4.7 Amino acids, peptides & proteins

Question 1

what is an a-amino acid?

Answer 1

a-amino acids are organic molecules containing a carboxylic acid group and amine group bonded to the same carbon atom

Question 2

displayed formula structure of an a-amino acid representing the side chain w/ R

Answer 2

HOOC-CH-R1-NH2

Question 3

Which amino acid is the only amino acid that is not chiral?

Answer 3

aminoethanoic acid (glycine): the R group on aminoethanoic acid is just a hydrogen atom so the carbon is not bonded to 4 diff grps.

Question 4

why are all amino acids (except glycine) chiral molecules?

Answer 4

all amino acids, except glycine, contain a chiral carbon atom bonded to four separate groups, it is bonded to an amino group, a carboxylic group a hydrogen atom and an R group

Question 5

how are amino acids amphoteric?

Answer 5

amino acids are amphoteric meaning they have both acidic and basic properties, they have a basic amino grp and an acidic carboxylic grp

Question 6

what is a zwitterion?

Answer 6

a zwitterion is a dipolar ion which means it has a positive charge in one part of the molecule and a negative part in another part on the molecule

Question 7

General displayed stricture of an amino acid zwitterion

Answer 7

N+H3-CRH-CO(minus) =O

Question 8

What happens when an alkali is added to an amino acid zwitterion?

Answer 8

The NH3+ grp donates a hydrogen ion to OH(minus) ions of the alkali to form water, the organic compound is no longer zwitterion as it only contains a -ve charge

Question 9

What happens when an acid is added to an amino acid zwitterion?:

Answer 9

The COO- grp accepts a hydrogen ion from the acid, the organic compound is no longer a zwitterion as it only contains a +ve charge

Question 10

General displayed structure of an amino acid in acidic conditions

Answer 10

N+H3-CRH-COH=O

Question 11

General displayed structure of an amino acid in alkaline conditions

Answer 11

NH2-CRH-CO(minus)=O

Question 12

Why do amino acids have relatively high melting points?

Answer 12

In the solid state the zwitterion is the usual form that an amino acid exists in, this means there are strong ionic attractions between neighbouring zwitterions in solid so a large amount of energy required to break the ionic attractions

Question 13

explain the solubility of amino acids in water

Answer 13

amino acids are generally soluble in water because strong ionic attractions form between the zwitterions and polar water molecules

Question 14

how does a peptide bond form?

Answer 14

a peptide bond forms during condensation reactions between two amino acids, a water molecule is lost

Question 15

what is the difference between a dipeptide and a polypeptide?

Answer 15

dipeptide is formed from 2 amino acids, polypeptide formed from more than 2 amino acids

Question 16

structure of peptide linkage

Answer 16

-C(=Oabve)-N(-Habve)- (+H2O in condensation rctns)

Question 17

how to polypeptides lead to protein formation?

Answer 17

polypeptides are chains of more than two amino acids joined together by peptide links, when the chain becomes very long, they can fold into proteins

Question 18

what does the primary structure of a protein involve?

Answer 18

the primary structure of a protein is the sequence of amino acids which make up the polypeptide chain

Question 19

what does the secondary structre of a protein involve?

Answer 19

the protein chain has peptide links which can form hydrogen bonds with each other leading to two possible shapes of the chain, a-helix spiral, B-pleated sheet, both stabilised by hydrogen bonding

Question 20

what does the tertiary structure of a protein involve?

Answer 20

the tertiary structure related to the extra bonds which can form between diff parts of the polypeptide chain, determining how the a-helices or B-pleated sheets of protein fold wrt each other, the types of extra bonds include, ionic & hydrogen bonds and disulphide bridges

Question 21

give examples of how proteins are essential for living systems:

Answer 21

-proteins are important for structural functions in living organisms, nails, feathers, skin, collagen are all made up of protein
-enzymes are proteins that are biological catalysts that speed up chemical rctns in the body
- some hormones are proteins e.g. insulin