U01.01.102 Amino acids

Understanding amino acids is critical for the USMLE Step 1, as they form the building blocks of proteins and play key roles in metabolism, growth, and cellular processes. Below is a structured breakdown of the key facts about amino acids:

Classification of Amino Acids

1. Protein-Forming Amino Acids

Only L-amino acids are incorporated into proteins.

2. Essential Amino Acids

Mnemonic: PVT TIM HaLL

Essential Amino Acid	Name
P	Phenylalanine
V	Valine
T	Tryptophan
T	Threonine
I	Isoleucine
M	Methionine
H	Histidine
L	Leucine
L	Lysine

Metabolic Classification

Type	Amino Acids	Mnemonic
Glucogenic	Methionine, Histidine, Valine	“We met his valentine, she is so sweet”
Glucogenic/Ketogenic	Isoleucine, Phenylalanine, Threonine, Tryptophan	No specific mnemonic
Ketogenic	Leucine, Lysine	“The onLy pureLy ketogenic amino acids”

Properties of Amino Acids

1. Acidic Amino Acids

Aspartic acid and glutamic acid
- Negatively charged at physiological pH.

2. Basic Amino Acids

Arginine, Histidine, Lysine
- Arginine is the most basic.
- Histidine has no charge at physiological pH.
- Found abundantly in histones, which bind to negatively charged DNA.

Special Considerations

Arginine and histidine are required during periods of growth.
Mnemonic: “His lys (lies) are basic.”

Table: Acidic vs. Basic Amino Acids

Property	Amino Acids	Notes
Acidic	Aspartic acid, Glutamic acid	Negatively charged at physiological pH
Basic	Arginine, Histidine, Lysine	Found in histones; bind to negatively charged DNA
		Arginine is the most basic; histidine has no charge

Points to Remember

L-amino acids are exclusively found in proteins.
Use the mnemonic PVT TIM HaLL to recall the essential amino acids.
Only leucine and lysine are purely ketogenic.
Basic amino acids (arginine, lysine, histidine) are critical for DNA binding due to their positive charge.
During growth, arginine and histidine are required in higher amounts.

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