Left/right amino acids

Fig. 13 Left. ELP carrier N-terminally fused to a cell-penetrating peptide and C-terminally fused to a therapeutic peptide. Right Amino acid sequences for several cell-penetrating peptides (see text for details). Reprinted from [83] with permission from Elsevier, cop)mght 2010...

By convention, the left-hand amino acid in this sequence is the one with a free amino group, the A-terminus, and the right-hand amino acid has the free carboxyl, the C-terminus. Thus, Ser-Phe-Ala is different from Ala-Phe-Ser, and represents a quite different molecule. Sometimes, the termini identities are emphasized by showing H- and -OH H- represents the amino group and -OH the carboxyl group. Some peptides are cyclic, and this convention can have no significance, so arrows are incorporated into the sequence to indicate peptide bonds in the direction CO— NH. As sequences become longer, one-letter abbreviations for amino... 

Life on Earth makes use of only a limited number of diastereomers of all those that are possible. Moreover, biotic processes display an enantiomeric excess e.g., left-handed amino acids and right-handed sugars almost exclusively predominate in living systems. 

Even so, as has been pointed out, silicon may have had a part to play in the origin of life on Earth. A curious fact is that terrestrial life forms utilize exclusively right-handed carbohydrates and left-handed amino acids. One theory to account for this is that the first prebiotic carbon compounds formed in a pool of "primordial soup" on a silica surface having a certain handedness. This handedness of the silicon compound determined the preferred handedness of the carbon compounds now found in terrestrial life. An entirely different possibility is that of artificial life or intelligence with significant silicon content. 

FIGURE 7 General. structure of amino acids (left) (R-groups are different for each amino acid), the amino acid serine (right), amino acids linked to a peptide chain (beyond). 

It is well-known in contemporary science that biological macromolecules diverge from other polymeric structures. Proteins are constructed only by left-handed amino acids, whereas DNA-RNA contain only right-handed sugars (Goldanskii Semenov,... 

The two dipeptides, like all peptides (and amino acids), contain ammonium and carboxylate end groups, referred to as N-terminal and C-terminal residues or units, respectively. The convention for drawing peptides is to place the N-terminal residue at the left and the C-terminal residue at the right. Names of peptides are obtained by naming the amino acid residues from left to right, usually with abbreviated symbols, e.g., Gly-Ala and Ala-Gly, as shown above. More expanded names use the full names of each amino acid. The far right amino acid residue retains the name of the amino acid. For all other amino acids except tryptophan, the -ine or -ic acid of the name is replaced by -yl. For tryptophan, -yl is added to the name. 

Give the expected hybridization around each atom designated with an arrow in glycine (left an amino acid) and para-aminobenzoic acid (right PABA used in sunscreens). 

Nature makes routine use of molecular chirality. We spoke above about proteins, which are made from units called amino acids. In fact, the two molecules resting in the two hands in Figure 1.8c are amino acids. So, amino acids are chiral. Interestingly, all the proteins in living creatures are made exclusively from left-handed amino acids, and when the creature dies and its protein disintegrates, one finds both left- and right-handed amino acids. How does the living matter know to select only left-handed amino acids ... 

Sason Sure, Roald. Proteins in living organisms and plants are made of left-handed amino acids, never of right-handed ones. So marvelous is this choice of nature that even in these days, there are heated arguments about how we ended up with this uniform chirality (called homochirality). 

FIGURE 7.8 Amino acids are chiral and snuggle nicely with their respective left and right hands. Proteins are made only from left-handed amino acids. This image was created by NASA. It is in the public domain, and it was obtained from http //en.wikipedia.org/wiki/ File Chirality with hands.svg. 

In nature, proteins have only left-handed amino acids (termed homochirality). This homochirality is established during tRNA loading and protein synthesis. The tRNA loading enzymes preferentially insert left-handed amino acids onto tRNA. Left-handed amino acids fit better into these enzymes than do right-handed amino acids. Also, nucleotides are chiral (see Problem 7.7). In studies of chiral recognition, in the loading enzyme, a tRNA model whose nucleotides had the chirality found in nature preferred to receive left-handed amino acids, and a model whose nucleotides had a... 

Right-handed and Left-handed Amino-acid Molecules... 

No one knows why living organisms are constructed of L-amino acids. We have no strong reason to believe that molecules resembling proteins could not be built up of equal numbers of right-handed and left-handed amino acid molecules. Perhaps the protein molecules that are made of amino acid molecules of one sort only are especially suited to the construction of a living organism —but if this is so, we do not know why. ... 

Figure 2-1 S3. The ViewerLite shows an elaborate depiction of hemoglobin on the right-hand side, with the amino acids in a cascade window on the left.

The charges used for calculations on proteins are best derived using a suitable fragment for each amino acid fleets the environment within the protein (right), rather than the isolated amino acid (left). 

An amino acid sequence is ambiguous unless we know the direction m which to read It—left to right or right to left We need to know which end is the N terminus and which IS the C terminus As we saw m the preceding section carboxypeptidase catalyzed hydrolysis cleaves the C terminal ammo acid and so can be used to identify it What about the N terminus ... 

An a helix can in theory be either right-handed or left-handed depending on the screw direction of the chain. A left-handed a helix is not, however, allowed for L-amino acids due to the close approach of the side chains and the CO group. Thus the a helix that is observed in proteins is almost always right-handed. Short regions of left-handed a helices (3-5 residues) occur only occasionally. 

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