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What are the Standard Twenty Amino Acids?

Updated on August 30, 2016

Below are some notes on amino acids that can be used for an understanding or refresher that is helpful and useful for the sciences; specifically biology, chemistry, and biochemistry.

Source

What is an Amino Acid?

Amino acid= [amino group] + [carboxyl group] attached to the alpha-carbon


The alpha-carbon in an amino acid:

  • Is bonded to an amino group
  • Is bonded to a carboxyl group
  • Is bonded to a hydrogen
  • Is bonded to a side chain group (R)

*The alpha-carbon is the carbon next to the carboxyl group.

*The R group determines the identity of a particular amino acid.

Key Terms and Ideas

Chiral= nonsuperimposable mirror image; an atom with four different groups bonded to it.

Achiral=not chiral; an atom that does not have four different groups bonded to it.

*All amino acids except glycine have a chiral center.

The amino acids found in proteins are of the L form.

The D form of amino acids occur in nature.

About Amino Acids

One of the most important properties of amino acids is their three-dimensional shape or stereochemistry.

With chiral centers, the amino acids can form two possible stereoisomers that are either in the L or D form.

Amino acids are classified according to:

  • The polar/non-polar nature of the side chain
  • The presence of an acidic or basic group in the side chain

Other useful criteria for classifying amino acids:

  • The presence of functional groups other than acidic or basic ones in the side chain
  • The nature of other functional groups in the side chain


A free amino acid at neutral pH:

  • carboxylate portion=negatively charged
  • amino group=positively charged

Zwitterion:

  • has equal positive and negative charges
  • is electrically neutral in solution

Neutral amino acids do not exist in the form:

NH2--CHR--COOH


(Neutral amino acids do not exist without charged groups).

Electrophoresis=a common method for separating molecules in an electric field.

  • This method is useful in determining the important properties of proteins and nucleic acids.

Amino Acids and Titration

Amino acids can act as both acids and bases.

In a free amino acid, the carboxyl group and the amino group are charged at neutral pH.

Amino acids without charged groups on their side chains exist as zwitterions with no net charge.

When an amino acid is titrated, its titration curve indicates the reaction of each functional group with hydrogen ion.

The classification of an amino group as acidic or basic depends on the pKa of the side-chain as well as on the chemical nature of the group.

Since amino acids, peptides, and proteins have different pKa values, there is the possibility that they can have different charges at a given pH.

The titratable groups of each of the amino acids have characteristic pKa values.


pI= [pKa1 + pKa2]/2

pKa1=the functional group that has dissociated at its isoelectric point; if two groups are dissociated at isoelectric pH, the pKa1 is the higher pH of the two.

pKa2=the functional group that has not dissociated at its isoelectric pH; if there are two groups that are not dissociated, the one with the lower pKa is used.



Isoelectric pH/Isoelectric Point (pI)

Isoelectric pH/isoelectric point (pI)= the pH at which a molecule has no net charge; at this point, a molecule will not migrate in an electric field. This property can be put to use in separation methods.

*While most amino acids have only two pKa values, we must be sure to average the correct pKa values for acidic and basic amino acids.

pKa Trends for Amino Acids

pKa of Alpha-Carboxyl Groups
pKa of Amino Groups
pKa of Side-Chain Groups (Including Carboxyl and Amino Groups)
Low; around 2.
High; from 9 to 10.5.
Depend on the groups' chemical nature.

Organizing and Learning Amino Acids

Below are different ways to organize, identify, and learn amino acids.

Names, Abbreviations, and Symbols for the 20 Standard Amino Acids

Name
Abbreviation
Symbol
Alanine
Ala
A
Arginine
Arg
R
Asparagine
Asn
N
Aspartic Acid
Asp
D
Cysteine
Cys
C
Glutamine
Gln
Q
Glutamic Acid
Glu
E
Glycine
Gly
G
Histidine
His
H
Isoleucine
Ile
I
Leucine
Leu
L
Lysine
Lys
K
Methionine
Met
M
Phenylalanine
Phe
F
Proline
Pro
P
Serine
Ser
S
Threonine
Thr
T
Tryptophan
Trp
W
Tyrosine
Tyr
Y
Valine
Val
V
Asparagine/Aspartic Acid*
Asx*
B*
Glutamine/Glutamic Acid*
Glx*
Z*
This chart was made according to the way the twenty amino acids were listed on: http://www.cryst.bbk.ac.uk/education/AminoAcid/the_twenty.html. The * =when it is not possible to differentiate or determine two amino acids.

Amino Acids Grouped According to Side-Chain Characteristics

Positively-Charged (Basic)
Negatively-Charged (Acidic)
Polar
Non-Polar (Hydrophobic)
Aliphatic
Aromatic
Hydroxylic (OH-Containing)
Sulphur-Containing
Amidic
Imino Acid
Arginine (Arg, R)
Aspartic Acid (Asp, D)
Serine (Ser, S)
Glycine (Gly, G)
Alanine (Ala, A)
Phenylalanine (Phe, F)
Serine (Ser, S)
Cysteine (Cys, C)
Asparagine (Asn, N)
Proline (Pro, P)
Histidine (His, H)
Glutamic Acid (Glu, E)
Cysteine (Cys, C)
Alanine (Ala, A)
Glycine (Gly, G)
Tryptophan (Trp, W)
Threonine (Thr,T)
Methionine (Met, M)
Glutamine (Gl, Q)
 
Lysine (Lys, K)
 
Threonine (Thr, T)
Valine (Val, V)
Isoleucine (Ile, I)
Tyrosine (Tyr, Y)
Tyrosine (Tyr, Y)
 
Lysine (Lys, K)
 
 
 
Asparagine (Asn, N)
Leucine (Leu, L)
Leucine (Leu, L)
 
 
 
 
 
 
 
Glutamine (Gln, Q)
Isoleucine (Ile, I)
Proline (Pro, P)
 
 
 
 
 
 
 
Tyrosine (Tyr, Y)
Proline (Pro, P)
Valine (Val, V)
 
 
 
 
 
 
 
 
Methionine (Met, M)
 
 
 
 
 
 
 
 
 
Phenylalanine (Phe, F)
 
 
 
 
 
 
 
 
 
Tryptophan (Tyr, Y)
 
 
 
 
 
 
This chart was made by me combined with the way the twenty amino acids were listed on: http://www.cryst.bbk.ac.uk/education/AminoAcid/the_twenty.html.

Basic=being basic in nature; being an [H+] acceptor

Hydroxylic=containing a hydroxyl group

Sulfur-Containing=containing a sulfur group

Amidic=having an amide group

Aliphatic=absence of a benzene ring or related structure

Aromatic=having a benzene ring or related structure

Acidic=being acidic in nature; being an [H+] donor


Categories of Amino Acids at pH 7

Acidic ("-" charged)
Basic ("+" charged)
Neutral ("0" charged)
Polar (Uncharged)
Non-Polar (Hydrophobic)
Glutamic Acid (Glu, E)
Lysine (Lys, K)
Alanine (Ala, A)
Asparagine (Asn, N)
Alanine (Ala, A)
Aspartic Acid (Asp, D)
Arginine (Arg, R)
Asparagine (Asn, N)
Cysteine (Cys, C)
Glycine (Gly, G)
--
Histidine (His, H)
Cysteine (Cys, C)
Glutamine (Gln, Q)
Isoleucine (Ile, I)
--
--
Glutamine (Gln, Q)
Serine (Ser, S)
Leucine (Leu, L)
--
--
Glycine (Gly, G)
Threonine (Thr, T)
Methionine (Met, M)
--
--
Isoleucine (Ile, I)
Tyrosine (Tyr, Y)
Phenylalanine (Phe, F)
--
--
Leucine (Leu, L)
--
Proline (Pro, P)
--
--
Methionine (Met, M)
--
Tryptophan (Trp, W)
--
--
Phenylalanine (Phe, F)
--
Valine (Val, V)
--
--
Proline (Pro, P)
--
--
--
--
Serine (Ser, S)
--
--
--
--
Threonine (Thr, T)
--
--
--
--
Tryptophan (Trp, W)
--
--
--
--
Tyrosine (Tyr, Y)
--
--
--
--
Valine (Val, V)
--
--
This chart was made based on: http://njms2.umdnj.edu/biochweb/education/bioweb/PreK2010/AminoAcids.htm and "Biochemistry" by Mary K. Campbell and Shawn O. Farrel, 7th edition.

Amino Acids and Behaviors

Amino Acids
Behavior
Alanine (Ala, A)
At very low pH it has a protonated and uncharged carboxyl group and a protonated and positively charged amino group; as base is added, the carboxyl group loses its proton to become a negatively charged carboxylate group and the pH of the solution increases.
Arginine (Arg, R)
Can accept a proton (basic).
Asparagine (Asn, N)
Has an amide group in its side chain that does not ionize in the range of pH usually found in biochemistry; is a derivative of aspartic acid.
Aspartic Acid (Asp, D)
Has an additional carboxy group in its side chain that can lose a proton, and forms the corresponding carboxylate anion; negatively charged at neutral pH.
Cysteine (Cys, C)
Polar side chain with thiol group (--SH) can react with other cysteine thiol groups to form disulfide bridges in proteins in an oxidation reaction. The thiol group can also lose a proton.
Glutamic Acid (Glu, E)
Has an additional carboxy group in its side chain that can lose a proton, and forms the corresponding carboxylate anion; negatively charged at neutral pH.
Glutamine (Gln, Q)
Has an amide group in its side chain that does not ionize in the range of pH usually found in biochemistry; is a derivative of glutamic acid.
Glycine (Gly, G)
 
Histidine (His, H)
Can be found in the protonated and unprotonated forms in proteins; can accept a proton (basic).
Isoleucine (Ile, I)
 
Leucine (Leu, L)
 
Lysine (Lys, K)
Can accept a proton (basic).
Methionine (Met, M)
 
Phenylalanine (Phe, F)
 
Proline (Pro, P)
 
Serine (Ser, S)
Would require a very high pH to lose a proton.
Threonine (Thr, T)
Would require a very high pH to lose a proton.
Tryptophan (Trp, W)
 
Tyrosine (Tyr, Y)
Hydroxyl group loses a proton at higher pH in titration.
Valine (Val, V)
 
Information for this chart taken from: "Biochemistry" by Mary K. Campbell and Shawn O. Farrel.

Buried and Surface Amino Acids

Buried
Surface
Alanine (Ala, A)
Arganine (Arg, R)
Cysteine (Cys, C)
Asparagine (Asn, N)
Isoleucine (Ile, I)
Aspartic Acid (Asp, D)
Leucine (Leu, L )
Glutamic Acid (Glu, E)
Methionine (Met, M)
Glutamine (Gln, Q)
Phenylalanine (Phe, F)
Glycine (Gly, G)
Tryptophan (Trp, W)
Histidine (His, H)
Valine (Val, V)
Lysine (Lys, K)
--
Proline (Pro, P)
--
Serine (Ser, S)
--
Threonine (Thr, T)
--
Tyrosine (Tyr, Y)
This chart is based off a chart found at http://www.geneinfinity.org/sp/sp_aaprops.html.

Amino Acids and What They Do

Amino Acid
What It Does
Alanine (Ala, A)
Supports glucose metabolism; helps to maintain a healthy nitrogen balance via the alanine cycle; is an important component of collagen.
Arginine (Arg, R)
Can be made by the body in adults but not in sufficient amounts in children; supports and speeds up wound healing/tissue repair; strengthens the immune system; promotes the release of hormones; helps to reduce blood pressure; precursor to nitric oxide (NO).
Asparagine (Asn, N)
Helps to keep the central nervous system (CNS) healthy; helps to maintain equilibrium/balance; plays an important role in changing one amino acid into another (transamination).
Aspartic Acid (Asp, D)
Participates in the urea cycle of the liver and helps to maintain a healthy nitrogen balance; participates in amino acide and nucleotide metabolism; plays an important role in the Citric Acid Cycle (TCA; Krebs Cycle) for energy production.
Cysteine (Cys, C)
Is important for healthy skin, hair, and nails; participates in the collagen production for healthy connective tissue; is a precursor for taurine to build healthy cell walls and myelin sheaths for neurons; assists liver in detoxification via the glutathione pathways; supports the metabolism of coenzyme A, heparin, biotin, lipoic acidm and glutathione.
Glutamic Acid (Glu, E)
Supports sugar and fat metabolism; is an important excitatory neurotransmitter; is used in the treatment of neurological illnesses.
Glutamine (Gln, Q)
Provides energy fuel for intestinal cells and thus promotes a healthy digestive system; supports GABA production for mental health and to stabilize moods; speeds up the recovery from muscle wasting during prolonged illness and post-surgery; helps to maintain a healthy nitrogen balance; supports a healthy acid/alkaline balance; participates in DNA/RNA synthesis.
Glycine (Gly, G)
Is an inhibitory neurotransmitter to alleviate hyperactivity, epilepsy, and manic depression; participates in the production of other amino acids, nucleic acids (RNA, DNA) and bile; is necessary for the conversion of glucose into energy; necessary for healthy collagen formation, which provides pliability and resilience to the connective tissue.
Histidine (His, H)
Is a precursor to histamine, which stimulates chloric acid production in the stomach, vital for healthy digestion and protection from food-borne pathogens; supports healthy growth and promotes tissue repair; helps to maintain the integrity of the myelin sheaths that cover many neurons for speedy nerve conduction; important for conversion on glucose into glycogen by the liver supporting blood sugar regulation; necessary for hemoglobin formation in red blood cells, as well as proper storage and movement of iron, helping to ward off anemia; serves as an important antioxidant and promotes cardiovascular health; removal of heavy metals.
Isoleucine (Ile, I)
Is necessary for hemoglobin formation; helps to regulate blood sugar; promotes muscle recovery after physical exercise; provides fuel for muscle work.
Leucine (Leu, L)
Supports and speeds up tissue repair/wound healing; is necessary for growth hormone production; helps to regulate blood sugar; promotes muscle recovery after physical exercise.
Lysine (Lys, K)
Supports healthy growth and bone development in children; promotes calcium absorption; important in the production of enzymes, hormones, antibodies; supports and speeds up tissue repair; provides fuel for muscle work.
Methionine (Met, M)
Facilitates lipid (fat) metabolism, healthy digestion, blood lipid regulation; important in heavy metal detoxification; is an important antioxidant.
Phenylalanine (Phe, F)
Is a precursor to tyrosine; is potentially helpful for chronic pain, Parkinson's, and mood disorders.
Proline (Pro, P)
Is an important component of collagen; promotes connective tissue health for bones, joints, tendons, and skin; helps to speed up recovery after injuries to the joints, tendons, and ligaments, as well as after severe burns to the skin.
Serine (Ser, S)
Is necessary for the production of tryptophan, which helps to calm the nerves and promote sleep; supports antibody and immunoglobulin production for a healthy immune system; helps to maintain healthy cell membranes as part of the phospholipid bilayer; participates in urine and pyrimidine metabolism; an important component in the myelination of nerves and nerve conduction.
Threonine (Thr, T)
Is a precursor to isoleucine; it can be transformed by the body into glycine; promotes healthy thymus growth and function, thus contributing to a healthy immune system; participates in the antibody formation to ward off infections and promote speedy recovery; is important to lipid (fat) metabolism and regulation.
Tryptophan (Trp, W)
Is a precursor to niacin (vitamin B3) and serotonin (neurotransmitter); promotes mental health and well being; is helpful for children/adults with ADHD; faciliates sleep.
Tyrosine (Tyr, Y)
Is a precuror to epinephrine, norepinephrinem dopamine; is used to treat mood disorders, Parkinson's, narcolepsy, mental/emotional stress; is necessary for the production of thyroid hormone; is a precursor to melanin and thus important for skin, hair, and eye pigmentation.
Valine (Val, V)
Provides fuel for muscle work; is vital for healthy muscle tissue and tissue repair; is helpful with alcohol related liver damage and hepatic encephalopathy, as well as degenerative neurological illness.
Information on functions of specific amino acids was taken from: http://www.whitetigernaturalmedicine.com/nutrition/proteins-amino-acids.

Source Information

The information used for this hub was taken from the following sources:

"Biochemistry" by Mary K. Campbell and Shawn O. Farrel; 7th edition.

My biochemistry lectures at school.

Knowledge and notes taken from previous courses in chemistry and biology.

http://www.whitetigernaturalmedicine.com/nutrition/proteins-amino-acids

http://en.wikipedia.org/wiki/Amino_acid

http://www.cryst.bbk.ac.uk/education/AminoAcid/the_twenty.html

http://njms2.umdnj.edu/biochweb/education/bioweb/PreK2010/AminoAcids.htm

http://www.geneinfinity.org/sp/sp_aaprops.html

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  • pharmacist2013 profile image

    pharmacist2013 4 years ago

    Great work, Amino acids are very important in biochemistry. keep on!

  • Nalini Marquez profile image
    Author

    Nalini Marquez 4 years ago

    Hi pharmacist2013, thank you! It's a lot of information so hopefully this method makes it stick. :-)

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