Proteins and Amino Acids Overview

Questions and Answers

What is a primary structural characteristic of proteins?

They are circular molecules of carbohydrates.

They are linear polymers of amino acids. (correct)

They consist of fatty acid chains.

They are composed of nucleotides.

Which of the following functions is NOT performed by proteins?

Transporting hydrophobic molecules.

Regulating metabolic processes.

Storing genetic information. (correct)

Facilitating cell adhesion.

Which type of protein acts as a signal carrier in the body?

Collagen

Polypeptide hormones (correct)

Structural proteins

Antibodies

What role do contractile proteins play in the body?

They facilitate muscle movement. Signup and view all the answers

In what way do proteins assist in the transport of substances in the body?

By acting as channels in membranes. Signup and view all the answers

Which protein is involved in the transport of oxygen in the blood?

Hemoglobin Signup and view all the answers

What is a function of cell adhesion molecules?

To facilitate cell-to-cell attachment. Signup and view all the answers

Which protein is specifically associated with the deposition of calcium phosphate in bones?

Collagen Signup and view all the answers

What is a primary protective function of proteins?

Protecting against pathogens. Signup and view all the answers

What characteristic of alanine and branched chain amino acids primarily contributes to their clustering within proteins?

Their high degree of hydrophobicity Signup and view all the answers

How do branched chain amino acid side chains primarily interact within proteins?

Via van der Waals forces and hydrophobic interactions Signup and view all the answers

Which property of the side chain of proline impacts the structure of peptide backbones?

The presence of a rigid ring structure Signup and view all the answers

What describes the state of free amino acids at physiologic pH (7.4)?

They exist as zwitterions. Signup and view all the answers

What is significant about the side chains of aromatic amino acids?

Their ring structure allows for variable polarity based on substituents Signup and view all the answers

Which aspect of the side chains of branched chain amino acids contributes to their lipid-like characteristics?

The aliphatic nature and bulkiness of the side chains Signup and view all the answers

What functional role do the side chains of amino acids primarily dictate?

The metabolic pathways of the amino acids. Signup and view all the answers

Which of the following statements is true about glycine?

It causes minimal steric hindrance in proteins. Signup and view all the answers

What occurs due to the unique geometry of proline within proteins?

It prevents the formation of alpha-helices Signup and view all the answers

Why can't the side chains of alanine and branched chain amino acids hydrogen bond with water?

They lack polar functional groups Signup and view all the answers

What is a characteristic of almost all carboxyl and amino groups in amino acids at physiological pH?

They are involved in peptide bonds. Signup and view all the answers

Which classification does glycine belong to?

Nonpolar, aliphatic amino acids. Signup and view all the answers

What force is primarily responsible for the packing of branched chain amino acids in close proximity?

Van der Waals forces between closely packed atoms Signup and view all the answers

What type of interaction do aromatic amino acid side chains engage in, dictated by their substituents?

Hydrophobic interactions based on polarity Signup and view all the answers

What is the predominant ionization state of the carboxyl group in free amino acids at pH 7.4?

Dissociated as a carboxylate ion (-COO-) Signup and view all the answers

What effect do the bulky side chains of branched chain amino acids have on protein structure?

They create hydrophobic cores that stabilize protein folding Signup and view all the answers

What role does the structural feature of amino acid side chains influence in proteins?

The folding and stability of proteins. Signup and view all the answers

What characteristic of glycine allows its frequent occurrence in certain protein structures?

Its minimal steric hindrance. Signup and view all the answers

Which property of amino acids aids in classifying them based on polarity?

The nature of the side chains. Signup and view all the answers

At physiological pH, what is the charge state of the amino group in free amino acids?

Protonated (-NH3+). Signup and view all the answers

What occurs to carboxylic acid groups as pH values decrease significantly below the pKa?

All of the carboxylic acid groups are protonated. Signup and view all the answers

At what pH do 50% of the molecules exist as carboxylate anions?

At pKa Signup and view all the answers

Which amino acid side chains are categorized as basic?

Histidine and lysine Signup and view all the answers

What happens to the carboxyl group of alanine at low pH?

It remains fully protonated. Signup and view all the answers

Which statement about the amine group (-NH3+) of amino acids is correct?

It is a weaker acid and has a smaller dissociation constant. Signup and view all the answers

How is isoelectric form represented for amino acids during titration?

The carboxyl group dissociates to form a dipolar zwitterion. Signup and view all the answers

What is the primary role of methionine in metabolism?

Transfers a methyl group to other compounds Signup and view all the answers

Which amino acid contains a sulfhydryl group that is important for forming disulfide bonds?

Cysteine Signup and view all the answers

At physiological pH, how is the side chain of aspartate typically charged?

Negatively charged Signup and view all the answers

What characterizes the side chains of aspartate and glutamate?

They are proton donors with negatively charged groups Signup and view all the answers

What is the significance of cystine in human physiology?

It plays a role in holding polypeptide chains together Signup and view all the answers

What happens to cysteine's side chain at an alkaline pH?

It loses a proton and becomes charged Signup and view all the answers

How can a cysteine molecule in solution affect other cysteine molecules?

By forming covalent disulfide bonds through oxidation Signup and view all the answers

Which sulfur-containing amino acid is classified as nonpolar?

Methionine Signup and view all the answers

What ionic interaction can aspartate and glutamate participate in?

Ionic bonds with positively charged amino acids Signup and view all the answers

What is the pKa of cysteine's sulfhydryl group for dissociation of hydrogen?

8.4 Signup and view all the answers

Study Notes

Overview of Proteins

Proteins are the most abundant biomolecules essential for every life process.
Composed of linear polymers formed by amino acids.
Functions include:
- Hormonal signal transmission.
- Catalyzing biochemical reactions as enzymes.
- Transporting hydrophobic compounds (e.g., hemoglobin, plasma albumin).
- Cell adhesion to extracellular matrix.
- Forming ion channels in lipid membranes.
- Facilitating movement through contractile proteins (muscles).
- Providing framework for calcium phosphate deposition (collagen in bone).
- Immune protection through immunoglobulins.

Structure of Amino Acids

20 different amino acids.
At physiological pH (7.4), amino acids exist as zwitterions:
- Carboxyl group loses a proton to become negatively charged.
- Amino group gains a proton, becoming positively charged.
Most functional groups involved in peptide bonds are non-reactive aside from hydrogen bonding.
The character of side chains determines the functional roles of amino acids in proteins.

Classification of Amino Acid Side Chains

Amino acids classified based on polarity and structure.

Nonpolar, Aliphatic Amino Acids

Glycine: Simplest amino acid, minimal steric hindrance; common in tight protein structures.
Alanine & Branched Chain Amino Acids (Valine, Leucine, Isoleucine):
- These have bulky hydrophobic side chains, promoting hydrophobic core formation in proteins.
- Hydrophobic interactions are enhanced by van der Waals forces.
Proline: Rigid structure causes kinks in peptide chains; crucial for collagen structure.

Aromatic Amino Acids

Side chains contain aromatic rings that impact polarity.
Aromatic structure consists of six-membered carbon-hydrogen rings with conjugated double bonds.
Threonine: Contains hydroxyl group, serves as attachment site for oligosaccharides in glycoproteins.

Sulfur-Containing Amino Acids

Methionine: Nonpolar, bulky with hydrophobic side chain, pivotal for metabolism due to methyl group transfer.
Cysteine: Critical for enzyme active sites; features a thiol group enabling disulfide bond formation (cystine).

Acidic and Basic Amino Acids

Aspartate and Glutamate:
- Function as proton donors, fully ionized at neutral pH.
- Form ionic bonds with positively charged molecules (lysine, arginine, histidine).
At low pH: Protonated carboxylic groups; at neutral pH: Mostly dissociated, negatively charged.

Amino Acids with Ionizable Groups

Acidic: Aspartate and Glutamate.
Basic: Histidine, Lysine, Arginine.

Titration of an Amino Acid

Dissociation of carboxyl group (-COOH) explains protonation states at different pH.
Applications of the Henderson-Hasselbalch equation for calculating pH and dissociation constants.
The amino group (-NH3+) is a weaker acid compared to carboxyl groups.

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Description

Explore the essential role of proteins and amino acids in biological processes. This quiz covers the structural features of proteins, their functions, and their significance in metabolism. Test your knowledge about these vital molecules that support life.