Unit 4: Amino Acids and Proteins - Subjective Questions

Amino acids are organic compounds that contain both an amino group and a carboxyl group. Most amino acids found in proteins have the general structure , where represents the variable side chain.

• The central carbon is called the -carbon.
• It is attached to an amino group, a carboxyl group, a hydrogen atom, and a variable group.
• The group determines the identity, size, polarity, charge, and chemical reactivity of the amino acid.
• At physiological pH, amino acids generally exist as zwitterions: .
• The side chain influences protein folding, solubility, catalytic activity, and interactions with other molecules.

Most amino acids, except glycine, possess a chiral -carbon because it is attached to four different groups.

• L-amino acids: In the Fischer projection, the amino group is placed on the left. Nearly all amino acids incorporated into proteins are L-amino acids.
• D-amino acids: In the Fischer projection, the amino group is placed on the right. They occur in some bacterial cell walls and specialized peptides.
• Glycine is achiral because its side chain is another hydrogen atom.
• L- and D-amino acids are enantiomers and have identical physical properties in achiral environments but differ in their interactions with enzymes and receptors.
• The biological preference for L-amino acids is essential for the uniform structure and function of proteins.

Amino acids can be classified according to the chemical nature of their side chains:

• Nonpolar amino acids: Their side chains are hydrophobic. Examples include glycine, alanine, valine, leucine, isoleucine, methionine, and proline.
• Polar uncharged amino acids: Their side chains can form hydrogen bonds but do not carry a net charge at physiological pH. Examples include serine, threonine, cysteine, asparagine, glutamine, and tyrosine.
• Acidic amino acids: They contain additional carboxyl groups and are negatively charged at physiological pH. Examples are aspartate and glutamate.
• Basic amino acids: They contain basic side chains and are positively charged or partly protonated at physiological pH. Examples are lysine, arginine, and histidine.

This classification helps explain protein solubility, folding, and interactions with other molecules.

Amino acids are classified nutritionally according to whether the human body can synthesize them:

• Essential amino acids: These cannot be synthesized in sufficient amounts and must be obtained from the diet. Examples include leucine, isoleucine, valine, lysine, methionine, threonine, tryptophan, and phenylalanine.
• Nonessential amino acids: These can be synthesized by the body in adequate amounts. Examples include alanine, aspartate, glutamate, and serine.
• Conditionally essential amino acids: These are normally synthesized by the body but become necessary in the diet during illness, trauma, rapid growth, or metabolic stress. Examples include arginine, cysteine, glutamine, glycine, proline, and tyrosine.

The classification depends on physiological conditions and nutritional requirements.

A zwitterion is a molecule containing both a positively charged group and a negatively charged group while having no net charge.

For a simple amino acid, the ionic forms change with pH:

• In a strongly acidic medium, the amino group is protonated and the carboxyl group is largely undissociated: .
• Near neutral pH, the carboxyl group loses a proton while the amino group remains protonated: . This is the zwitterionic form.
• In a strongly alkaline medium, the amino group also loses its proton: .

The predominant ionic form depends on the pH and the values of the ionizable groups.

The isoelectric point, represented by , is the pH at which an amino acid or protein has zero net electrical charge.

• At the , the zwitterionic form is usually predominant.
• The molecule shows minimum movement in an electric field.
• Solubility is often lowest at the isoelectric point, so precipitation may occur.
• For a neutral amino acid, the isoelectric point is calculated using the two relevant ionization constants:
  
• For acidic or basic amino acids, the two values surrounding the neutral species are used.

The concept of is important in protein purification, electrophoresis, and formulation of biological products.

A titration curve shows the change in pH of an amino acid solution as a strong acid or base is added.

• At very low pH, the amino acid is mainly in the fully protonated cationic form.
• As base is added, the carboxyl group loses a proton near its .
• The zwitterion becomes the predominant form between the two dissociation steps.
• At higher pH, the amino group loses a proton and the anionic form predominates.
• The curve contains relatively flat regions near each . These are buffering regions, where the amino acid resists changes in pH.
• The midpoint of each buffering region corresponds approximately to a value.
• The equivalence points occur when one ionizable proton has been removed in nearly stoichiometric quantity.

Consider a neutral amino acid with two ionizable groups: a carboxyl group with and an amino group with .

• At low pH, the molecule is positively charged.
• On addition of base, the carboxyl group dissociates first, producing the zwitterion with zero net charge.
• Further addition of base causes the amino group to dissociate, producing the negatively charged form.
• Therefore, the neutral zwitterion lies between the two dissociation steps.
• The isoelectric point is the average of the two values flanking the zwitterion:
  

For example, if and , then:

Thus, the amino acid has zero net charge at approximately pH .

The Henderson–Hasselbalch equation relates pH to the of a weak acid and the ratio of its conjugate base to acid forms:

For an amino acid carboxyl group, the equation can be written as:

For an amino group, it can be written as:

• When , the concentrations of the protonated and deprotonated forms are equal.
• Amino acids buffer most effectively within approximately one pH unit of their .
• This buffering property helps biological systems resist sudden pH changes.

A peptide bond forms between the carboxyl group of one amino acid and the amino group of another amino acid through a condensation reaction.

The resulting linkage is called a peptide bond.

• The bond is covalent and planar because of partial double-bond character caused by resonance.
• Rotation around the peptide bond is restricted.
• A chain with two amino acids is a dipeptide, while longer chains are polypeptides.
• Polypeptide sequences are written from the amino-terminal, or -terminal, end to the carboxyl-terminal, or -terminal, end.
• The sequence of amino acids determines the higher levels of protein structure and function.

Proteins are large biological macromolecules composed of one or more polypeptide chains made from amino acids linked by peptide bonds.

Major functions include:

• Catalysis: Enzymes accelerate biochemical reactions.
• Structure: Collagen, keratin, and elastin provide support and strength.
• Transport: Hemoglobin transports oxygen, while membrane proteins transport ions and molecules.
• Movement: Actin and myosin participate in muscle contraction.
• Defense: Antibodies recognize and neutralize foreign substances.
• Regulation: Hormones and receptors control cellular processes.
• Storage: Ferritin stores iron, and some proteins store amino acids.
• Signal transduction: Receptors transmit information across cell membranes.

Protein function depends on its three-dimensional structure and chemical properties.

The primary structure of a protein is the specific linear sequence of amino acids in its polypeptide chain.

• Amino acids are joined by peptide bonds.
• The sequence is conventionally written from the -terminus to the -terminus.
• It determines the positions of charged, polar, nonpolar, and special residues such as cysteine and proline.
• The primary sequence directs the formation of secondary, tertiary, and quaternary structures.
• Even a single amino acid substitution can alter folding, stability, or activity.
• For example, a change in the amino acid sequence of hemoglobin can affect red blood cell shape and oxygen transport.

Therefore, the primary structure contains the information required for the final functional conformation of a protein.

The secondary structure of a protein refers to regularly repeating local conformations of the polypeptide backbone, stabilized mainly by hydrogen bonds between backbone carbonyl and amide groups.

• Alpha helix: The polypeptide backbone coils into a right-handed helix. Hydrogen bonds form between the carbonyl oxygen of one peptide bond and the amide hydrogen about four residues away. Side chains project outward.
• Beta sheet: Extended polypeptide segments called beta strands are arranged side by side and stabilized by hydrogen bonds. Sheets may be parallel, antiparallel, or mixed.
• Turns and loops: These connect helices and sheets and often occur on the protein surface.
• Proline can disrupt alpha helices, while glycine provides flexibility.

Secondary structures contribute to protein strength, flexibility, and overall folding.

The tertiary structure is the complete three-dimensional conformation of a single polypeptide chain.

It is stabilized by several interactions:

• Hydrophobic interactions: Nonpolar side chains cluster away from water, usually inside the protein.
• Hydrogen bonds: These form between polar side chains or between side chains and backbone groups.
• Ionic interactions: Oppositely charged side chains form salt bridges.
• Disulfide bonds: Oxidation of two cysteine residues produces a covalent bond.
• Van der Waals forces: Close contacts between atoms contribute to stability.

The tertiary structure creates active sites, binding pockets, and functional surfaces. Changes in environmental conditions can disrupt these interactions and impair function.

The quaternary structure of a protein is the arrangement and interaction of two or more polypeptide subunits in a functional protein complex.

• Each subunit has its own primary, secondary, and tertiary structure.
• Subunits are held together by hydrophobic interactions, hydrogen bonds, ionic interactions, and sometimes disulfide bonds.
• Hemoglobin is a tetramer composed of two alpha and two beta subunits.
• A protein composed of only one polypeptide chain does not have quaternary structure, even though it may possess primary, secondary, and tertiary structures.
• Quaternary organization can produce cooperativity, allosteric regulation, and increased functional efficiency.

Thus, quaternary structure describes the organization of multiple folded chains rather than the folding of one chain.

The four levels of protein organization are related but distinct:

• Primary structure: The linear amino acid sequence joined by peptide bonds.
• Secondary structure: Local arrangements such as alpha helices, beta sheets, and turns, stabilized mainly by backbone hydrogen bonds.
• Tertiary structure: The complete three-dimensional folding of one polypeptide chain, stabilized by side-chain interactions and disulfide bonds.
• Quaternary structure: The association of multiple folded polypeptide subunits into one functional protein.

The primary structure determines the possible higher-level structures. Secondary and tertiary structures create the protein's three-dimensional shape, while quaternary structure is present only when multiple subunits associate. Disruption of higher-level interactions can cause loss of activity without necessarily breaking peptide bonds.

Fibrous and globular proteins differ in shape, solubility, and biological roles.

• Fibrous proteins:

  • Have elongated, repetitive structures.
  • Usually contain extensive secondary structure.
  • Are generally insoluble in water.
  • Provide mechanical strength and support.
  • Examples include collagen, keratin, and fibroin.

• Globular proteins:

  • Fold into compact, roughly spherical structures.
  • Have complex tertiary or quaternary structures.
  • Are often soluble in water or aqueous solutions.
  • Perform dynamic functions such as catalysis, transport, regulation, and defense.
  • Examples include enzymes, hemoglobin, and antibodies.

The difference arises mainly from the distribution of hydrophobic and hydrophilic amino acid residues.

Denaturation is the disruption of the native three-dimensional structure of a protein without extensive hydrolysis of its peptide bonds.

Common causes include:

• Heat: Increases molecular motion and disrupts weak interactions.
• Extreme pH: Alters the ionization of amino acid side chains and breaks salt bridges.
• Organic solvents: Interfere with hydrophobic interactions.
• Detergents: Disrupt hydrophobic interactions and membranes.
• Chaotropic agents: Urea and guanidinium ions disrupt hydrogen bonding and hydrophobic effects.
• Heavy metal ions: Bind to functional groups, especially sulfhydryl groups.
• Mechanical agitation: May unfold proteins at air–liquid interfaces.

Denaturation commonly affects secondary, tertiary, and quaternary structures while leaving the primary structure largely intact.

Denaturation changes the native conformation of a protein and usually causes loss of biological activity.

• Hydrogen bonds, ionic interactions, hydrophobic interactions, and other noncovalent forces may be disrupted.
• The protein may unfold and expose normally buried hydrophobic residues.
• Solubility may decrease, causing aggregation or precipitation.
• Active sites and binding sites may lose their correct shape.
• Enzymes may no longer bind substrates or catalyze reactions.
• Peptide bonds are generally not broken during ordinary denaturation, so primary structure remains intact.
• Denaturation can be reversible or irreversible, depending on the protein and the severity of treatment.

For example, heating egg albumin causes irreversible denaturation and coagulation.

Renaturation is the restoration of a protein's native conformation and biological activity after removal of the denaturing agent.

Renaturation is possible when:

• The primary amino acid sequence remains intact.
• The denaturing treatment has not caused irreversible chemical changes.
• The denaturing agent is removed gradually or under suitable conditions.
• Temperature, pH, ionic strength, and solvent conditions are returned to appropriate values.
• Correct disulfide bonds are re-formed when necessary.

The process demonstrates that the amino acid sequence contains information for proper folding. However, many proteins aggregate during unfolding or require molecular chaperones, so renaturation is not always successful. Ribonuclease A is a classic example of a protein capable of renaturation under suitable conditions.