39 Questions
Which of the following is a characteristic of polar amino acids?
They have a zero net charge at neutral pH
What is the purpose of disulfide bonds in many extracellular proteins?
To stabilize the protein structure
Which of the following is a characteristic of nonpolar (hydrophobic) amino acid side chains?
They are repelled by water molecules
What is the significance of the asymmetric carbon atom in amino acids?
It determines the optical activity of the amino acid
Which of the following is the correct way to represent the formation of a peptide bond between two amino acids?
The carboxyl group of one amino acid reacts with the amino group of another amino acid
What is the defining characteristic of alpha-amino acids?
They have an amino group attached to the alpha-carbon
Which of the following amino acids has a nonpolar, aliphatic side chain?
Valine
Which amino acid is classified as both nonessential and conditional?
Arginine
What is the primary covalent bond that links amino acids together in a polypeptide chain?
Peptide bond
Which of the following statements about the side chains of alpha amino acids is correct?
The side chain is always attached to the alpha carbon
Which amino acid has an aromatic side chain?
Phenylalanine
Which of the following amino acids has a sulfur-containing side chain?
Cysteine
What is the primary determinant of an amino acid's unique biochemical properties?
The side chain (R group)
Which of the following statements about peptide linkages is correct?
Peptide linkages are formed between the carboxyl and amino groups of adjacent amino acids.
Which of the following is true about alpha amino acids?
They have the amino group bonded directly to the alpha carbon.
At physiological pH (pH=7.4), what is the state of the carboxyl and amino groups in amino acids?
The carboxyl group is dissociated, forming a negatively charged carboxylate ion, and the amino group is protonated.
Which of the following amino acids would be classified as nonpolar?
Alanine
What is the general formula for amino acids?
NH2-CHR-COOH
Which of the following is not a key function of amino acids in the body?
Storing genetic information
Which type of side chain is typically found in nonpolar amino acids?
Aliphatic side chains
Which of the following is not a common classification of amino acids?
Cationic amino acids
What type of chemical bond links the amino acids together in a polypeptide chain?
Peptide bond
Which of the following is not a characteristic of alpha-amino acids?
Have a linear structure with the amino and carboxyl groups at opposite ends
What is the primary function of the side chains in amino acids?
To determine the chemical properties of the amino acid
Which of the following statements about non-polar amino acid side chains is correct?
They are lipid-like in nature and promote hydrophobic interactions within proteins.
What is the primary factor that determines the role an amino acid plays in a protein?
The properties of the amino acid side chain.
Which of the following categories are amino acids typically classified into based on their side chain properties?
Polar, non-polar, basic, and acidic.
What is the primary function of hydrophobic interactions in protein structure?
To promote the folding of proteins into their three-dimensional shapes.
Which of the following statements about hydrophobes is correct?
They are non-polar molecules with long carbon chains that do not interact with water.
Which of the following is a characteristic of amino acids with polar side chains?
They have an uneven distribution of electrons and can form hydrogen bonds.
Which of the following statements about amino acid side chains (R groups) is correct?
The chemical nature of the side chain determines the unique properties and function of each amino acid.
Which of the following best describes nonpolar, hydrophobic side chains?
They are repelled by water molecules and tend to cluster together within a protein.
What is the primary factor that determines whether an amino acid is classified as essential or nonessential?
The ability of the human body to synthesize it.
Which of the following statements accurately describes the peptide linkage between amino acids in a polypeptide chain?
It is a covalent bond formed by the removal of a water molecule between the amino and carboxyl groups.
Which of the following statements about alpha-amino acids is true?
They have both an amino and a carboxyl group attached to the same carbon atom.
What is the primary structural difference between a peptide and a polypeptide?
A peptide is a short chain of amino acids, while a polypeptide is a longer chain.
Which of the following statements about conditional amino acids is correct?
They are only required in certain medical conditions or stages of life.
What is the primary role of peptide bonds in the structure of proteins?
They link the individual amino acids together in a linear chain.
Which of the following statements about stereoisomers or optical isomers is true?
They have different arrangements of atoms in the molecule and different properties.
Study Notes
Amino Acids
- Amino acids are organic compounds that contain amine (-NH2) and carboxyl (-COOH) functional groups, along with a side chain (R group) specific to each amino acid.
- At physiologic pH (pH=7.4), the carboxyl group is dissociated, forming a negatively charged carboxylate ion, and the amino group is protonated.
- The carbon atom next to the carboxyl group (which is therefore numbered 2 in the carbon chain starting from that functional group) is called the α–carbon.
Classification of Amino Acids
- Amino acids are classified into four categories based on the properties of their side chains: non-polar, polar, acidic, and basic.
- Non-polar amino acids have an even distribution of electrons and do not gain or lose protons or participate in hydrogen or ionic bonds.
- Polar amino acids have an uneven distribution of electrons, such as acids and bases.
- Acidic amino acids have acidic side chains.
- Basic amino acids have basic side chains.
Essential and Non-Essential Amino Acids
- Essential amino acids cannot be made by the body and must come from food.
- There are 9 essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
- Non-essential amino acids are produced by the body and include: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.
- Conditional amino acids are usually not essential, except in times of illness and stress, and include: arginine, cysteine, glutamine, tyrosine, glycine, ornithine, proline, and serine.
Protein Structure
- A polypeptide chain is a series of amino acids joined together in a specific sequence.
- Each amino acid unit in a polypeptide is called a residue.
- Proteins are long chains of amino acids held together by peptide bonds.
- A peptide is two or more amino acids joined together by peptide bonds.
Hydrophobic Interactions
- Hydrophobic interactions describe the relations between water and hydrophobes (low water-soluble molecules).
- Hydrophobes are nonpolar molecules and usually have a long chain of carbons that do not interact with water molecules.
- Non-polar side chains fill up the interior of the folded protein and help give it its three-dimensional shape.
Optical Properties of Amino Acids
- Amino acids that have an asymmetric center at the α–carbon can exist in two forms, designated D and L.
- The two forms in each pair are termed stereoisomers, optical isomers, or enantiomers.
Test your knowledge on essential and nonessential amino acids with this quiz. Learn about the 9 essential amino acids that must be obtained from food and the nonessential amino acids that our bodies can produce. Identify the differences between essential and nonessential amino acids.
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