Characteristics Of ProteinsㅡPeptides

Question 1

A ______________ is a naturally occurring, unbranched polymer in which the monomer units are amino acids. Thus the starting point for a discussion of proteins is an understanding of the structures and chemical properties of amino acids.

Answer 1

protein

Question 2

An _______________ is an organic compound that contains both an amino (!NH2) group
and a carboxyl (ㅡCOOH) group. The amino acids found in proteins are always
a-amino acids.

Answer 2

amino acid

Question 3

An ____________ is an amino acid in which the amino group and the carboxyl group are attached to the a-carbon atom.

Answer 3

a-amino acid

Question 4

A __________________ is one of the 20 a-amino acids normally found in proteins.

Answer 4

standard amino acid

Question 5

A _____________ is an amino acid that contains one amino group, one carboxyl group, and a nonpolar side chain. When incorporated into a protein, such amino acids are hydrophobic (“water-fearing”); that is, they are not attracted to water molecules. They are generally found in the interior of proteins, where there is limited contact with water.

Answer 5

nonpolar amino acid

Question 6

A ___________________ is an amino acid that contains one amino group, one carboxyl group, and a side chain that is polar but neutral. In solution at physiological pH, the side chain of a ____________ is neither acidic nor basic.

Answer 6

polar neutral amino acid

Question 7

A ________________ is an amino acid that contains one amino group and two carboxyl groups, the second carboxyl group being part of the side chain. In solution at physiological pH, the side chain of a polar acidic amino acid bears a negative
charge; the side-chain carboxyl group has lost its acidic hydrogen atom.

Answer 7

polar acidic amino acid

Question 8

A _________________ is an amino acid that contains two amino groups and one carboxyl group, the second amino group being part of the side chain. In solution at physiological pH, the side chain of a polar basic amino acid bears a positive charge; the nitrogen atom of the amino group has accepted a proton.

Answer 8

polar basic amino acid

Question 9

Adequate amounts of 11 of the 20 standard amino acids can be synthesized from carbohydrates and lipids in the body if a source of nitrogen is also available. However, the adult human body cannot produce adequate amounts of the other nine standard amino acids. These amino acids, which are called _____________, must be obtained from dietary protein.

Answer 9

essential amino acids

Question 10

A _______________ is a protein that contains all of the essential amino acids in the same relative amounts in which the body needs them. A ________________ may or may not contain all of the nonessential amino acids.

Answer 10

complete dietary protein

Question 11

An ____________ is a protein that does not contain adequate amounts, relative to the body’s needs, of one or more of the essential amino acids.

Answer 11

incomplete dietary protein

Question 12

A ______________ is an essential amino acid that is missing, or present in inadequate amounts, in an incomplete dietary protein.

Answer 12

limiting amino acid

Question 13

Protein from __________ is usually complete dietary protein. Casein from milk and proteins found in meat, fish, and eggs are complete dietary proteins.

Answer 13

animal sources

Question 14

There is one common incomplete dietary protein that comes from animal sources. It is __________, a protein in which tryptophan is the limiting amino acid.

Answer 14

gelatin

Question 15

Protein from plant sources tends to be incomplete dietary protein. With plant proteins, three amino acids are often limiting: _________ (wheat, rice, oats, and corn), ___________ (beans and peas), and ____________ (corn and beans). Note that both corn and beans have two limiting amino acids.

Answer 15

lysine
methionine
tryptophan

Question 16

______ is the only common plant protein that is a complete dietary protein.

Answer 16

Soy

Question 17

______________ are two or more incomplete dietary proteins that, when combined, provide an adequate amount of all essential amino acids relative to the body’s needs.

Answer 17

Complementary dietary proteins

Question 18

The rules for drawing Fischer projection formulas for amino acid structures follow.

Answer 18

1. The ㅡCOOH group is put at the top of the projection formula, the R group at the bottom. This positions the carbon chain vertically.
2. The ㅡNH2 group is in a horizontal position. Positioning it on the left denotes the L isomer, and positioning it on the right denotes the D isomer.

Question 19

A ____________ is a molecule that has a positive charge on one atom and a negative charge on another atom, but which has no net charge.

Answer 19

zwitterion

Question 20

Guidelines for amino acid form as a function of solution pH follow.

Answer 20

Low pH
☆ All acid groups are protonated (ㅡCOOH).
All amino groups are protonated (ㅡN+H3).
High pH
☆ All acid groups are deprotonated (ㅡCOO2). All amino groups are deprotonated (ㅡNH2).
Neutral pH
☆ All acid groups are deprotonated (ㅡCOO2). All amino groups are protonated
(ㅡN+H3).

Question 21

___________ is the only standard amino acid that has a side chain that contains a sulfhydryl group (ㅡSH group)

Answer 21

Cysteine

Question 22

Cysteine, in the presence of mild oxidizing agents, readily a. ___________, that is, reacts with another cysteine molecule to form a cystine molecule.
(A b. _________ is a molecule that is made up of two like subunits.)

Answer 22

a. dimerizes
b. dimer

Question 23

A _________ is an unbranched chain of amino acids.

Answer 23

peptide

Question 24

Peptides are further classified by the number of
amino acids present in the chain. A compound containing two amino acids is specifically called a __________; three amino acids joined together in a chain constitute a ___________; and so on.

Answer 24

dipeptide
tripeptide

Question 25

The name __________ is loosely used to refer to peptides with 10 to 20 amino acid residues, and the name ____________ is used to refer to longer
peptides, a long unbranched chain of amino acids.

Answer 25

oligopeptide
polypeptide

Question 26

In amino acid chemistry, amide bonds that link amino acids together are given the specific name of ___________. A ___________ is a covalent bond between the carboxyl group of one amino acid and the amino group of another amino acid.

Answer 26

peptide bonds

Question 27

In all peptides, long or short, the amino acid at one end of the amino acid sequence has a free H3Nㅡ group, and the amino acid at the other end of the sequence has a free COOㅡ group. The end with the free H3Nㅡgroup is called the _____________ and the end with the free COOㅡ group is called the ___________.

Answer 27

N-terminal end
C-terminal end

Question 28

The individual amino acids within a peptide chain are called _________________. An ___________ is the portion of an amino acid structure that remains, after the release of H2O, when an amino acid participates in peptide bond formation as it becomes part of a peptide chain.

Answer 28

amino acid residues

Question 29

The abbreviated formula for the tripeptide which contains the amino acids _________, _______, and _________, is ____________.

Answer 29

glycine, alanine, and serine
Gly–Ala–Ser

Question 30

Peptides that contain the same amino acids but in different order are different molecules (constitutional isomers) with different properties.

Answer 30

Isomeric Peptides