MCAT Content / Amino Acids Peptides Proteins / Amino Acids Description

Amino acids: description

Topic: Amino Acids Peptides Proteins

Amino acids are the monomers (building blocks) of proteins.

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

The way these groups are arranged around the α-carbon determines the amino acid’s absolute configuration, and helps distinguish the amino acid’s two possible enantiomers (non-superimposable mirror image forms). The absolute configuration is denoted as either L or D, which does not necessarily correlate to S and R, depending on the R group. In general if the “priority” is NH2>COOH>R, then S=L-amino acid and R=D-amino acid, but if the priority is NH2>R>COOH, then S=D-amino acid and R=L-amino acid. L-amino acids are more common in nature and are the only type found in proteins. D-amino acids are less common and never found in proteins.

There are 21 different amino acids which can be present in proteins, each with a specific side chain. Ten of these are considered essential amino acids in humans because the human body cannot produce them and they must be obtained from the diet. All organisms have different essential amino acids based on their physiology.

In the aqueous environment of the cell and under physiological conditions, both the amino group and the carboxyl group are ionized, so they have the structures -NH3+ and -COO, respectively. This is why amino acids are considered dipolar ions. Further, at low pH, amino acids are in their cationic form, and at high pH, they exist in their anionic form. 

The R group, or side chain, which is bound to the α-carbon, gives each amino acid specific characteristics, including size, polarity, and pH.

The chemical composition of the side chain determines the characteristics of the amino acid. Amino acids such as valine, methionine, and alanine are nonpolar (hydrophobic), while amino acids such as serine, threonine, and cysteine are polar (hydrophilic). Hydrophobicity can be measured using a hydropathy index, where a positive index indicates a more hydrophobic animo acid. The side chains of lysine, histidine, and arginine are positively charged under physiological conditions (and contain amine groups in their side chains), so these amino acids are basic. The side chains of aspartate and glutamate are negatively charged under physiological conditions (they contain a carboxylate/carboxylic acid in their side chain), so they are acidic. 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.

Amino acids can be synthesized through the Gabriel synthesis, which is a chemical reaction that transforms primary alkyl halides into primary amines.

Amino acids can also be made via the Strecker synthesis, which involves the reaction of an aldehyde with ammonium chloride followed by condensation to yield an α-aminonitrile, which is then hydrolyzed in acidic conditions.


Practice Questions

 

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MCAT Official Prep (AAMC)

Official Guide C/P Section Passage 1 Question 1

Section Bank C/P Section Passage 4 Question 31

Section Bank C/P Section Passage 10 Question 74

Section Bank C/P Section Question 83

Section Bank C/P Section Question 84

Section Bank C/P Section Passage 11 Question 88

Sample Test C/P Section Passage 1 Question 4

Sample Test C/P Section Question 17

Sample Test C/P Section Passage 6 Question 33

Sample Test C/P Section Passage 9 Question 47

Sample Test C/P Section Passage 9 Question 49

Practice Exam 1 C/P Section Passage 5 Question 23

Practice Exam 1 C/P Section Passage 5 Question 25

Practice Exam 1 C/P Section Passage 7 Question 34

Practice Exam 2 C/P Section Passage 2 Question 5

Practice Exam 2 C/P Section Passage 3 Question 13

Practice Exam 2 C/P Section Passage 3 Question 14

Practice Exam 2 C/P Section Passage 4 Question 17

Practice Exam 2 C/P Section Passage 4 Question 19

Practice Exam 2 C/P Section Passage 4 Question 20

Practice Exam 3 C/P Section Passage 6 Question 33

Practice Exam 3 C/P Section Passage 7 Question 36

Practice Exam 4 C/P Section Passage 7 Question 36

Practice Exam 4 C/P Section Passage 9 Question 48

Online Flashcards Biochemistry Question 3

Online Flashcards Biochemistry Question 13

Online Flashcards Biochemistry Question 14

Online Flashcards Biochemistry Question 25

Official Guide B/B Section Passage 5 Question 17

Official Guide B/B Section Passage 5 Question 19

Section Bank B/B Section Passage 1 Question 5

Section Bank B/B Section Passage 2 Question 8

Section Bank B/B Section Question 13

Section Bank B/B Section Passage 3 Question 21

Section Bank B/B Section Passage 5 Question 35

Section Bank B/B Section Passage 7 Question 49

Section Bank B/B Section Question 56

Section Bank B/B Section Passage 9 Question 66

Section Bank B/B Section Passage 9 Question 67

Section Bank B/B Section Passage 10 Question 79

Sample Test B/B Section Passage 4 Question 19

Sample Test B/B Section Passage 5 Question 26

Sample Test B/B Section Question 45

Sample Test B/B Section Question 57

Section Bank B/B Section Passage 9 Question 66

Practice Exam 1 B/B Section Passage 2 Question 7

Practice Exam 3 B/B Section Passage 2 Question 8

Practice Exam 3 B/B Section Passage 5 Question 24

Practice Exam 4 B/B Section Passage 2 Question 8

Practice Exam 4 B/B Section Passage 3 Question 20

Practice Exam 4 B/B Section Passage 5 Question 25


 

Key Points

• Amino acids are the monomers that make up 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), a hydrogen atom, and a side chain (R group).

• Amino acids with L absolute configuration are more common in nature and are the only type found in proteins.

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

• Amino acids can be made through the Gabriel or Strecker synthesis.


Key Terms

Amino acid: monomer of proteins

Side chain: R group unique to each amino acid

Absolute configuration at α-carbon: the arrangement of amino, caboxyl, and R-group around the α-carbon give it either L- or D-configuration.

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

Zwitterion: an amino acid which has separate positively and negatively charged groups

Gabriel synthesis: chemical reaction that transforms primary alkyl halides into primary amines

Strecker synthesis: chemical reaction that transforms an aldehyde to an amino acid

Hydropathy index: measurement of the hydrophobicity of a specific amino acid in a protein (higher = more hydrophobic)



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