MCAT Content / Amino Acids / Description

Description

Topic: Amino Acids

Amino acids are the building blocks of proteins.

There are 21 amino acids present in proteins and each contains an amino group and carboxyl-acid-group in their basic structure. 10 of the 21 amino acids are considered essential amino acids for humans as our bodies cannot produce them; they must be obtained from the diet. All organisms have different essential amino acids based on their physiology.

Each amino acid has the same fundamental structure, which consists of a central carbon atom, also known as the alpha (α) carbon, bonded to an amino group (-NH2), a carboxyl group (-COOH), hydrogen atom, and a side chain (-R). The alpha carbon is a chiral carbon atom (with the exception of glycine) which has two indistinguishable hydrogen atoms on the alpha carbon. Every amino acid except cysteine has S configuration (cysteine has R).

Every amino acid also has another atom or group of atoms bonded to the central atom known as the R group. This R group, or side chain, gives each amino acid protein-specific characteristics including size, polarity, and pH.

In the aqueous environment of the cell, both the amino group and the carboxyl group are ionized under physiological conditions, and so have the structures -NH3+ and -COO, respectively. This is why amino acids are viewed as dipolar ions. Amino acids are cationic at low pH and anionic at high pH. At the isoelectric point (pI), the amino acid is zwitterionic (neutral).

The chemical composition of the side chain determines the characteristics of the amino acid. The side chains of lysine and arginine are positively charged so these amino acids are also known as basic amino acids. The side chains of aspartate and glutamate are negatively charged and are known as acidic amino acids. Proline is an exception to the standard structure of an amino acid because its R group is linked to the amino group, forming a ring-like structure. Generally, if the side chain contains carboxylic acids, then it is acidic; if it contains amines, then it is basic. Amino acids such as valine, methionine, and alanine are nonpolar (hydrophobic), while amino acids such as serine, threonine, and cysteine are polar (hydrophilic).

 


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Key Points

• Amino acids are the monomers that comprise proteins.

• Each amino acid has the same fundamental structure, which consists of a central carbon atom, also known as the alpha (α) carbon, bonded to an amino group (-NH2), a carboxyl group (-COOH), hydrogen atom, and a side chain (-R).

• The chemical composition of the side chain determines the characteristics of the amino acid (hydrophilic or phobic, basic or acidic).

• The R groups can be divided into four categories based on distinct chemical properties: acidic, basic, polar, nonpolar.

• All acidic and basic R groups are polar.

• If the side chain contains carboxylic acids, then it is acidic; if it contains amines, then it is basic.


Key Terms

amino acid: monomer of proteins

dipolar: having an electric dipole (one slightly positive end and one slightly negative end of a molecule)

cationic: a positively charged ion

anionic: a negatively charged ion

zwitterionic: A molecule with functional groups of which at least one has a positive and one has a negative electrical charge; the net charge of the entire molecule is zero



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