Proteins. Proteins serve many functions, including the following: 1. Structure: Collagen and keratin are the chief constituents of skin, bone, hair, and nails. 2. Catalysts: Virtually all reactions in living systems are catalyzed by proteins called enzymes.
Proteins serve many functions, including the following:
Amino acid: A compound that contains both an amino group and a carboxyl group.
With the exception of glycine, all protein-derived amino acids have at least one stereocenter (the -carbon) and are chiral.
A comparison of the configuration of L-alanine and D-glyceraldehyde (as Fischer projections):
Nonpolar side chains. Each ionizable group is shown in the form present in highest concentration at pH 7.0).
Acidic and basic side chains (at pH 7.0)
1. For 19 of the 20, the -amino group is primary; for proline, it is secondary.
2. With the exception of glycine, the a-carbon of each is a stereocenter.
3. Isoleucine (left) and threonine (right) contain a second stereocenter.
The net charge on an amino acid depends on the pH of the solution in which it is dissolved.
The pH at which the majority of molecules of a compound in solution have no net charge.
The -SH (sulfhydryl) group of cysteine is easily oxidized to an -S-S- (disulfide).
The amino acids phenylalanine, tryptophan, and tyrosine have aromatic rings on their side chains.
Tryptophan is the precursor to the neurotransmitter serotonin.
Phenylalanine and tyrosine are precursors to norepinephrine and epinephrine, both of which are stimulatory.
Figure 22.3 Hydroxylation (oxidation) of proline, lysine, and tyrosine, respectively and iodination for tyrosine, give these uncommon amino acids.
In 1902, Emil Fischer proposed that proteins are long chains of amino acids joined by amide bonds.
By convention, peptides are written from the left to right, beginning with the free -NH3+ group and ending with the free -COO- group.
Figure 22.4 A small peptide showing the direction of the peptide chain (N-terminal to C-terminal).
Proteins behave as zwitterions.
Proteins also have an isoelectric point, pI.
at this pH.
Primary structure: The sequence of amino acids in a polypeptide chain.
The number peptides possible from the 20 protein-derived amino acids is enormous.
Figure 22.8 The hormone insulin consists of two polypeptide chains, A and B, held together by two disulfide bonds. The sequence shown here is for bovine insulin.
How important is the exact amino acid sequence?
Secondary structure: Conformations of amino acids in localized regions of a polypeptide chain.
In a section of -helix
Figure 22.10(b) The -pleated sheet structure.
In a section of b-pleated sheet;
Figure 22.11 A random coil.
Schematic structure of the enzyme carboxypeptidase. The -pleated sheet sections are shown in blue, the -helix portions in green, and the random coils as orange strings.
Many globular proteins contain -helices, -pleated sheets, and random coils.
The collagen triple helix.
Tertiary structure: the overall conformation of an entire polypeptide chain.
Tertiary structure is stabilized in four ways:
Forces that stabilize tertiary structures of proteins.
Quaternary structure: the arrangement of polypeptide chains into a noncovalently bonded aggregation.
The quaternary structure of hemoglobin. The structure of heme is shown on the next screen.
The structure of heme
Integral membrane proteins form quaternary structures in which the outer surface is largely nonpolar (hydrophobic) and interacts with the lipid bilayer. Two of these are shown on the next screens.
Integral membrane protein of rhodopsin, made of -helices.
An integral membrane protein from the outer mitochondrial membrane forming a -barrel from eight -pleated sheets.
Denaturation: The process of destroying the native conformation of a protein by chemical or physical means.
Denaturing agents include: