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In solution it is the nature of the amino
acid R-groups that dictate structure-function relationships of peptides
and proteins. The hydrophobic amino acids will generally be encountered
in the interior of proteins shielded from direct contact with water.
Conversely, the hydrophilic amino acids are generally found on the
exterior of proteins as well as in the active centers of enzymatically
active proteins. Indeed, it is the very nature of certain amino
acid R-groups that allow enzyme reactions to occur.
The imidazole ring of histidine allows it to act as either a proton
donor or acceptor at physiological pH. Hence, it is frequently found
in the reactive center of enzymes. Equally important is the ability
of histidines in hemoglobin to buffer the H+ ions from carbonic
acid ionization in red blood cells. It is this property of hemoglobin
that allows it to exchange O2 and CO2 at the tissues or lungs, respectively.
The primary alcohol of serine and threonine as well as the thiol
(-SH) of cysteine allow these amino acids to act as nucleophiles
during enzymatic catalysis. Additionally, the thiol of cysteine
is able to form a disulfide bond with other cysteines:
Cysteine-SH + HS-Cysteine <-------->
Cysteine-S-S-Cysteine
This simple disulfide is identified as cystine.
The formation of disulfide bonds between cysteines present within
proteins is important to the formation of active structural domains
in a large number of proteins. Disulfide bonding between cysteines
in different polypeptide chains of oligomeric proteins plays a crucial
role in ordering the structure of complex proteins, e.g. the insulin
receptor.
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Optical Properties of the Amino
Acids
A tetrahedral carbon atom with 4 distinct constituents
is said to be chiral. The one amino acid not exhibiting chirality
is glycine since its '"R-group" is a hydrogen atom. Chirality
describes the handedness of a molecule that is observable by the
ability of a molecule to rotate the plane of polarized light either
to the right (dextrorotatory) or to the left (levorotatory). All
of the amino acids in proteins exhibit the same absolute steric
configuration as L-glyceraldehyde. Therefore, they are all L-a-amino
acids. D-amino acids are never found in proteins, although they
exist in nature. D-amino acids are often found in polypetide antibiotics.
The aromatic R-groups in amino acids absorb ultraviolet light with
an absorbance maximum in the range of 280nm. The ability of proteins
to absorb ultraviolet light is predominantly due to the presence
of the tryptophan which strongly absorbs ultraviolet light.
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The Peptide Bond
Peptide bond formation is a condensation reaction
leading to the polymerization of amino acids into peptides and proteins.
Peptides are small consisting of few amino
acids. A number of hormones
and neurotransmitters are peptides. Additionally, several antibiotics
and antitumor agents are peptides. Proteins are polypeptides of
greatly divergent length. The simplest peptide, a dipeptide, contains
a single peptide bond formed by the condensation of the carboxyl
group of one amino acid with the amino group of the second with
the concomitant elimination of water. The presence of the carbonyl
group in a peptide bond allows electron resonance stabilization
to occur such that the peptide bond exhibits rigidity not unlike
the typical -C=C- double bond. The peptide bond is, therefore, said
to have partial double-bond character. |
Referred to as the "building blocks of life, Amino
Acids make
up proteins in every tissue of the body, and contribute to the formation
of hormones, immunoglobulins, hemoglobin, collagen, muscles, neurotransmitters,
enzymes, antibodies, and receptors and are involved in cellular
energy production. They play a major role in nearly every chemical
process that affects physical and mental function. Amino
Acid deficiencies
may manifest as fatigue, allergic sensitivity, arthritis, digestive
disorder, cognitive function, cardiovascular health, athletic performance,
and neurologic imbalance.
All peptides and polypeptides are polymers of alpha-amino acids.
There are 20 a-amino acids that are relevant to the make-up of mammalian
proteins (see below). Several other amino
acids are found in the
body free or in combined states (i.e. not associated with peptides
or proteins). These non-protein associated amino
acids perform specialized
functions. Several of the amino acids found in proteins also serve
functions distinct from the formation of peptides and proteins,
e.g., tyrosine in the formation of thyroid hormones or glutamate
acting as a neurotransmitter.
The a-amino acids in peptides and proteins (excluding proline) consist
of a carboxylic acid (-COOH) and an amino (-NH2) functional group
attached to the same tetrahedral carbon atom. This carbon is the
a-carbon. Distinct R-groups, that distinguish one amino
acid from
another, also are attached to the alpha-carbon (except in the case
of glycine where the R-group is hydrogen). The fourth substitution
on the tetrahedral a-carbon of amino acids is hydrogen.
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