Q: A student is asked to draw the structure of an L-amino acid using the Fischer projection. Which of the following is a correct representation?

$$\begin{array}{c} \mathrm{COOH} \\ | \\ \mathrm{H} - \mathrm{C} - \mathrm{NH_2$} \\ | \\ \mathrm{R} \end{array}$$

Question 1

Which of the following statements about amino acid configuration in proteins is correct?

Accepted Answer

Life almost exclusively uses L-amino acids to build proteins.

Question 2

Glycine is unique among the standard amino acids because:

Accepted Answer

It is achiral and thus does not have L or D configuration.

Question 3

In the Fischer projection of an L-amino acid, which group is typically placed at the top?

Accepted Answer

The carboxyl group ($$\mathrm{COOH})$$

Question 4

Which of the following is true regarding the configuration of L-amino acids (excluding cysteine)?

Accepted Answer

They all have an S configuration.

Question 5

A researcher is analyzing a protein sample and finds that all amino acids except glycine are chiral. What is the implication for the configuration of these amino acids in the protein?

Accepted Answer

They are almost exclusively L-amino acids.

Question 6

Which method is commonly used to determine the L/D configuration of amino acids?

Accepted Answer

Standard Fischer projection

Question 7

Consider the following Fischer projection of an amino acid:

$$
\begin{array}{c}
\mathrm{COOH} \
| \
\mathrm{H} - \mathrm{C} - \mathrm{NH_2$} \
| \
\mathrm{R}
\end{array}
$$

Which configuration does this represent?

Accepted Answer

L-amino acid

Question 8

Why is cysteine an exception to the rule that all L-amino acids have an S configuration?

Accepted Answer

Its side chain contains a sulfur atom, which changes the priority order in the Cahn-Ingold-Prelog system.

Question 9

A student is asked to draw the structure of an L-amino acid using the Fischer projection. Which of the following is a correct representation?

Accepted Answer

$$\begin{array}{c} \mathrm{COOH} \ | \ \mathrm{H} - \mathrm{C} - \mathrm{NH_2$} \ | \ \mathrm{R} \end{array}$$

Question 10

Which of the following best explains why proteins in living organisms are composed almost exclusively of L-amino acids?

Accepted Answer

Enzymes responsible for protein synthesis are stereospecific for L-amino acids.

Question 11

A patient is found to have a rare genetic disorder that incorporates D-amino acids into their proteins. What is the most likely consequence?

Accepted Answer

Altered protein folding and loss of biological activity.

Question 12

Which of the following is NOT a correct way to represent an L-amino acid?

Accepted Answer

Fischer projection with $$\mathrm{NH_2$} at $$the top and $$\mathrm{COOH} on $$the left.

Question 13

If an amino acid has the following configuration: $$\mathrm{COOH} at $$the top, $$\mathrm{NH_2$} on $$the left, $$\mathrm{H} on $$the right, and $$\mathrm{R} at $$the bottom in a Fischer projection, what is its configuration?

Accepted Answer

L-amino acid

Question 14

Which of the following amino acids is achiral?

Accepted Answer

Glycine

Question 15

A biochemist is comparing the stereochemistry of amino acids in proteins and finds that all except cysteine have the same configuration. What is the configuration for most amino acids, and why is cysteine different?

Accepted Answer

Most have S configuration; cysteine has R due to the sulfur atom's priority.

Question 16

Which of the following is a correct statement about the use of amino acid configurations in life?

Accepted Answer

Proteins are built almost exclusively from L-amino acids.

Question 17

Which of the following representations is NOT used to depict L-amino acids?

Accepted Answer

Circular dichroism spectrum

Question 18

A scientist uses the Cahn-Ingold-Prelog priority rules to assign configuration to an amino acid. For most L-amino acids, what is the configuration assigned?

Accepted Answer

S

Question 19

Which of the following is a consequence of using D-amino acids in protein synthesis instead of L-amino acids?

Accepted Answer

Proteins would have altered structure and potentially lose function.

L- and D- Amino Acids: Configuration, Representation, and Biological Relevance

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