4. Amino acids Flashcards
An amino acid is a carbon compound with both what?
An amine group (-NH2) and a carboxyl group (-COOH).
Both of these groups are attached to the same carbon, which we call the central carbon atom. What are the other two groups attached?
A hydrogen atom (H) and a variable amino acid side chain (when unspecified, we use R)
What makes each amino acid unique?
Side chain or R group.
Scientists have discovered up to five hundred naturally occurring amino acids. Out of this number, the human body only uses how many amino acids to make all the proteins that keep us alive?
20
In the amino acid glycine, the side chain is what?
Hydrogen.
Most naturally occurring amino acids are what?
α-amino acids.
An α-amino acid is one where the two functional groups (−NH2 and −COOH) are attached to the same carbon atom. Cells use these amino acids to synthesise proteins.
Amino acids are chiral, meaning that amino acids are capable of forming what?
Enantiomers—mirror image molecules.
Which enantiomer is more common in nature, which cells use to build proteins?
The L form.
What are the 4 general groups of amino acid?
- Non-polar amino acids have non-polar side chains, typically alkyl groups.
- Polar amino acids contain polar groups such as −OH or −CONH2.
- Positively-charged amino acids typically contain additional −NH2 groups.
- Negatively-charged amino acids typically contain additional −COOH groups.
Amino acids also share some common properties, namely their ability to form what?
Zwitterions.
Amino acids may act as an acid or a base depending on the pH of the solution.
When the solution is acidic, there is an excess of what?
Protons (H+).
Because amino acids can act both as an acid or a base, we call them what?
Amphoteric.
The isoelectric point (pI) is the pH where the amino acid is at a net neutral charge, i.e. where the zwitterionic form is most dominant.
This is not the same as neutral pH (7), because some amino acids contain extra −COOH or −NH2 groups in their side chains, skewing the balance in one direction. If there are more −COOH groups, the amino acid needs more acidic conditions to become fully neutral; if there are more −NH2 groups, the amino acid needs more basic conditions to become fully neutral. As a result, each amino acid has a differentwhat?
Isoelectric point.
We can make use of the isoelectric point in electrophoresis.
Electrophoresis is a procedure that separates molecules based on their movement under the influence of what?
An electric field.
To perform this procedure, scientists insert a solution of mixed molecules in the middle of a medium (typically a gel). They buffer the pH of the medium to a set value, and then apply an electric field.
At one end of the medium lies the anode, a positive electrode which attracts negatively-charged particles; on the other, the cathode, a negative electrode which attracts positively-charged particles.
How does this apply for an amino acid?
When the pH is more acidic than the pI, the amino acid becomes positively charged, so it moves to the cathode.
When the pH is more basic than the pI, the amino acid becomes negatively charged, so it moves to the anode.
Glycine has a pI of 6.1, while cysteine has a pI of 5. Say we buffer the solution to a pH of 5.5.
This is more acidic than 6.1, so glycine will be in the positively-charged acid form.
This is more basic than 5, so cysteine will be in the negatively-charged base form.
Under the influence of the electric field, glycine will move to the _____, while cysteine moves to the _____.
Cathode, anode.
Fortunately, humans can synthesise eleven of the twenty amino acids that make up our proteins. But what about the other nine?
These nine amino acids are termed essential because we cannot synthesise them—they must come from where?
Our diets.
In a condensation reaction, the carboxyl group (−COOH) of one amino acid reacts with the _____ group (−NH2) of the other amino acid.
Amino.
The dipeptide begins with an amino group and ends on which group?
A carboxyl group.
Peptide chains can be many amino acids long.
As the peptide chain grows longer, the terminology we use also changes.
What is the name of the longest chain?
Protein.
The peptide link is the bond between the carboxyl group and the amino group, abbreviated as −CONH−. As we mentioned before, the positioning of the hydrogen atom next to the C=O bond does not matter.
The easiest way to identify the peptide link is to keep an eye out for nitrogen atoms. Once you find one, look for a neighbouring carbonyl group (C=O). Be sure to circle both the ___ and the ___.
Nitrogen atom and the carbonyl group.
