Amino Acids: Structure, Groups and Function

Amino acids are organic molecules that, when linked together with other amino acids, form a protein. Amino acids are essential to life because the proteins they form are involved in virtually all cell functions. Some proteins function as enzymes, some as antibodies, while others provide structural support. Although there are hundreds of amino acids found in nature, proteins are constructed from a set of 20 amino acids. The classification of amino acids depends on what functions they serve in the human body.

Key Takeaways

• Almost all cell functions involve proteins. These proteins are composed of organic molecules called amino acids.
• While there are many different amino acids in nature, our proteins are formed from 20 amino acids.
• From a structural perspective, amino acids are typically composed of a carbon atom, a hydrogen atom, a carboxyl group along with an amino group and a variable group.
• Based on the variable group, the classification of amino acids falls into four categories: nonpolar, polar, negatively charged, and positively charged.
• Of the set of 20 amino acids, eleven can be made naturally by the body and are termed nonessential amino acids. Amino acids that can not be naturally made by the body are called essential amino acids.

Structure

Generally, amino acids have the following structural properties:

• A carbon (the alpha carbon)
• A hydrogen atom (H)
• A Carboxyl group (-COOH)
• An Amino group (-NH₂)
• A "variable" group or "R" group

All amino acids have the alpha carbon bonded to a hydrogen atom, carboxyl group, and amino group. The "R" group varies among amino acids and determines the differences between these protein monomers. The amino acid sequence of a protein is determined by the information found in the cellular genetic code. The genetic code is the sequence of nucleotide bases in nucleic acids (DNA and RNA) that code for amino acids. These gene codes not only determine the order of amino acids in a protein, but also determine a protein's structure and function.

Classification of Amino Acids

Amino acids can be classified into four general groups based on the properties of the "R" group in each amino acid. Amino acids can be polar, nonpolar, positively charged, or negatively charged. Polar amino acids have "R" groups that are hydrophilic, meaning that they seek contact with aqueous solutions. Nonpolar amino acids are the opposite (hydrophobic) in that they avoid contact with liquid. These interactions play a major role in protein folding and give proteins their 3-D structure. Below is a listing of the 20 amino acids grouped by their "R" group properties. The nonpolar amino acids are hydrophobic, while the remaining groups are hydrophilic.

Nonpolar Amino Acids

Polar Amino Acids

Polar Basic Amino Acids (Positively Charged)

Polar Acidic Amino Acids (Negatively Charged)

While amino acids are necessary for life, not all of them can be produced naturally in the body. Of the 20 amino acids, 11 can be produced naturally. These nonessential amino acids are alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, and tyrosine. With the exception of tyrosine, nonessential amino acids are synthesized from products or intermediates of crucial metabolic pathways. For example, alanine and aspartate are derived from substances produced during cellular respiration. Alanine is synthesized from pyruvate, a product of glycolysis. Aspartate is synthesized from oxaloacetate, an intermediate of the citric acid cycle. Six of the nonessential amino acids (arginine, cysteine, glutamine, glycine, proline, and tyrosine) are considered conditionally essential as dietary supplementation may be required during the course of an illness or in children. Amino acids that can not be produced naturally are called essential amino acids. They are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Essential amino acids must be acquired through diet. Common food sources for these amino acids include eggs, soy protein, and whitefish. Unlike humans, plants are capable of synthesizing all 20 amino acids.

Amino Acids and Protein Synthesis

Proteins are produced through the processes of DNA transcription and translation. In protein synthesis, DNA is first transcribed or copied into RNA. The resulting RNA transcript or messenger RNA (mRNA) is then translated to produce amino acids from the transcribed genetic code. Organelles called ribosomes and another RNA molecule called transfer RNA help to translate mRNA. The resulting amino acids are joined together through dehydration synthesis, a process in which a peptide bond is formed between the amino acids. A polypeptide chain is formed when a number of amino acids are linked together by peptide bonds. After several modifications, the polypeptide chain becomes a fully functioning protein. One or more polypeptide chains twisted into a 3-D structure form a protein.

Biological Polymers

While amino acids and proteins play an essential role in the survival of living organisms, there are other biological polymers that are also necessary for normal biological functioning. Along with proteins, carbohydrates, lipids, and nucleic acids constitute the four major classes of organic compounds in living cells.

Sources

• Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011.