Chapter 3- Amino Acids

Question 1

Amino acids

Answer 1

• the building blocks of proteins
• alpha amino acids consists of a central alpha carbon atom
• linked to an amino group, a carboxylic acid group, a hydrogen atom, and a side chain (R group)
• each kind of amino acid has a different R group
• with 4 different groups connected to the tetrahedral carbon, alpha amino acids are chiral (May exist in L or D isomers)
• only L amino acids are constituents of protein

Question 2

Amino Acid Charges

Answer 2

• all amino acids have at least two charged groups
• free aminos in solution at neutral pH exist predominantly as dipolar ions called “zwitterions”
• in the dipolar form, the amino group is pronated (NH3+), the carboxyl group is depronated (COO-)
• the ionization state of the amino varies with pH

Question 3

Aminos & pH

Answer 3

• in acid solution, the amino group is pronated (NH3+) and the carboxyl group is not dissociated (COOH)
• as the pH is raised, the carboxyl group is the first to give up a proton, bc its pka is near 2.
• the dipolar form persists until the pH approaches 9, when the protonated amino group loses a proton.

*Under physiological conditions (pH 7.4) aminos exist in the dipolar “zwitterionic” form.

Question 4

20 aminos, sorted into 4 groups

Answer 4

1. Alkyl, Aromatic (hydrocarbons)–Hydrophobic w/nonpolar R groups
2. Neutral–Polar w/neutral R groups (but charge not evenly distributed)
3. Acidic (carboxylic acid)–Positively charged with R groups that are positively charged at physiological pH (7.4)
4. Basic (N)–Negatively charged with R groups that are negatively charged at physiological pH

Question 5

Hydrophobic aminos

Answer 5

• side chains consisting of only hydrogen and carbon
• 9 hydrophobic aminos: glycine, alanine, valine, leucine, isoleucine, methionine, proline, phenylalanine, tryptophan

Gav Limp PT: a guy named Gav has a limp and needs PT, but he is hydrophobic of the water exercises.

Question 6

Polar aminos

Answer 6

• neutral overall, yet they are polar because the R group contains an electronegative atom that hoards electrons
• the hydroxyl groups on serine, threonine, & tyrosine make them more hydrophilic (water loving) and reactive
• the sulfhydryl group is much more reactive than a hydroxyl group and can completely lose a proton at slightly basic pH
• includes: serine, threonine, tyrosine, cysteine, asparagine, glutamine

Question 7

Positive aminos

Answer 7

• highly hydrophilic (water loving)
• lysine and arginine have long side chains w groups that are positively charged at neutral pH
• histidine contains a imidazole group (aromatic ring) that can also be positively charged- can be uncharged or positive at neutral pH depending on local environment. It is often found on the active sites of enzymes

Question 8

Negative aminos

Answer 8

• have acidic side chains
• made up of 2 aminos: aspartic acid & glutamic acid
• acidic side chains are usually negatively charged under intercellular conditions
• often called aspartate & glutamate to emphasize the negative charge

Question 9

7 of the 20 aminos have readily available ionizable side chains able to form ionic bonds as well as to donate or accept protons (acid-base catalyst) to facilitate reactions. Which 7 aminos are these?

Answer 9

Tyrosine
Cysteine
Arginine
Lysine
Histidine
Aspartic acid
Glutamic acid