Proteins Can Be Denatured or Rendered Inactive by Which of the Following Factors?

ix.one Proteins

Learning Objectives

1. Describe the four levels of protein structure.
2. Identify the types of attractive interactions that hold proteins in their near stable three-dimensional structure.
3. Explain what happens when proteins are denatured.
4. Identify how a poly peptide tin can be denatured.

Each of the thousands of naturally occurring proteins has its own feature amino acid composition and sequence that result in a unique three-dimensional shape. Since the 1950s, scientists have determined the amino acid sequences and three-dimensional conformation of numerous proteins and thus obtained important clues on how each protein performs its specific function in the body.

Proteins are compounds of high molar mass consisting largely or entirely of chains of amino acids. Because of their keen complication, protein molecules cannot be classified on the basis of specific structural similarities, as carbohydrates and lipids are categorized. The two major structural classifications of proteins are based on far more general qualities: whether the protein is (1) fiberlike and insoluble or (2) globular and soluble. Some proteins, such as those that compose hair, skin, muscles, and connective tissue, are fiberlike. These fibrous proteins are insoluble in water and normally serve structural, connective, and protective functions. Examples of fibrous proteins are keratins, collagens, myosins, and elastins. Pilus and the outer layer of skin are composed of keratin. Connective tissues comprise collagen. Myosins are muscle proteins and are capable of contraction and extension. Elastins are institute in ligaments and the rubberband tissue of artery walls.

Globular proteins, the other major course, are soluble in aqueous media. In these proteins, the bondage are folded so that the molecule equally a whole is roughly spherical. Familiar examples include egg albumin from egg whites and serum albumin in blood. Serum albumin plays a major role in transporting fatty acids and maintaining a proper balance of osmotic pressures in the body. Hemoglobin and myoglobin, which are of import for binding oxygen, are also globular proteins.

Levels of Protein Structure

The structure of proteins is mostly described every bit having iv organizational levels. The first of these is the principal construction, which is the number and sequence of amino acids in a protein'southward polypeptide chain or chains, outset with the costless amino group and maintained by the peptide bonds connecting each amino acrid to the adjacent. The master construction of insulin, composed of 51 amino acids, is shown in Figure 9.1 "Master Structure of Human Insulin".

Figure 9.1 Primary Construction of Human Insulin

Human insulin, whose amino acid sequence is shown here, is a hormone that is required for the proper metabolism of glucose.

A protein molecule is non a random tangle of polypeptide chains. Instead, the chains are arranged in unique only specific conformations. The term secondary construction refers to the fixed arrangement of the polypeptide backbone. On the footing of X ray studies, Linus Pauling and Robert Corey postulated that certain proteins or portions of proteins twist into a spiral or a helix. This helix is stabilized by intrachain hydrogen bonding betwixt the carbonyl oxygen atom of 1 amino acrid and the amide hydrogen atom four amino acids up the chain (located on the side by side turn of the helix) and is known every bit a correct-handed α-helix. Ten ray data point that this helix makes ane plow for every iii.6 amino acids, and the side chains of these amino acids projection outward from the coiled courage (Figure 9.2 "A Brawl-and-Stick Model of an α-Helix"). The α-keratins, found in pilus and wool, are exclusively α-helical in conformation. Some proteins, such as gamma globulin, chymotrypsin, and cytochrome c, have little or no helical structure. Others, such every bit hemoglobin and myoglobin, are helical in certain regions merely not in others.

Figure 9.2 A Ball-and-Stick Model of an α-Helix

This brawl-and-stick model shows the intrachain hydrogen bonding between carbonyl oxygen atoms and amide hydrogen atoms. Each plow of the helix spans 3.six amino acids. Note that the side chains (represented as green spheres) point out from the helix.

Another common type of secondary structure, chosen the β-pleated sheet conformation, is a sheetlike arrangement in which two or more than extended polypeptide chains (or separate regions on the aforementioned concatenation) are aligned side by side. The aligned segments can run either parallel or antiparallel—that is, the N-terminals can face in the same direction on adjacent chains or in different directions—and are connected by interchain hydrogen bonding (Effigy 9.3 "A Ball-and-Stick Model of the β-Pleated Sheet Construction in Proteins"). The β-pleated sheet is especially important in structural proteins, such as silk fibroin. It is also seen in portions of many enzymes, such as carboxypeptidase A and lysozyme.

Figure nine.three A Ball-and-Stick Model of the β-Pleated Sheet Construction in Proteins

The side bondage extend above or below the sheet and alternate along the chain. The protein chains are held together by interchain hydrogen bonding.

Tertiary structure refers to the unique three-dimensional shape of the protein as a whole, which results from the folding and bending of the protein backbone. The third construction is intimately tied to the proper biochemical operation of the poly peptide. Effigy 9.four "A Ribbon Model of the Three-Dimensional Structure of Insulin" shows a depiction of the iii-dimensional construction of insulin.

Figure nine.iv A Ribbon Model of the Three-Dimensional Structure of Insulin

The spiral regions represent sections of the polypeptide chain that have an α-helical structure, while the wide arrows stand for β-pleated sheet structures.

Four major types of bonny interactions determine the shape and stability of the tertiary structure of proteins.

1. Ionic bonding. Ionic bonds upshot from electrostatic attractions between positively and negatively charged side bondage of amino acids. For example, the common allure between an aspartic acrid carboxylate ion and a lysine ammonium ion helps to maintain a item folded surface area of a protein (part (a) of Figure nine.5 "Tertiary Poly peptide Construction Interactions").

2. Hydrogen bonding. Hydrogen bonding forms between a highly electronegative oxygen atom or a nitrogen atom and a hydrogen atom attached to another oxygen atom or a nitrogen atom, such as those constitute in polar amino acrid side bondage. Hydrogen bonding (likewise as ionic attractions) is extremely important in both the intra- and intermolecular interactions of proteins (role (b) of Figure 9.five "Tertiary Poly peptide Structure Interactions").

iii. Disulfide linkages. Two cysteine amino acid units may exist brought close together as the protein molecule folds. Subsequent oxidation and linkage of the sulfur atoms in the highly reactive sulfhydryl (SH) groups leads to the germination of cystine (office (c) of Figure 9.5 "Third Protein Structure Interactions"). Intrachain disulfide linkages are found in many proteins, including insulin (yellow bars in Figure nine.i "Principal Structure of Human Insulin") and accept a strong stabilizing effect on the third structure.

four. Dispersion forces. Dispersion forces arise when a normally nonpolar cantlet becomes momentarily polar due to an uneven distribution of electrons, leading to an instantaneous dipole that induces a shift of electrons in a neighboring nonpolar atom. Dispersion forces are weak but can exist important when other types of interactions are either missing or minimal (part (d) of Figure ix.v "Tertiary Protein Structure Interactions"). This is the case with fibroin, the major protein in silk, in which a high proportion of amino acids in the protein take nonpolar side chains. The term hydrophobic interaction is often misused every bit a synonym for dispersion forces. Hydrophobic interactions ascend because water molecules engage in hydrogen bonding with other h2o molecules (or groups in proteins capable of hydrogen bonding). Because nonpolar groups cannot appoint in hydrogen bonding, the protein folds in such a manner that these groups are buried in the interior part of the protein structure, minimizing their contact with water.

Figure  9.5 Tertiary Protein Structure Interactions

4 interactions stabilize the tertiary structure of a protein: (a) ionic bonding, (b) hydrogen bonding, (c) disulfide linkages, and (d) dispersion forces.

When a protein contains more than one polypeptide chain, each concatenation is called a subunit. The system of multiple subunits represents a fourth level of structure, the quaternary construction of a protein. Hemoglobin, with four polypeptide chains or subunits, is the most often cited instance of a protein having quaternary structure (Effigy 9.vi "The Quaternary Structure of Hemoglobin"). The quaternary structure of a poly peptide is produced and stabilized by the aforementioned kinds of interactions that produce and maintain the third structure. A schematic representation of the four levels of protein structure is in Figure 9.seven "Levels of Structure in Proteins".

Figure 9.six The Quaternary Construction of Hemoglobin

Hemoglobin is a protein that transports oxygen throughout the torso.

Source: Prototype from the RCSB PDB (world wide web.pdb.org) of PDB ID 1I3D (R.D. Kidd, H.M. Bakery, A.J. Mathews, T. Brittain, E.N. Baker (2001) Oligomerization and ligand binding in a homotetrameric hemoglobin: two high-resolution crystal structures of hemoglobin Bart's (gamma(4)), a marker for blastoff-thalassemia. Poly peptide Sci. 1739–1749).

Figure 9.7 Levels of Construction in Proteins

The principal structure consists of the specific amino acid sequence. The resulting peptide chain can twist into an α-helix, which is one blazon of secondary structure. This helical segment is incorporated into the tertiary structure of the folded polypeptide chain. The unmarried polypeptide chain is a subunit that constitutes the quaternary construction of a poly peptide, such equally hemoglobin that has four polypeptide chains.

Denaturation of Proteins

The highly organized structures of proteins are truly masterworks of chemical architecture. Merely highly organized structures tend to have a certain delicacy, and this is true of proteins. Denaturation is the term used for whatsoever change in the iii-dimensional structure of a protein that renders information technology incapable of performing its assigned role. A denatured protein cannot exercise its chore. (Sometimes denaturation is equated with the precipitation or coagulation of a protein; our definition is a bit broader.) A wide variety of reagents and conditions, such every bit heat, organic compounds, pH changes, and heavy metal ions tin can cause protein denaturation (Tabular array 9.i "Protein Denaturation Methods").

Table 9.i Poly peptide Denaturation Methods

Method	Effect on Protein Structure
Heat higher up 50°C or ultraviolet (UV) radiation	Rut or UV radiations supplies kinetic energy to poly peptide molecules, causing their atoms to vibrate more rapidly and disrupting relatively weak hydrogen bonding and dispersion forces.
Use of organic compounds, such equally ethyl alcohol	These compounds are capable of engaging in intermolecular hydrogen bonding with protein molecules, disrupting intramolecular hydrogen bonding inside the poly peptide.
Salts of heavy metal ions, such as mercury, silver, and lead	These ions form strong bonds with the carboxylate anions of the acidic amino acids or SH groups of cysteine, disrupting ionic bonds and disulfide linkages.
Alkaloid reagents, such as tannic acid (used in tanning leather)	These reagents combine with positively charged amino groups in proteins to disrupt ionic bonds.

Anyone who has fried an egg has observed denaturation. The articulate egg white turns opaque as the albumin denatures and coagulates. No one has yet reversed that process. However, given the proper circumstances and enough fourth dimension, a poly peptide that has unfolded under sufficiently gentle weather can refold and may again exhibit biological activity (Figure nine.viii "Denaturation and Renaturation of a Protein"). Such evidence suggests that, at least for these proteins, the chief structure determines the secondary and tertiary structure. A given sequence of amino acids seems to prefer its particular iii-dimensional arrangement naturally if conditions are correct.

Effigy ix.8 Denaturation and Renaturation of a Poly peptide

The denaturation (unfolding) and renaturation (refolding) of a protein is depicted. The red boxes correspond stabilizing interactions, such every bit disulfide linkages, hydrogen bonding, and/or ionic bonds.

The primary structures of proteins are quite sturdy. In general, fairly vigorous weather condition are needed to hydrolyze peptide bonds. At the secondary through quaternary levels, however, proteins are quite vulnerable to attack, though they vary in their vulnerability to denaturation. The delicately folded globular proteins are much easier to denature than are the tough, gristly proteins of hair and peel.

Concept Review Exercises

1. What is the predominant attractive forcefulness that stabilizes the formation of secondary structure in proteins?

2. Distinguish between the tertiary and 4th levels of poly peptide structure.

3. Briefly describe four means in which a protein could be denatured.

Answers

1. hydrogen bonding

2. 3rd structure refers to the unique three-dimensional shape of a single polypeptide chain, while quaternary construction describes the interaction between multiple polypeptide chains for proteins that take more than than ane polypeptide chain.

3. (one) heat a protein above fifty°C or expose information technology to UV radiation; (2) add together organic solvents, such every bit ethyl alcohol, to a protein solution; (3) add salts of heavy metal ions, such as mercury, silverish, or lead; and (4) add alkaloid reagents such equally tannic acrid

Fundamental Takeaways

• Proteins tin exist divided into two categories: gristly, which tend to exist insoluble in water, and globular, which are more soluble in water.
• A protein may have up to four levels of structure. The primary structure consists of the specific amino acid sequence. The resulting peptide chain can course an α-helix or β-pleated sheet (or local structures not every bit easily categorized), which is known equally secondary structure. These segments of secondary structure are incorporated into the tertiary structure of the folded polypeptide chain. The fourth construction describes the arrangements of subunits in a protein that contains more than one subunit.
• Four major types of attractive interactions determine the shape and stability of the folded protein: ionic bonding, hydrogen bonding, disulfide linkages, and dispersion forces.
• A wide variety of reagents and conditions can cause a poly peptide to unfold or denature.

Exercises

ane. Classify each poly peptide as fibrous or globular.

a. albumin

b. myosin

c. fibroin

2. Allocate each protein as gristly or globular.

a. hemoglobin

b. keratin

c. myoglobin

3. What name is given to the predominant secondary construction found in silk?

4. What name is given to the predominant secondary construction establish in wool protein?

five. A poly peptide has a 3rd construction formed by interactions betwixt the side chains of the following pairs of amino acids. For each pair, identify the strongest type of interaction betwixt these amino acids.

a. aspartic acid and lysine

b. phenylalanine and alanine

c. serine and lysine

d. two cysteines

half-dozen. A protein has a 3rd structure formed by interactions between the side chains of the following pairs of amino acids. For each pair, place the strongest type of interaction between these amino acids.

a. valine and isoleucine

b. asparagine and serine

c. glutamic acid and arginine

d. tryptophan and methionine

7. What level(s) of poly peptide construction is(are) ordinarily disrupted in denaturation? What level(s) is(are) not?

8. Which class of proteins is more easily denatured—fibrous or globular?

Answers

ane.

a. globular

b. fibrous

c. fibrous

three. β-pleated canvas

v.

a. ionic bonding

b. dispersion forces

c. dispersion forces

d. disulfide linkage

7. Poly peptide denaturation disrupts the secondary, 3rd, and quaternary levels of structure. Just primary construction is unaffected by denaturation.

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Source: https://guides.hostos.cuny.edu/che120/chapter9

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