MCAT Amino Acids: What You Need to Know

October 20, 2023
5 min read


Reviewed by:

Akhil Katakam

Third-Year Medical Student, Lewis Katz School of Medicine at Temple University

Reviewed: 10/20/23

Preparing for the MCAT and are wondering how you should study for the biochemistry segment? Read on to learn how to tackle MCAT amino acids.

Male and female student studying together

To put it bluntly: we need amino acids to function. They’re involved in the production of proteins and metabolism molecules and the prevention of various diseases. In fact, they make up 20% of our body weight, the largest component after water. 

Becoming a doctor requires knowledge of the molecules essential to optimal health. The MCAT wants to make sure that you know the foundations of the human body and have the capacity to learn more. Read on to find out what you need to know about amino acids for the MCAT. 

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What are Amino Acids?

Amino acids are organic molecules that join together to make proteins. The human body uses proteins for growth, the breakdown of food, repair, and many other functions. This is why amino acids are often referred to as the “building blocks” of both proteins and life.

Twenty amino acids are necessary for the proper functioning of the human body. Each one is made up of a central carbon atom that is bound to three separate regions:

  • An amino group, or N-terminus 
  • A carboxylic acid group, or C-terminus 
  • A variable side chain, or R-group, that is unique to each amino acid

What Do I Need to Know About Amino Acids MCAT? 

There are 20 unique R-groups, meaning that there are 20 unique amino acids that you could be tested on in the MCAT. What do you need to know about each of these amino acids? Read online, and it seems most people shrug and say “everything.” 

With the comprehensiveness of the MCAT, I see where they’re going on that, but it’s definitely not the most comforting advice for someone trying to figure out where to start studying for the biochemistry segment. Below we will break down how you should approach studying amino acids on the MCAT.

Know the Basics

It’s probably not surprising to you that you should know the basics of each of the 20 amino acids. But what are they?

They are:

  • The one-letter code 
  • The three-letter code
  • Structure of the side chain

For instance, the structure of cysteine is shown below in an amino acid structure diagram. C and Cys are its one-letter and three-letter code, respectively. 

You should also know key properties, such as if an amino acid, depending on its side chain, is polar (aka hydrophilic) or nonpolar (aka hydrophobic) and basic or acidic. If applicable, you should know the side chain’s pKa (which, in general terms, is a value that indicates how strong or weak an acid is).

Being familiar with the structure of an amino acid can help you determine some of its properties. For instance, nonpolar amino acids mainly consist of hydrocarbons. 

What Does the MCAT Website Have to Say?

Beyond the basics, the AAMC Content Guide indicates that you should know:

  • Absolute configuration at the α position
  • Reactions involving amino acids, such as the formation of sulfur linkages for cysteine and cystine, and peptide linkages for polypeptides and proteins
  • How α-amino acids are synthesized, such as the Strecker Synthesis and Gabriel Synthesis 
  • The primary, secondary, and tertiary structures of proteins

Amino Acid Structure

Aside from just memorizing the amino acids MCAT chart, it’s important you actually understand the amino acid structures, as they play a crucial role in biochemistry and the functioning of biological molecules. Some of the major topics to familiarize yourself with are:

  • General structure: Amino amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain or R group. The R group differentiates one amino acid from another.
  • Amino Acid Nomenclature: Understand the one-letter and three-letter abbreviations for amino acids.
  • Amino Acid Stereoisomerism: Amino acids are chiral molecules that exist in two enantiomeric forms: L-amino acids and D-amino acids.
  • Primary Structure: The primary structure of a protein is the sequence of amino acids linked together. This sequence determines the protein's function and structure.
  • Secondary Structure: Amino acids can form secondary structures like alpha helices and beta sheets due to hydrogen bonding between the peptide bonds in the backbone.
  • Tertiary Structure: Tertiary structure is the three-dimensional folding of a protein due to interactions between the R groups of amino acids. These interactions include disulfide bonds, hydrogen bonds, hydrophobic interactions, and more.
  • Quaternary Structure: The arrangement of multiple polypeptide chains.
  • Ionization of Amino Acids: Amino and carboxyl groups are protonated or deprotonated under different pH conditions.
  • pH and Isoelectric Point (pI): You should understand how pH affects the charge of amino acids. There will certainly be questions related to isoelectric points on the MCAT!
  • Titration Curves: You might be asked to interpret titration curves that show how the pH changes as acid or base is added to a solution containing amino acids.
  • Amino Acid Side Chains: Familiarize yourself with the properties and classifications of different R groups, as they play a significant role in protein function and structure.
  • Amino Acid Biosynthesis: Some amino acids can be synthesized by the human body, while others must be obtained through the diet. Understanding these pathways is important.
  • Amino Acid Modifications: Some amino acids can undergo post-translational modifications, which can affect protein function (phosphorylation, glycosylation, and acetylation).

Amino Acid Configuration

Amino acid configuration, particularly the L and D forms, is important to understand for the MCAT. Here’s what to focus on:

  • Chirality: Amino acids are chiral molecules, except glycine. This is because they are carbon atoms, with four substituents bonded to them.
  • L- and D-Amino Acids: Amino acids are classified into two enantiomeric forms: L-amino acids (levorotatory) and D-amino acids (dextrorotatory). In biological systems, proteins are composed of L-amino acids, and they are the predominant form;  D-amino acids are rare.
  • L-Amino Acids in Proteins: Proteins are composed of amino acids in the L-configuration. The L-form is important for protein structure and function in living organisms.
  • D-Amino Acids in Nature: D-amino acids can be found in certain contexts in nature but are generally rare in proteins. They are more commonly found in the cell walls of some bacteria.
  • Optical Activity: L- and D-amino acids are optically active and rotate plane-polarized light. This property can be used to distinguish between the two forms in a laboratory setting.
  • Meso Compounds: Some molecules can have a meso configuration, which means they are achiral even though they have chiral centers.
  • Racemization: Under certain conditions, amino acids can undergo racemization, converting L-amino acids to D-amino acids.

Amino Acid Classification

Amino acids can be classified into different groups based on the properties of their R groups. These groups include nonpolar (hydrophobic), polar (hydrophilic), acidic, and basic amino acids. These are the basic groups that you should know, as well as some related topics:

  • Aromatic Amino Acids: Aromatic amino acids are a subset of nonpolar amino acids and include phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp). They have aromatic ring structures.
  • Sulfur-Containing Amino Acids: Methionine (Met) and Cysteine (Cys) contain sulfur in their side chains. Cysteine is known for its ability to form disulfide bonds, which play a critical role in stabilizing protein structures.
  • Amino Acid Side Chain Reactivity: Understand the reactivity of amino acid side chains.
  • Titration and pKa Values: You should be familiar with the concept of pKa values to better understand the ionization state of amino acids at different pH levels.

Peptide Bonds

Amino acids are linked together through peptide bonds. This involves the dehydration synthesis process and the release of water, which is an important process to know for the MCAT. Additionally, you should familiarize yourself with the following topics:

  • Peptide Bond Notation: Know the notation used to represent peptide bonds, typically shown as a line or dash (-) between the nitrogen of the amino group and the carbon of the carboxyl group of two adjacent amino acids.
  • Amine and Carbonyl Groups: The amino group of one amino acid attacks the carbonyl carbon of the carboxyl group in another amino acid. This leads to the formation of the peptide bond.
  • Planar and Rigid Structure: Peptide bonds have a planar and rigid structure due to the partial double bond character of the carbon-nitrogen bond. This rigidity influences the overall protein structure.
  • Residue: Each amino acid in a peptide chain is often referred to as an amino acid residue after the peptide bond forms.
  • Hydrolysis: Peptide bonds can be hydrolyzed, breaking the bond between amino acids. This process is essential for protein digestion and degradation. Enzymes facilitate the formation and hydrolysis of peptide bonds in protein molecules. 
  • Secondary Structure Formation: Peptide bonds are critical in the formation of secondary protein structures.

MCAT Amino Acids Chart

Take a look at the structures of the 20 amino acids that you could be tested on in the MCAT. 

MCAT Amino Acids Question Examples

Here are some examples of amino acid practice questions:

Question 1

“The unique cyclic structure of which of the following amino acids plays a central role in the formation of alpha helices and beta sheets?

Choose one answer:

  1. Lysine
  2. Valine
  3. Proline
  4. Arginine”

The answer is B. Proline. Proline is the only cyclic amino acid, which you can see from its structure above.

Question 2

“Which of the following amino acids has a net negative charge at physiologic pH (~7.4)?

  1. Glutamic Acid
  2. Histidine
  3. Lysine
  4. Asparagine”

The answer is A. Glutamic Acid.

Question 3

“The amino acids in hemoglobin (or any protein) uniformly have which of the following configurations?

  1. L
  2. R
  3. S
  4. D”

The correct answer is A. L. 

Question 4

“Which of the following properties of a protein is least likely to be affected by changes in pH?

  1. Tertiary structure
  2. Primary structure
  3. Secondary structure
  4. Net charge”

The answer to this question is B. Primary structure.

Question 5

“Hydrogen bonding between separate subunits of DNA polymerase is an example of which of the following?

  1. 1 degree structure
  2. 2 degree structure
  3. 3 degree structure
  4. 4 degree structure”

Option D. 4 degree structure is correct.  

Amino Acids MCAT FAQs 

Still have questions? Let’s explore some answers to frequently asked questions about amino acids.

1. How Should You Memorize Amino Acids for the MCAT?

Mnemonics is a simple and fun way to memorize the different properties of amino acids. For instance, three amino acids have basic side chains: Histidine (H), Lysine (K), and Arginine (R). 

Remember these three by combining their one-letter code into a memorable sentence, such as “Harry’s Red Kite.”

Beyond mnemonics, you should have a clear mental image of each amino acid. If you enjoy creativity, you can draw each amino acid as something funny to make them more memorable. 

Online quizzes that test the 20 amino acids are also a great option.

2. In What Way Do Nonpolar Amino Acids Differ From Polar Ones? 

Nonpolar amino acids generally contain hydrocarbon side chains made up of carbon and hydrogen, and sometimes sulfur. Carbon, sulfur, and hydrogen atoms have a slight electronegative difference, meaning that these atoms share electrons equally. This small difference results in non-polarized bonds in the side chains of nonpolar amino acids. 

Polar amino acids, on the other hand, have a side chain with the central carbon bonded to an oxygen or nitrogen atom. Nitrogen and oxygen are more electronegative than carbon and hydrogen, meaning that they pull electrons away from carbon and hydrogen atoms. This results in a charge difference across the side chains of polar amino acids, with a partial positive charge on the carbon and a partial negative charge on the oxygen or nitrogen.

These oxygen and nitrogen atoms also can make hydrogen bonds with water, making these polar amino acids hydrophilic. Nonpolar amino acids are hydrophobic and do not interact with water.

3. What Are Peptide Bonds, and Will They Be on the MCAT? 

Peptide bonds are covalent bonds that link together two amino acids, resulting in the polymerization process in which a protein is formed. These bonds occur when the carboxylic group of one amino acid reacts with the amino group of the other, releasing a water molecule in the process. 

The MCAT website says that peptide linkages can be tested.

4. What is Chirality, and What Should I Know for the MCAT? 

A molecule is chiral if it cannot be superposed on its mirror image, just like we cannot superpose our right hand onto our left hand. Like our hands, 19 out of the 20 amino acids have chirality, meaning they are also not superposable with their mirror images. 

Asymmetry around the central carbon of amino acids, referred to as the α- carbon, is responsible for this chirality. The exception is glycine, which only has a hydrogen molecule as its R group. 

The other 19, due to their chiral nature, have a “right hand” form and a “left hand” form, referred to as the D- and L-enantiomers, respectively. The amino acids in our bodies are all in the L-form. In visual representations of amino acids, called Fischer projections, L-enantiomers are represented as having the amino group on the left. 

The MCAT site indicates that you can be tested on the absolute configurations at the α position of amino acids. You should be able to answer questions on the spatial arrangement of atoms (aka absolute configuration) of chiral amino acids.

These absolute configurations use an R/S nomenclature system, which you should become familiar with, as well as with the D/L system and general chirality.

For instance, you could be shown a picture of a molecule and asked to count how many chiral centers it contains. 

5. Should I Know the pKa of Every Amino Acid for the MCAT? 

No, you should only know the pKa  (which is the negative log of the acid dissociation constant or Ka value) of glutamate, glutamine, arginine, lysine, histidine, and aspartic acid. These amino acids are more sensitive to changes in pH. 

Quiz your knowledge on pKAs here!

Final Thoughts

There is a lot to cover with amino acids, but once you understand their underlying patterns and structures, the more difficult concepts can begin to come together. Practice, patience, and confidence is your key to tackling MCAT amino acids.

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