Note that there are three ways to refer to an amino acid: by its full name, its three-letter abbreviation, or its one-letter abbreviation. (D-amino acids are not naturally found in eukaryotic metabolic pathways.) b) Classifying amino acidsĮach amino acid has a characteristic side chain, and the properties of these side chains are essential for the function of proteins. The chirality of biological molecules becomes quite important, as only L-configuration (left-handed) amino acids can be used by the body. (The exception happens to be glycine, as the R group is simply a hydrogen atom.) Chirality refers to right- or left-handedness, denoted as D- and L- molecules, respectively. Note that for 19 of the 20 amino acids, the alpha carbon itself is chiral, or attached to four different constituent groups. This carbon is connected to every constituent of the amino acid: the amino group (-NH3+), the carboxylic acid part (-COO-), the R group, and a hydrogen atom (H). The R group is connected to the central carbon, which is known as the alpha carbon. We’ll discuss these side chains further in the next section. There are 20 different R groups-each of which you should commit to memory. These R groups, or side chains, can be as simple as a single hydrogen atom or as complex as an imidazole ring. Exceptions arise when accounting for charges on the R-group, or side chain, of the amino acid. Most amino acids have a net charge of zero. Note that at physiological pH, amino acids are zwitterions they contain both positive and negative charges on the same molecule. At physiological pH (pH ~7), this carboxylic acid is deprotonated, leaving a negative charge on the functional group. The acid on every amino acid is a carboxylic acid, a functional group composed of COOH. This results in a positive charge on the functional group.
#Bulky hydrophobic amino acids free
At physiological pH (pH ~7), this free electron pair is able to accept a bond to a single hydrogen atom. Note that this results in one free electron pair on the nitrogen atom. It is similar to ammonia (NH4), except that at one position, the nitrogen is attached to a carbon instead of a hydrogen (NH2C instead of NH3). Recall that an amino group is a functional group composed of NH3+. At the end of this guide, you will also find several passage-based and standalone questions to sharpen your skills. Their definitions are particularly important and can also be found in Part 4 of this guide. Throughout the guide, you will encounter several bolded terms. Be sure to refer to our other biochemistry guides for further information on proteins, enzymes, and other biological molecules. While it will not be a comprehensive handbook to everything about proteins, it will be a good place to start studying these basic principles of biochemistry. This guide will serve as an introduction to amino acids, protein structure, and protein interactions. These topics are especially intimidating because there is virtually no limit to what you can learn about proteins, amino acids, and everything else.
Proteins are an incredibly high-yield concept on the MCAT, but like a lot of biochemistry topics, they aren’t easily mastered without a great deal of practice.
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