Chapter 3: Amino Acid

Question 1

All amino acids are precursors to
other —

Answer 1

biomolecules

Question 2

Fischer projection

Answer 2

In a Fischer projection, every atom is identified and the bonds to the central carbon atom are represented by horizontal and vertical lines. By convention, the horizontal
bonds are assumed to project out of the page toward the viewer, whereas the vertical bonds are assumed to project behind the page away from the viewer.

Question 3

An α-amino acid contains: (5)

Aka the 20 amino acids

Answer 3

• a central carbon atom, called the α carbon,
• an amino group
• a carboxylic acid group
• a hydrogen atom
• a side chain, called the R group

Question 4

With four different groups connected to the tetrahedral αcarbon atom, α-amino acids are —- and may exist ….

Answer 4

• chiral
• they may exist in one or the other of two mirror-image forms, called the L isomer and the D isomer

Question 5

The L and D isomers are —– of each other

Answer 5

mirror images

Question 6

Only —- amino acids are constituents of proteins

Answer 6

L

Question 7

Free amino acids in solution at neutral pH exist predominantly as —-

Answer 7

dipolar ions (also called zwitterions)

Question 8

Dipolar ions (also called zwitterions) forms:

Answer 8

In the dipolar form, the amino group NH3+ is protonated and the carboxyl group COO- is deprotonated.

Question 9

Acids ___ hydrogen ions

Answer 9

donates

Question 10

Explain what happens to an amino acid when its in acid

Answer 10

• the amino group is protonated NH3+ and the carboxyl group is not dissociated (-COOH)

Question 11

As the PH of the amino acid solution is raised…

Answer 11

• ## As the pH is raised, the carboxylic acid is the first group to give up a proton, because its pka is near 2.

Question 12

The dipolar form persists until—-

Answer 12

• the pH approaches 9, when the protonated amino group loses a proton. Under physiological conditions, amino acids exist in the dipolar form.

Question 13

Under physiological conditions, amino acids exist in the — form at a PH of —-

Answer 13

• dipolar/ zwitterionic
• 7.4

Question 14

Twenty kinds of side chains varying in size, shape, charge,
hydrogen-bonding capacity, hydrophobic character, and chemical reactivity are commonly found in proteins. Many of these properties are conferred by ——

Answer 14

• functional groups

Question 15

Although there are many ways to classify amino acids, we
will assort these molecules into four groups on the basis of the general chemical characteristics of their R groups (4):

Answer 15

1. Hydrophobic amino acids with nonpolar R groups
2. Polar amino acids with neutral R groups but the charge is not evenly distributed
3. Positively charged amino acids with R groups that have a positive charge at physiological PH
4. Negatively charged amino acids with R groups that have a negative charge at physiological pH

Question 16

hydrophobic amino acids (10)

What they are + who they consist of

Answer 16

• The amino acids having side chains consisting only of hydrogen and carbon
• phenylalanine, leucine, isoleucine, alanine, glycine, tryptophan, valine, methionine, and proline,

Question 17

The amino acids are all chiral, with the exception of —, whose side chain is —.

Answer 17

• glycine
• H

Question 18

Larger aliphatic side chains are found in the branched-chain amino acids (4)

Answer 18

• valine
• leucine
• isoleucine
• Methionine

Question 19

Why is proline special (2)?

side chain+ amino group

Answer 19

• Proline is unique in that it is the only amino acid where the side chain is connected to the protein backbone twice (both the α-carbon and the nitrogen atom), forming a five-membered nitrogen-containing ring.
• its amino group, through which it links to the other amino acids, is a secondary amine, rather than a primary amine group (−NH2), as in the other nineteen amino acids.

Question 20

The —-isomer of proline, is the only form that is involved in protein synthesis

Answer 20

L

Question 21

The hydrophobic amino acids, particularly the larger
aliphatic and aromatic ones, tend to —-

Answer 21

cluster together inside the protein away from the aqueous environment of the cell.

Question 22

Name the amino acids that contain hydroxyl groups (-OH) (3):

Answer 22

—serine, threonine, and tyrosine

Question 23

The hydroxyl groups on serine, threonine, and tyrosine make them more hydrophilic (water-loving) and reactive than their respective nonpolar counterparts ——.

Answer 23

alanine, valine, and phenylalanine

Question 24

Cysteine vs Serine (2)

struturally+ the difference causes

Answer 24

• Cysteine is structurally similar to serine but contains a sulfhydryl (-SH), or thiol group in place of the hydroxyl group.
• The sulfhydryl group is much more reactive than a hydroxyl group and can completely lose a proton at slightly basic pH to form the reactive thiolate group

Question 25

the set of polar amino acids includes —- and —-, which contain a terminal carboxamide

Answer 25

asparagine and glutamine

Question 26

Name all the polar amino acids (6)

Answer 26

• serine (Ser)
• threonine (Thr)
• cysteine (Cys)
• asparagine (Asn)
• glutamine (Gln)
• tyrosine (Tyr)

Question 27

What characteristics do lysine and arginine share?

Answer 27

• have long side chains that terminate with groups that are positively charged at neutral pH

Question 28

Histidine properties:

Answer 28

• contains an imidazole group, an aromatic ring that also can be positively charged
• the imidazole group of histidine is unique in that it can be uncharged or positively charged near neutral pH, depending on its local environment

Question 29

Negatively Charged Amino Acids Have —- Side Chains

Answer 29

Acidic

Question 30

Negative amino acids (2):

List em

Answer 30

aspartic acid and glutamicacid

In some proteins, these side chains accept protons, which neutralize the negative charge.

Question 31

—– are positively charged at neutral pH.

Answer 31

Lysine, arginine, and histidine

Question 32

Seven of the 20 amino acids——have readily ionizable side chains. These seven amino acids are able to form
— bonds as well as to donate or accept protons to facilitate reactions

Answer 32

• tyrosine
• cysteine
• arginine
• lysine
• histidine
• aspartic acid
• glutamic acid
• ionic

Question 33

acid–base catalysis

Answer 33

The ability to donate or accept protons

Question 34

Essential amino acids

Answer 34

Amino acids that cannot be generated in the body and must be supplied by the diet

Question 35

Lower the pka the more

Answer 35

acidic

Question 36

At a pH below the pKa for each functional group on the amino acid, the functional group is —–. At a pH above the pKa for the functional group it is —–. If the pH equals the pKa, the functional group is —–.

Answer 36

• protonated
• deprotonated
• 50% protonated and 50% deprotonated

Question 37

How do you differentiate between D and L isomer?

Answer 37

1. Look down the H-C (alpha) bond
2. Clockwise CORN = L isomer

Question 38

PKa of carboxyl group

Question 39

Lysine pka of positive charge

Answer 39

Question 40

Arginine pka of positive charge

Answer 40

12.5

Question 41

How to find Isoelectric point (pI)

Answer 41

Question 42

Which amino acid side chains are capable of ionization

Answer 42

• lysine
• histidine
• arginine
• asparatate
• glutamate
• cysteine
• tyrosine