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1. PURPOSE

  1. The third part of our metabolism review focuses primarily on amino acids and proteins

  2. Amino acids are used in many pathways beyond protein synthesis, including energy production and neurotransmitter synthesis

  3. Diseases in specific pathways do show up on Step 1, but overall, questions focus more on the function of each pathway and effects of diet on their function

2. AMINO ACID PROPERTIES

  1. Unlike in biochemistry classes, knowing specifics about each amino acid is not necessary for Step 1

  2. What you learn depends on how much time you want to put into it

  3. Charged amino acids: Highest value

    1. Basic (+): Lysine, histidine, arginine (lies, hiss, arg)

    2. Acidic (–): Glutamic acid (glutamate), aspartic acid (aspartate)

  4. Polarity: Can help judge severity of genetic mutation

    1. Nonpolar: Glycine, alanine, valine, leucine, isoleucine, methionine, proline, phenylalanine, tryptophan

    2. Polar: All others (including acidic and basic)

  5. Essential: Required in the diet vs. synthesized in the body

    1. Valine, leucine, isoleucine, phenylalanine, tryptophan, lysine, arginine, histidine, methionine, threonine

      • Grouping by similarity may help: Leucine/isoleucine; phenylalanine, tryptophan; methionine, threonine; histidine (basic), lysine (basic), and valine

      • Becomes important for common diseases, particularly phenylketonuria

3. AMINO ACIDS AS ENERGY

  1. All amino acids can be used for energy if necessary

  2. Amino acids can be glucogenic, ketogenic, or both, based on where they enter the pathway

    1. Glucogenic: Converted into compounds that can be used to make glucose by gluconeogenesis

      • Converted into pyruvate, or TCA cycle intermediates

    2. Ketogenic: Converted directly to acetyl-CoA or acetoacetate

      • Remember that neither acetyl-CoA nor ketone bodies can be used to make glucose

      • Leucine and lysine—exclusively ketogenic

  3. Clinical: In pyruvate dehydrogenase deficiency, pyruvate cannot be converted to acetyl-CoA

    1. Shunt to lactic acid leads to acidosis with neurologic symptoms

    2. Appears in infancy: Failure to thrive, developmental delay, seizures

    3. Treatment: Reduce glucogenic amino acids in diet, favor leucine and lysine

Table 7-1.Glucogenic and ketogenic amino acids.
Figure 7-1.

Examples of glucogenic and ketogenic amino acids.

4. OTHER AMINO ACID METABOLIC PATHWAYS

  1. We've reviewed amino acids’ role in (1) protein synthesis (Chapter 3) and (2) energy production

  2. There are several additional uses of amino acids, especially for neurotransmitter production

    1. Knowing exactly which amino acids are made into which compounds is not important here

    2. Some easy ones to remember are histamine and glutamate/GABA

    3. These will come up in other organ systems ...

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