Unicellular Organisms

Amino Acids

	The building blocks of proteins are amino acids. An amino acid contains an amino group (-NH₂), a carboxylic acid group (-COOH), and a side chain (R). The carbon at the center is called the alpha-carbon (Figure 11-16). Although there are many amino acids, only 20 different amino acids are present in humans. The unique characteristics of the 20 amino acids are due to the side chain.
Figure 11-16 Amino Acid

Figure 11-17 shows the 20 amino acids (with three-letters and one-letter abbreviations following the full name). Nonpolar amino acids are not soluble in water, which makes them hydrophobic. Polar amino acids have hydrophilic side chain, which forms hydrogen bonds³ with water. Acidic amino acids have side chains that can ionize as a weak acid. The side chains of the basic amino acids contain an amino group that can ionize as a weak base. The numbers at the bottom of each graph is the value of isoelectric point (pI). The isoelectric point is a value of pH at which the amino acid gives an overall charge of zero and not accepting or donating any H⁺ ion in a solution. The hexagon is the benzene ring C₆H₆. Amino acids on earth are all left-handed with the NH₂ group to the left. Essential (E in Figure 11-17) amino acids cannot be synthesized by the human body and must be provided through diet, while non-essential (NE) amino acids are synthesized by the body from carbon, nitrogen, hydrogen, oxygen, and sulphur.

There are more amino acids beside those 20's used by biological system as listed in Figure 11-17.

Figure 11-17 The 20 Amino Acids [view large image]

	The bond that joins 2 amino acids is called a peptide bond. The NH₂ and OH group at the end of the peptide are available for adding more amino acids to the chain. (Figure 11-18a). Combining amino acids to form peptide will release water, while adding water to peptide will break it up into individual amino acids.
Figure 11-18a Peptide Formation [view large image]

Isomers are two similar forms of molecule being the mirror image of each other. They exhibit chirality if the two forms cannot be superimposed as shown by the amino acid (the left-handed form) in Figure 11-16 and the D-amino acid (the right-handed form) in Figure 11-18b. The laws of quantum chemistry do not favor either variety over the other. But life on Earth uses almost exclusively left-handed amino acids to build proteins, and right-handed sugars to build nucleic acids. Such facts imply that all life on Earth today is descended from a common ancestor. Since the handedness affects the polarization of light by rotating the polarization angle to the left or to the right, it is found that asymmetry in handedness extends all the way to the Orion molecular cloud with the detection of circularly polarized light (in the infrared) from there. The implication is that a characteristic pattern of handedness will be imprinted on all the material from which

a group of stars forms together. But since circularly polarized light can itself be either left-handed or right-handed, depending on how it rotates, molecules in different interstellar clouds (or even in different parts of the same cloud) may be affected in different ways. Thus, there is still a chance that stranded space travelers will starve amidst plenty because their metabolisms could not cope with the food found on other worlds.

Figure 11-18b D-amino acid

³The bonding energy for the various chemical bonds are roughly in the ratio - Van der Waals : Hydrogen Bonds : Covalent Bonds = 1 : 10 : 100. Formation of hydrogen bonds releases 3 - 10 kcal/mole (~ 0.1 - 0.4 ev). Hydrogen bonds are found between only a few elements of the periodic table. The most common are those in which H connects two atoms from the group F, O, N, and, less commonly, Cl. The hydrogen bond in water has the configuration: H-O-H(+)ЕЕ..(-)O=H₂. Covalent bonds are created with sharing electrons in between two atomic nuclei. A stable configuration can be achieved by sharing up to three pairs of electrons. Van der Waals forces are the intermolecular attractions produced by temporary dipoles (shifting of electrons).