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Chapter 7: Amino Acid Metabolism

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.
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Table 7-1. Glucogenic and ketogenic amino acids.
Glucogenic Both Ketogenic
Essential
Histidine Isoleucine Leucine
Methionine Phenylalanine Lysine
Threonine Tryptophan
Valine
Nonessential
Alanine Tyrosine
Arginine
Aspartate
Asparagine
Cysteine
Glutamine
Glutamate
Glycine
Proline
Serine
Figure 7-1.

Examples of glucogenic and ketogenic amino acids.

A set of reactions and the T C A cycle showing the amino acids that form different substrates of the TCA cycle. At the top level, alanine, glycine, and tryptophan are shown to form pyruvate, which forms acetyl coenzyme A. This acetyl coenzyme A is also formed from leucine and tryptophan. The acetyl coenzyme A forms acetoacetyl coenzyme A, which can also be formed from the amino acids, leucine, lysine, and tryptophan. The acetyl coenzyme A reacts with oxaloacetate from the T C A cycle to form the next substrate in the T C A cycle, citrate. The citrate forms isocitrate, which forms alpha ketoglutarte. This alpha ketoglutarate can also be formed from histidine and glutamine. The alpha ketoglutarate forms succinyl coenzyme A, which forms succinate. The succinate forms fumarate, which finally forms oxaloacetate. The oxaloacetate completes the T C A cycle. In the reactions, alanine, glycine, histidine, and glutamate are shown as glucogenic. Leucine and lysine are shown as ketogenic, while tryptophan is shown as both. The caption reads, “examples of glucogenic and ketogenic amino acids,"
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Figure 7-2.

Neurotransmitter synthesis.

A set of four reactions show the synthesis of neurotransmitters such as epinephrine, norepinephrine, dopamine, histamine, G A B A, serotonin, and melatonin. In the first set of reactions, phenylalanine reacts with B H subscript 4 in the presence of phenylalanine hydroxylase to form tyrosine, which further reacts with B H subscript 4 in the presence of tyrosine hydroxylase to form Dopa. Dopa reacts with B subscript 6 in the presence of dopa decarboxylase to form dopamine, which in turn forms norepinephrine. Norepinephrine reacts with S A M in the presence of P N M T to form epinephrine. In the second reaction histidine in the presence of B subscript 6 forms histamine. In the third reaction, glutamate in the presence of B subscript 6 forms G A BA. In the fourth reaction tryptophan in the presence of B subscript 6 and B H subscript 4 forms serotonin, which in turn forms melatonin. The caption reads, “neurotransmitter synthesis.”
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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 when relevant ...

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