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Amino Acid

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Amino Acid Structure
Amino Acid Structure

Basic Amino Acid Structure

Credit: Yassine Mrabet

Amino Acids

An amino acid is an organic molecule that, when linked together with other amino acids, forms 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.

Amino Acid 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 they also determine a protein's structure and function.

Amino Acid Groups

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

  • Ala: Alanine           Gly: Glycine           Ile: Isoleucine            Leu: Leucine
  • Met: Methionine     Trp: Tryptophan     Phe: Phenylalanine     Pro: Proline
  • Val: Valine
    Polar Amino Acids

  • Cys: Cysteine         Ser: Serine            Thr: Threonine
  • Tyr: Tyrosine          Asn: Asparagine     Gln: Glutamine
    Polar Basic Amino Acids (Positively Charged)

  • His: Histidine           Lys: Lysine            Arg: Arginine
    Polar Acidic Amino Acids (Negatively Charged)

  • Asp: Aspartic acid     Glu: Glutamic acid
While amino acids are necessary for life, not all of them can be produced naturally in the body. Of the 20 amino acids, 10 can be produced naturally. These amino acids are alanine, proline, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, serine, glycine, and tyrosine. The amino acids that can not be produced naturally are called essential amino acids. They are arginine (essential for children), histidine, threonine, isoleucine, methionine, leucine, lysine, phenylalanine, tryptophan, and valine. The essential amino acids must be acquired through diet. Unlike humans, plants are capable of synthesizing all 20 amino acids.

Protein Synthesis

Proteins are produced through the processes of DNA transcription and translation. In protein synthesis, DNA is first copied into RNA. The RNA transcript, messenger RNA (mRNA), is then translated into amino acids. Cell structures called ribosomes along with another RNA molecule called transfer RNA help to translate mRNA into amino acids. 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.

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