Week 4: Protein Design

Protein Design I

Questions from Shuguang Zhang

Question 1: How many molecules of amino acids do you take with a piece of 500 grams of meat? (on average an amino acid is ~100 Daltons)

Answer: In 500 grams of meat, you consume approximately 8.2 × 10²³ molecules of amino acids. This refined calculation uses a standard protein content of 30% for cooked meat and an adjusted average molecular weight of 110 Daltons per amino acid residue.

Step-by-Step Refined Calculation

1. Calculate Total Protein Mass
   Raw meat typically contains about 20–22% protein, but 500 g of cooked lean meat (like chicken breast or steak) averages roughly 30% protein due to water loss during cooking.
   Protein mass = 500 g × 0.30 = 150 g.

2. Adjust for Amino Acid Residue Weight
   The average weight of the 20 free amino acids is ~100 Da, but when they link to form protein chains, they lose a water molecule (18 Da). Biochemists use an adjusted average of 110 Da per residue in a protein chain.

3. Convert Mass to Daltons
   Using the conversion factor where 1 g = 6.022 × 10²³ Da (Avogadro’s number):
   Total mass in Daltons = 150 g × (6.022 × 10²³ Da/g) = 9.033 × 10²⁵ Da.

4. Calculate Total Molecules
   Divide the total mass in Daltons by the average mass of a single amino acid residue:
   Number of molecules = (9.033 × 10²⁵ Da) / (110 Da/residue) = 8.21 × 10²³ residues.

Comparison of Meat Types (500 g cooked)

The specific amino acid count fluctuates depending on the lean‑to‑fat ratio of the meat:

Question 2: Why do humans eat beef but do not become a cow, eat fish but do not become fish?

Answer:
Humans do not “become” the animals they eat because the body does not absorb tissues whole; it reduces them to universal chemical precursors before rebuilding them according to a strictly human genetic blueprint.

1. The Principle of Molecular Deconstruction (Digestion)

Proteolytic Breakdown: Enzymes like pepsin and trypsin hydrolyze the peptide bonds of animal proteins, reducing them into their smallest constituents: amino acids.
Universal Currency: At the molecular level, a leucine molecule from a cow is chemically identical to a leucine molecule in a human. By the time these nutrients reach the bloodstream, all species-specific “identity” has been stripped away.

2. Genetic Blueprint and Protein Synthesis (The “Human” Template)

Once absorbed, these universal amino acids enter the cellular amino acid pool. The reason we rebuild these into human muscle rather than bovine muscle lies in our DNA.
The Instruction Manual: Your DNA contains the specific sequences (templates) for human proteins. Through the processes of transcription and translation, your ribosomes use the “imported” amino acids to assemble exclusively human proteins (like human collagen or human hemoglobin).
Biological Identity: The body’s metabolic pathways are genetically “hard-coded.” Even if you consume fish DNA, your cells lack the biological machinery to incorporate foreign genetic material into your own genome; instead, foreign DNA is broken down into harmless nucleotides and recycled as raw material.

3. Evolutionary Barrier and Immune Guarding

From an evolutionary standpoint, the ability to maintain a stable biological identity while consuming diverse organic matter is a survival necessity.
Immune Surveillance: The human immune system is designed to recognize and destroy “non-self” proteins. If a cow protein were to enter the bloodstream intact, the body would treat it as a pathogen (an allergen or invader) rather than a building block.
Metabolic Adaptability: Humans are physiologically omnivorous, meaning our systems are specialized in extracting energy from varied sources without compromising our own structural integrity.

Summary

In short: We eat cow structure, but we follow human instructions.