Characteristics Of ProteinsㅡPeptides Flashcards
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.
protein
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.
amino acid
An ____________ is an amino acid in which the amino group and the carboxyl group are attached to the a-carbon atom.
a-amino acid
A __________________ is one of the 20 a-amino acids normally found in proteins.
standard amino acid
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.
nonpolar amino acid
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.
polar neutral amino acid
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.
polar acidic amino acid
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.
polar basic amino acid
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.
essential amino acids
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.
complete dietary protein
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.
incomplete dietary protein
A ______________ is an essential amino acid that is missing, or present in inadequate amounts, in an incomplete dietary protein.
limiting amino acid
Protein from __________ is usually complete dietary protein. Casein from milk and proteins found in meat, fish, and eggs are complete dietary proteins.
animal sources
There is one common incomplete dietary protein that comes from animal sources. It is __________, a protein in which tryptophan is the limiting amino acid.
gelatin
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.
lysine
methionine
tryptophan
______ is the only common plant protein that is a complete dietary protein.
Soy
______________ 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.
Complementary dietary proteins
The rules for drawing Fischer projection formulas for amino acid structures follow.
- The ㅡCOOH group is put at the top of the projection formula, the R group at the bottom. This positions the carbon chain vertically.
- 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.
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.
zwitterion
Guidelines for amino acid form as a function of solution pH follow.
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).
___________ is the only standard amino acid that has a side chain that contains a sulfhydryl group (ㅡSH group)
Cysteine
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.)
a. dimerizes
b. dimer
A _________ is an unbranched chain of amino acids.
peptide
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.
dipeptide
tripeptide
