Ch 4 pH Buffer and Amino Acids

Question 1

What does K_a represent?

Answer 1

A numerical value of acid strength

K_a is a measure of the dissociation of an acid into hydrogen ion and conjugate base when dissolved in water.

Question 2

What is the formula for K_a?

Answer 2

K_a = [H⁺][A⁻]/[HA]

This formula represents the concentrations of hydrogen ions, conjugate base, and undissociated acid.

Question 3

How does the K_a value relate to acid strength?

Answer 3

The larger the K_a value, the stronger the acid.

Question 4

What is pK_a?

Answer 4

pK_a = -log K_a

pK_a is often used to express the strength of an acid.

Question 5

What does a lower pK_a value indicate?

Answer 5

The stronger the acid.

Question 6

What is the relationship between pH and pK_a?

Answer 6

pH can be altered by adding a base or acid.

Question 7

What is a buffer?

Answer 7

A weak acid whose pH resists change upon addition of either more acid or more base.

Question 8

What components make up a buffer?

Answer 8

A weak acid and its conjugate base.

Question 9

What is the common buffer found in cells?

Answer 9

H₂PO₄⁻/HPO₄²⁻

This buffer is critical for maintaining the pH of biological fluids.

Question 10

What happens when acid is added to a buffer?

Answer 10

The buffer binds H⁺ ions.

Question 11

What happens when a base is added to a buffer?

Answer 11

The buffer releases H⁺ ions.

Question 12

What is buffer capacity?

Answer 12

Buffers do not have unlimited ability to withstand pH change if large amounts of acid or base are added.

Question 13

When is a buffer’s maximum capacity reached?

Answer 13

When pK_a is equal to the pH, the acid is 50% dissociated.

Question 14

How does pH relate to pK_a in the Henderson-Hasselbalch equation?

Answer 14

If [A⁻] > [HA], then pH > pK_a; if [A⁻] < [HA], then pH < pK_a.

Question 15

What is the effective buffer range?

Answer 15

A buffer is effective in a range of about 1 pH unit above or below the pK_a.

Question 16

What is the principal buffer in blood?

Answer 16

H₂CO₃/HCO₃⁻.

Question 17

What can hyperventilation result in?

Answer 17

Increased blood pH.

Question 18

What can hypoventilation result in?

Answer 18

Decreased blood pH.

Question 19

What is the structure of an amino acid?

Answer 19

Contains an α-carboxyl group, an α-amino group, and an α-carbon.

Question 20

What distinguishes one amino acid from another?

Answer 20

The R group.

Question 21

How many different R groups are commonly found in proteins?

Answer 21

20.

Question 22

What is unique about glycine?

Answer 22

It has no side chain and is the only achiral amino acid.

Question 23

What type of amino acid is proline?

Answer 23

An imino acid.

Question 24

What are the classifications of amino acids based on their side chains?

Answer 24

• Non-polar
• Polar
• Charged
• Uncharged
• Acidic
• Basic
• Aromatic

Question 25

List the non-polar amino acids.

Answer 25

* Ala (Alanine) * Val (Valine) * Leu (Leucine) * Ile (Isoleucine) * Pro (Proline) * Phe (Phenylalanine) * Trp (Tryptophan) * Met (Methionine)

Question 26

What are polar amino acids?

Answer 26

* Gly (Glycine) * Asn (Asparagine) * Gln (Glutamine) * Ser (Serine) * Thr (Threonine) * Cys (Cysteine) * Tyr (Tyrosine)

Question 27

What characterizes acidic amino acids?

Answer 27

They donate protons and become negatively charged at physiological pH.

Question 28

What characterizes basic amino acids?

Answer 28

They can accept a proton and are positively charged at physiological pH.

Question 29

What is the representation of acidic amino acids?

Answer 29

* Asp (Aspartic Acid) * Glu (Glutamic Acid)

Question 30

What is the representation of basic amino acids?

Answer 30

* Arg (Arginine) * Lys (Lysine) * His (Histidine)

Question 31

What are the acidic amino acids that are negatively charged at physiological pH?

Answer 31

* Asp * Glu * Cys (somewhat) * Tyr (somewhat) ## Footnote Acidic amino acids have a carboxyl group that can donate a proton, leading to a negative charge.

Question 32

Which amino acids are classified as basic amino acids that are positively charged at physiological pH?

Answer 32

* Arg * Lys * His ## Footnote Basic amino acids have side chains that can accept protons, resulting in a positive charge.

Question 33

What group in amino acids has a neutral charge?

Answer 33

The amide group ## Footnote The side chain amide has O=C bonded to an amine, resulting in no net charge.

Question 34

Why aren't Asparagine or Glutamine considered basic amino acids?

Answer 34

They do not have a positively charged side chain at physiological pH.

Question 35

How many pK_a values do all amino acids have at least?

Answer 35

Two pK_a values ## Footnote This includes the pK_a of the amino group and the carboxylic acid group.

Question 36

What is the pK_a of the amino group in amino acids?

Answer 36

About 9.5

Question 37

What is the pK_a of the carboxylic acid group in amino acids?

Answer 37

About 2

Question 38

At neutral pH, what happens to the amino group and the carboxyl group?

Answer 38

* Amino group gains a proton * Carboxyl group loses its proton

Question 39

What does the titration curve of an amino acid represent?

Answer 39

The reaction of each functional group with hydroxide ions.

Question 40

What is the Zwitterion?

Answer 40

The state of an amino acid where the positive NH3+ is balanced by the negative COO- ## Footnote This occurs at the isoelectric point (pI).

Question 41

What are the two types of peptide bond formation reactions?

Answer 41

Condensation reaction ## Footnote A condensation reaction occurs between the carboxylate ion of one amino acid and the amino group of another.

Question 42

What is released during the formation of a peptide bond?

Answer 42

Water

Question 43

Why is there little rotation around the peptide bond?

Answer 43

Due to its partial double bond character

Question 44

Define 'Peptide'.

Answer 44

A relatively short string of amino acids.

Question 45

Define 'Polypeptide or protein'.

Answer 45

A longer string of amino acids, often 100 or more.

Question 46

What is the N-terminus of a peptide?

Answer 46

The start or left end of a peptide or protein, free amino terminus.

Question 47

What is the C-terminus of a peptide?

Answer 47

The end or right end of a peptide or protein, free carboxyl terminus.

Question 48

Why are the side chains of amino acids important?

Answer 48

They contribute most to localized charges on proteins.

Question 49

At physiological pH, what is the charge of the side chains of Asp and Glu?

Answer 49

Normally deprotonated (COO-)

Question 50

At physiological pH, what is the charge of the side chains of Lys and Arg?

Answer 50

Normally protonated (NH3+)

Question 51

What happens to the charge of Cysteine at pH 9.0?

Answer 51

Cysteine deprotonates from its neutral form.

Question 52

What are essential amino acids?

Answer 52

* Phenylalanine * Valine * Tryptophan * Threonine * Isoleucine * Methionine * Histidine * Arginine * Lysine * Leucine ## Footnote Essential amino acids must be supplied by the diet.

Question 53

What are non-essential amino acids?

Answer 53

* Alanine * Asparagine * Aspartic acid * Cysteine * Glutamic acid * Glutamine * Glycine * Proline * Serine * Tyrosine ## Footnote Non-essential amino acids can be synthesized by the body.

Question 54

What is the net charge of a peptide at physiological pH if the pK values are less than 7.4?

Answer 54

They tend to deprotonate.

Question 55

What is the net charge of a peptide at physiological pH if the pK values are greater than 7.4?

Answer 55

They tend to protonate.

Question 56

What is the net charge on the peptide 'ASP-GLY-ALA-VAL-ILE-THR-MET-TRP-LYS-PRO-ASN-GLN-PHE-LEU-LYS-GLU-SER-ARG' at neutral pH?

Answer 56

8

Question 57

What is the total net charge of the peptide 'ASP-GLY-ALA-VAL-ILE-THR-MET-TRP-LYS-PRO-ASN-GLN-PHE-LEU-LYS-GLU-SER-ARG' at neutral pH?

Answer 57

-2