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Amino Acid Classification and Structure

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Amino Acids

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Amino Acids

An amino acid is a compound that contains both an amino group (−NH 2 ) and a carboxyl group (−COOH) in the same molecule. While any number of amino acids can possibly be imagined, biochemists generally reserve the term for a group of 20 amino acids which are formed and used by living organisms. The Figure below shows the general structure of an amino acid.

An amino acid is an organic molecule that contains an amino group, a carboxyl group and a side chain (R), all bonded to a central carbon atom.

The amino and carboxyl groups of an amino acid are both covalently bonded to a central carbon atom. That carbon atom is also bonded to a hydrogen atom and an R group. It is this R group which varies from one amino acid to another and is called the amino acid side chain.

The nature of the side chains accounts for the variability in physical and chemical properties of the different amino acids. Some side chains consist of nonpolar aliphatic or aromatic hydrocarbons. Other side chains are polar, while some are acidic or basic.

Five of the twenty biologically relevant amino acids, each having a distinctive side chain (R). Alanine’s side chain is nonpolar, while threonine’s is polar. Tryptophan is one of several amino acids whose side chain is aromatic. Aspartic acid has an acidic side chain, while lysine has a basic side chain.

The Table below lists the names of the 20 naturally occurring amino acids along with a three-letter abbreviation which is used to describe sequences of linked amino acids.

Amino Acids and Abbreviations

Amino acid

Abbreviation

Amino acid

Abbreviation

Alanine

Ala

Leucine

Leu

Arginine

Arg

Lysine

Lys

Asparagine

Asn

Methionine

Met

Aspartic acid

Asp

Phenylalanine

Phe

Cysteine

Cys

Proline

Pro

Glutamine

Gln

Serine

Ser

Glutamic acid

Glu

Threonine

Thr

Glycine

Gly

Tryptophan

Trp

Histidine

His

Tyrosine

Tyr

Isoleucine

Ile

Valine

Val

Another more recent set of abbreviations employs only one letter. Leucine would be designated by L, serine by S, tyrosine by Y. The advantage of this system comes when listing the amino acid sequence of a protein that may contain over 100 amino acids in its chain.

Summary

  • Amino acid is defined.
  • The generic amino acid structure is shown.
  • Common amino acids are listed.

Practice

Read the material at the link below and answer the following questions:

http://hyperphysics.phy-astr.gsu.edu/hbase/organic/essam.html

  1. What are essential amino acids?
  2. What are nonessential amino acids?
  3. What happens if you are deficient in an amino acid?

Review

  1. What is an amino acid?
  2. What is the three-letter abbreviation for histidine?
  3. Why would the one-letter abbreviation system have an advantage?

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