Lab Molecular

Amino acids, Peptides and Protein

Dr. Alejandro Yévenes


•  Some function of proteins:

•  The light produced by fireflies is the result of the reaction involving the protein luciferin and ATP, catalyzed by the enzyme Luciferase.


•  Some function of proteins:

•  Erythrocytes contain large amounts of the oxygentrasporting protein hemoglobin.


•  Some function ofproteins:

•  The protein Keratin is the chief structural component of hair, sclaes, horn, wool, nails and feathers.


•  General structure of an amino acid.

•  This structure is common to all but one of the amino acids. (Proline, a cyclic amino acid, is the exception.) The R group or side chain (red) attached to the carbon (blue) is different in each amino acid.


La fórmulageneral de un aminoácido es:

carbono α

grupo α-amino

grupo carboxilo

cadena lateral

20 tipos diferentes de cadena lateral (R). A pH 7, los grupos amino y carboxilo se encuentran ionizados.

•  The additional carbons in an R group are commonly designated β, γ, δ, ε, and so forth, proceeding out from the carbon.


For all the common amino acids except glycine, the carbonis bonded to four different groups: a carboxyl group, an amino group, an R group, and a hydrogen atom (in glycine, the R group is another hydrogen atom). The carbon atom is thus a chiral center. Because of the tetrahedral arrangement of the bonding orbitals around the -carbon atom, the four different groups can occupy two unique spatial arrangements, and thus amino acids have two possiblestereoisomers. Since they are nonsuperimposable mirror images of each other, the two forms represent a class of stereoisomers called enantiomers. All molecules with a chiral center are also

optically active —that is, they rotate planepolarized light.


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Special nomenclature has been developed to specify the absolute configuration of the four substituents of asymmetric carbon atoms. Theabsolute configurations of simple sugars and amino acids are specified by the D, L system (Fig. 3–4), based on the absolute configuration of the three-carbon sugar glyceraldehyde, a convention proposed by Emil Fischer in 1891


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The amino acid residues in protein molecules are exclusively L stereoisomers. D Amino acid residues have been found only in a few, generally small peptides, includingsome peptides of bacterial cell walls and certain peptide antibiotics.


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Absorption of ultraviolet light by aromatic amino acids.

Comparison of the light absorption spectra of the aromatic amino acids tryptophan and tyrosine at pH 6.0. The amino acids are present in equimolar amounts (10-3 M) under identical conditions. The measured absorbance of tryptophan is asmuch as four times that of tyrosine.

Note that the maximum light absorption for both tryptophan and tyrosine occurs near a wavelength of 280 nm. Light absorption by the third aromatic amino acid, phenylalanine (not shown), generally contributes little to the spectroscopic properties of proteins.


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Fluorescencia


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The principal components of a spectrophotometer. A lightsource emits light along a broad spectrum, then the monochromator selects and transmits light of a particular wavelength. The monochromatic light passes through the sample in a cuvette of path length l and is absorbed by the sample in proportion to the concentration of the absorbing species. The transmitted light is measured by a detector.


Reversible formation of a disulfide bond by the oxidationof two molecules of cysteine. Disulfide bonds between Cys

residues stabilize the structures of many proteins.


Amino acid sequence of bovine insulin.



The two polypeptide chains are joined by disulfide crosslinkages.



The A chain is identical in human, pig, dog, rabbit, and sperm

whale insulins.



The B chains of the cow, pig, dog, goat, and...