct answer:

Amide

Explanation:

Polypeptide formation involves the C-terminus of one amino acid attaching to the N-terminus of another. This polymerization results in a dipeptide with the byproduct of one water molecule. The newfound combination results in a carbonyl being attached to a nitrogen. This functional group is called an amide.

Example Question #31 : Proteins

Hypersensitivity reactions occur when body tissues are affected by an abnormal immune reaction. The result is damage to normal tissues and clinical illness. A peanut allergy is an example of a hypersensitivity reaction, but there are three additional broad classes.

One class involves the abnormal production or deposition of antibodies. Antibodies are B-cell derived molecules that normally adhere to pathogens, rendering them unable to continue an infection. When antibodies are produced against normal tissues, however, disease can result. Figure 1 depicts a schematic structure of an antibody.

Antibodies can be divided into two peptide chains: heavy and light. Heavy chains form the backbone of the antibody, and are attached to light chains via covalent bonding. Each heavy and light chain is then further divided into constant and variable regions. Variable regions exhibit molecular variety, generating a unique chemical identity for each antibody. These unique patterns help guarantee that the body can produce antibodies to recognize many possible molecular patterns on invading pathogens.

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The polypeptides that make up the heavy and light chains of antibodies are most likely connected by covalent bridges involving atoms of which element?

Possible Answers:

Sulfur

Carbon

Hydrogen

Oxygen

Nitrogen

Correct answer:

Sulfur

Explanation:

Covalent bridges can be found in organic molecules, linking one region of the molecule to another. These bridges are almost invariably disulfide linkages, in which two sulfur atoms form a covalent linkage that provides a great deal of stability between peptide chains. Disulfide bridges are commonly involved in protein tertiary structure and other organic structural linkages, such as the joining of the heavy and light chains in antibodies.

Example Question #1591 : Mcat Biological Sciences

Hypersensitivity reactions occur when body tissues are affected by an abnormal immune reaction. The result is damage to normal tissues and clinical illness. A peanut allergy is an example of a hypersensitivity reaction, but there are three additional broad classes.

One class involves the abnormal production or deposition of antibodies. Antibodies are B-cell derived molecules that normally adhere to pathogens, rendering them unable to continue an infection. When antibodies are produced against normal tissues, however, disease can result. Figure 1 depicts a schematic structure of an antibody.

Antibodies can be divided into two peptide chains: heavy and light. Heavy chains form the backbone of the antibody, and are attached to light chains via covalent bonding. Each heavy and light chain is then further divided into constant and variable regions. Variable regions exhibit molecular variety, generating a unique chemical identity for each antibody. These unique patterns help guarantee that the body can produce antibodies to recognize many possible molecular patterns on invading pathogens.

 

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Antibodies are made of proteins, which form one of the broad classes of biological macromolecules. A glycoprotein is different from other kinds of proteins principally because __________.

Possible Answers:

glycoproteins always interact with other proteins via van der Waals interactions

glycoproteins always act as hormonal messengers

glycoproteins always contain sugar moieties

glycoproteins are always heavier than other proteins

glycoproteins are always lighter than other proteins

Correct answer:

glycoproteins always contain sugar moieties

Explanation:

The prefix "glyco-" indicates that some substrate has had a carbohydrate moiety added to its structure. Glycolipids are thus lipids bound to saccharide units, and glycoproteins are proteins bound to saccharide units.

Example Question #31 : Proteins

Drain cleaners are a common household staple, used to open clogged drains in bathtubs and sinks. Human hair is a common culprit that clogs pipes, and hair is made predominately of protein. Drain cleaners are effective at breaking down proteins that have accumulated in plumbing. Drain cleaners can be either acidic or basic, and are also effective at breaking down fats that have accumulated with proteins.

A typical reaction—reaction 1—which would be expected for a drain cleaner on contact with human hair, would be as follows in an aqueous solution:

Pic_1

Another reaction that may occur, reaction 2, would take place as follows in an aqueous solution:

Pic_2

In the proteins depicted in both reactions in the preceeding passage, which portions of the molecule are shown?

I. Amino terminus

II. Carboxy terminius

III. Side chain

Possible Answers:

III, only

I, only

II and III, only

I and III, only

II, only

Correct answer:

I, only

Explanation:

The protein that is reacting with the drain cleaner in both instances shows the NH2 end, or amino terminus. The side chain and carboxy terminus are not shown.

Example Question #1 : Biochemistry

 

An enzyme that cleaves disulfide bridges would most disrupt a protein containing which amino acid sequence?

Possible Answers:

Val–Leu–Leu–Cys–Tyr–Thr

Tyr–Cys–Val–Val–Leu–Thr

Cys–Leu–Val–Tyr–Tyr–Thr

All of the answers would be equally affected

Tyr–Cys–Cys–Thr–Val–Leu

Correct answer:

Tyr–Cys–Cys–Thr–Val–Leu

Explanation:

Disulfide bridges are made between two cysteine amino acids. An enzyme that cleaves disulfide bonds would disrupt a protein containing the most cysteine residues; therefore, Tyr–Cys–Cys–Thr–Val–Leu is the correct answer.

Example Question #1 : Gre Subject Test: Biology

Which of the following is an example of the secondary structure of a protein?

Possible Answers:

Hydrophobic interactions

Peptide bonding between amino acids

Hydrogen bonding between R groups

Hydrogen bonding between an amine and carbonyl group

Disulfide bonds between cysteine residues

Correct answer:

Hydrogen bonding between an amine and carbonyl group

Explanation:

By definition, the secondary structure of a protein is the hydrogen bonding between the amine and carbonyl groups in the amino acid chain. This usually occurs in the form of alpha-helices or beta-pleated sheets.

The linear sequence of the amino acids formed by peptide bonds is the primary protein structure. Interactions of R groups determines the tertiary structure. These interactions can be in the form of disulfide bonds, hydrogen bonding, or hydrophobic interactions.

Example Question #11 : Protein Structure

You are given the amino acid sequence Ala-Gly-His-Tyr. This is an example of which level of protein structure?

Possible Answers:

Secondary

None of the other answers are correct

Primary

Tertiary

Quaternary

Correct answer:

Primary

Explanation:

Primary protein structure refers to the linear sequence of amino acids, which is the information given in this question. Secondary structure includes interactions between nearby parts of the linear chain, and includes the common examples of alpha-helices and beta-pleated sheets. Tertiary structure is the protein's three-dimensional shape, and quaternary structure refers to interactions between tertiary subunits.

Example Question #12 : Protein Structure

The term "denaturation," when used in conjunction with proteins or nucleic acids, refers to a change in structural characteristics primarily due to __________.

Possible Answers:

changes in primary structure

the binding of toxic compounds

the disruption of non-covalent bonds

the disruption of covalent bonds

Correct answer:

the disruption of non-covalent bonds

Explanation:

The denaturation of proteins and nucleic acids occurs due to the disruption of non-covalent bonds, especially hydrogen bonds. In the case of nucleic acids, covalent bonds can be disrupted by specific enzymes, but this is not a form of denaturation. Changes in the primary structure of nucleic acids and proteins simply result in the net production of different proteins due to sequential changes of amino acids and nucleotides, not denaturation. Toxic compounds can interfere with nucleic acid and protein formation, but they do so by interrupting the non-covalent forces of each, such as hydrophobic forces and hydrogen bonding. Thus, the correct answer is the disruption of non-covalent bonds.

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