Chapter 2 - Protein Composition and Structure

Question 1

Proteins are linear polymers built of monomer units called

Answer 1

amino acids.

Question 2

Primary structure of a protein

Answer 2

The sequence of amino acids linked end to end.

Question 3

Secondary structure of a protein

Answer 3

The 3d structure formed by hydrogen bonds between amino acids near one another.

Question 4

Tertiary structure of a protein

Answer 4

Formed by long-range interactions between amino acids.

Question 5

Enzymes

Answer 5

The proteins that catalyze specific chemical reactions in biological systems.

Question 6

Zwitterions

Answer 6

A molecule or ion having separate positively and negatively charged groups.

Question 7

The R group of an amino acid is often referred to as the

Answer 7

side chain.

Question 8

Which isomer of amino acids are constituents of proteins?

Answer 8

L amino acids

Question 9

For all amino acids, the L isomer has ___ absolute configuration.

Answer 9

S

Question 10

In acidic solution the amino group of an amino acid is

Answer 10

protonated (-NH₃⁺).

Question 11

In acidic solution, the carboxyl group of the amino acid is

Answer 11

not dissociated (-COOH).

Question 12

As the pH is raised, the first group to give up a proton in an amino acid is the

Answer 12

carboxylic acid.

Question 13

The protonated amino group in an amino acid loses a proton around pH

Answer 13

9

Question 14

The only achiral amino acid is

Answer 14

glycine.

Question 15

Which amino acids have more than 1 chiral center?

Answer 15

Isoleucine and threonine.

Question 16

How many amino acids have readily ionizable side chains?

Answer 16

7

Question 17

Peptide bond (or amide bond)

Answer 17

Formed by linking the alpha-carboxyl group of one amino acid to the alpha-amino group of another amino-acid.

Question 18

The formation of a dipeptide is accompanied by the loss of a

Answer 18

water molecule.

Question 19

The lifetime of a peptide bond in aqueous solution in the absence of a catalyst approaches ______ years.

Answer 19

1000

Question 20

Each amino acid unit in a polypeptide is called a

Answer 20

residue.

Question 21

Main chain (or backbone)

Answer 21

Regularly repeating part of a polypeptide chain.

Question 22

The polypeptide backbone is rich in ______ bonding potential.

Answer 22

Hydrogen

Question 23

The largest known polypeptide is ______ which consists of more than ______ amino acids.

Answer 23

Titin; 27000

Question 24

Oligopeptides

Answer 24

Peptide chains made of small numbers of amino acids.

Question 25

The most common cross links are

Answer 25

disulfide bonds.

Question 26

Disulfide bonds form by

Answer 26

the oxidation of a pair of cysteine residues.

Question 27

Cystine

Answer 27

The result of two linked cysteines.

Question 28

The amino acid sequences of proteins are determined by

Answer 28

the nucleotide sequences of genes.

Question 29

A peptide bond is essentially planar or nonplanar?

Answer 29

planar

Question 30

A peptide bond cannot rotate because

Answer 30

the bond resonates between a single and double bond with the C=O group.

Question 31

Almost all peptide bonds are (cis or trans)

Answer 31

trans.

Question 32

The most common cis peptide bonds are

Answer 32

X-Pro linkages

Question 33

phi

Answer 33

The angle of rotation about the bond between the nitrogen and the alpha-carbon atoms.

Question 34

psi

Answer 34

The angle of rotation about the bond between the alpha-carbon and the carbonyl carbon atoms.

Question 35

The phi and psi angles determine

Answer 35

the path of the polypeptide chain.

Question 36

Some diseases resulting from improperly folded proteins.

Answer 36

• Parkinson disease
• Huntington disease
• transmissible spongiform encephalopathies (prion disease)

Question 37

Alpha helices, beta strands, and turns are formed by

Answer 37

a regular patter of hydrogen bonds between the peptide N-H and C=O groups of amino acids near each other in the linear sequence.

Question 38

The alpha helix is stabilized by

Answer 38

hydrogen bonds between the NH and CO groups of the main chain.

Question 39

The CO group of each amino acid forms a hydrogen bond with the NH group of the amino acid that is ______ residues ahead in the sequence.

Answer 39

four

Question 40

Number of amino acids residues per turn in an alpha helix.

Answer 40

3.6

Question 41

The pitch of the alpha helix is the

Answer 41

length of one complete turn along the helix axis.

Question 42

The pitch of the alpha helix is equal to

Answer 42

the product of the rise (1.5 A) and the number of residues per turn (3.6), or 5.4 Angstroms.

Question 43

Essentially, all alpha helixes formed in proteins are ______ handed.

Answer 43

right

Question 44

Beta sheets are stabilized by

Answer 44

hydrogen bonding between polypeptide strands.

Question 45

A beta sheet is formed by

Answer 45

linking two or more beta strands lying next to one another through hydrogen bonds.