If you are into supplements and protein powders, I'm sure you have seen packages with the word "amino acids" on them. For those who don't know what they are, amino acids (AA) play an essential role in the human body; acting both as the building blocks of peptides and proteins as well as precursors in metabolic pathways. Proteins that are formed from these AA have distinct three-dimensional structures and sequences that allow them to have different roles in various biological and chemical processes.
Only 10 out of the 20 AA can be produced by the human body. They are: Alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine and tyrosine. The other 10 amino acids that we need to obtain from our foods are: Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These are considered to be essential amino acids because we HAVE to obtain them from different foods.
The reason why we call them "amino acids" is because each structure contains two distinct functional groups. An amino acid has a central chiral carbon called the alpha carbon (Cα), a hydrogen atom, an amino (-NH2) group, a carboxylic acid (-COOH) group, as well as a variable side chain (R).
**Be aware that because of the intramolecular acid-base reactions, a proton is transferred from the carboxyl group to the amino group to produce a zwitterion (dipolar ion). Therefore, an amino acid normally has an NH3+ group on one end and a COO- group on the other end!
1) Amino acids are crystalline solids with high melting points. They are generally soluble in water and are insoluble in nonpolar solvents.
2) The carboxyl group has a low pKa of 2 and is negatively charged at physiological pH. The amino group, on the other hand, has a high pKa of approx 9-10 and therefore is positively charged at the physiological pH.
3) When we say that amino acids have chiral carbons, what it essentially means is that these amino acids are optically active molecules. Therefore, if you shoot a plain, polarized light at these AAa, they will rotate to that light.
***Note: Out of all the AA, only glycine is not optically active. Explanation below.
There are 20 standard amino acids and they all have very unique side chains that make them distinctive from one another. In order to make it a little simpler, we can divide these amino acids into 2 different categories: Hydrophobic (water-hating) and hydrophillic (water-loving). Amino acids that have nonpolar R groups are hydrophobic and will bury inside the proteins so that they can stay as far away from water as possible. AAs that have polar side chains, on the other hand, will fold themselves so that their R groups will be on the surface so that they can interact with other water molecules.
A. NONPOLAR HYDROPHOBIC AMINO ACIDS
1) Glycine (gly, G) is a unique amino acid because it only has a hydrogen as its "R group" - making it the smallest AA out of the 20 standard ones that we will be discussing about today. Because it has 2 different hydrogen atoms - one that already attaches to the backbone and another one that attaches to the side chain, this means that the AA is symmetrical and thus is achiral. Because of its high conformational flexibility, glycine favors the unfolded conformation over the helix conformation and is considered as a "helix breaker".
2) Alanine (ala, A) has a methyl (CH3) group as its side chain. Interestingly enough, it's also the analog of the α-keto acid pyruvate - an intermediate in sugar metabolism.
3) Valine (val, V) has an isopropyl R group.
4) Leucine (leu, L) has an iso-butyl side chain. What this means is that it has a similar side chain to valine, but with an extra CH2 group.
5) Isoleucine (Ile, I) is an isomer of leucine and has sec-butyl instead of iso-butyl as its side chain. Since our body cannot synthesize this AA, we need to obtain it from foods such as eggs, poultry, fish, and soy products.
6) Methionine (met, M) has a sulfur that's embedded in its linear aliphatic side chain (also known as the thioether side chain). Now, you may think that it's a polar AA because of the sulfur. However, if you look at where it's located, you will see that it is partially hidden and that the difference in electronegativity is extremely small! Foods that consist high levels of methionine include eggs, seeds, fish, and meats.
7) Proline (pro, P), unlike other AAs, is the only one that has the backbone in its side chain. The 3-carbon chain from the alpha carbon loops around and attaches itself to the amino group. Because it doesn't have a hydrogen on the alpha amino group, it cannot donate a hydrogen bond and thus cannot stabilize an alpha helix or a beta sheet. Because of its unique structure, alpha helices that have this AA often have "kinks"" as a result. Proline, along with glycine, are known as "helix breakers".
8) Phenylalanine (phe, F) is an essential amino acid that cannot be synthesized by the body. Because of its hydrophobicity, this AA always bury itself inside of a protein.
9) Tryptophan (Trp, W) is another essential amino acid that we must obtain from foods. At a glance, it almost looks like a polar amino acid. However, if you take a closer look, you will see that the N-H bond makes up a very small part of a rather large side chain. The electrons from the nitrogen is so integrated into the side chain of the tryptophan that it doesn't really interact with water molecules.
I'm Linh - a science geek who loves experimenting and tinkering with recipes! I hope that this blog brings more ideas into your kitchens! Happy eating folks! XOXO