Week 4 HW: Protein Design Part 1
Conceptual Questions
— Q1: 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)
First, we need to calculate the number of moles and multiply by Avogadro’s number (NA=6.022×1023 mol−1). An amino acid has an average mass of ~100 Daltons (Da), which is roughly equivalent to 100 g/mol. Meat is mostly protein (~20% of its weight is protein). → 500 g of meat contains approximately 100 g of protein. Since 1 mole of amino acids weighs ~100 g, there are ~1 mole of amino acids in 100 g of protein. → 1 mole is equivalent to 6.022 × 1023 molecules (Avogadro’s number). So you consume approximately 6 × 1023 amino acids in 500 g of meat.
Q2: Why do humans eat beef but do not become a cow, eat fish but do not become fish?
When you consume animal proteins — whether from beef, fish, or any other source — your body does not simply absorb and repurpose those proteins wholesale. Instead, the digestive system dismantles them through a carefully orchestrated process involving enzymes such as pepsin and various proteases, which hydrolyze the peptide bonds holding amino acids together, reducing complex foreign proteins into their simplest building blocks: individual amino acids and small peptides. These generic molecular units are then absorbed through the intestinal lining into the bloodstream, where they are transported to cells throughout the body. Once inside the cell, the process shifts from digestion to construction, guided by the Central Dogma of molecular biology — the principle that genetic information flows from DNA to RNA to protein. Using the instructions encoded in human DNA, the cell’s ribosomes read messenger RNA transcripts and reassemble those same basic amino acids into entirely new, distinctly human proteins. This elegant process explains why eating cow or fish protein does not make you more “cow-like” or “fish-like” — because by the time those amino acids are rebuilt into proteins, they are following your body’s own genetic blueprint, not the animal’s.
Q3: Why are there only 20 natural amino acids?
The 20 protein amino acids were chosen during evolution for their chemical stability, functionality and compatibility with the ribosomal machinery. Furthermore, their structure allows them to perform a wide variety of enzymatic and structural functions without being excessively complex.
Q5: Where did amino acids come from before enzymes that make them, and before life started?
Long before life existed on Earth and biological enzymes evolved to synthesize proteins, amino acids were being formed through entirely abiotic — or non-living — chemical processes. The landmark Miller-Urey experiment provided the first compelling laboratory evidence for this, demonstrating that when the conditions of early Earth are simulated — a reducing atmosphere composed of methane, ammonia, hydrogen, and water vapor — and electrical discharges are introduced to mimic lightning strikes, amino acids spontaneously emerge from inorganic chemistry alone. This groundbreaking result suggested that the fundamental building blocks of life were not the product of biology, but rather an inevitable outcome of basic chemistry under the right environmental conditions. Further supporting this idea, astrobiological research has revealed that amino acids are not exclusive to Earth — they have been discovered on carbonaceous chondrite meteorites, most notably the Murchison meteorite, which was found to contain a diverse array of extraterrestrial amino acids.
Q6: If you make an α-helix using D-amino acids, what handedness (right or left) would you expect?
If a protein were constructed entirely from D-amino acids rather than the L-amino acids found in nature, you would expect it to fold into a left-handed α-helix. In natural proteins, L-amino acids preferentially adopt right-handed α-helical conformations because this geometry minimizes steric clashes between the side chains and the peptide backbone, making it the energetically favorable configuration. D-amino acids, however, are the exact mirror images — or enantiomers — of their L counterparts, meaning their stereochemical constraints are precisely reversed. As a result, a polymer built entirely from D-amino acids would experience the same stabilizing forces and steric considerations as a natural protein, but reflected in the opposite direction, driving the chain to fold into a left-handed helix instead. This mirror-image relationship illustrates just how profoundly the chirality of individual amino acids influences the three-dimensional architecture of the proteins they compose, and highlights why the near-universal selection of L-amino acids in biological systems is so fundamental to the structural consistency of life as we know it.
Q7 Can you discover additional helices in proteins?
Beyond the well-known α-helix, proteins and engineered peptides can adopt a surprising variety of helical conformations, and scientists have developed several creative strategies to discover and design them. While naturally occurring proteins predominantly utilize the standard α-helix, the 3-helix, and the rarer π-helix, the conformational space available to polypeptide chains is far broader than what evolution has sampled. One powerful approach involves the use of unnatural amino acids — such as β- or γ-amino acids, which carry extra carbon atoms in their backbone — to build synthetic peptides called foldamers that fold into entirely novel helical geometries, such as the 14-helix or 12-helix, structures with no natural counterpart. Another strategy exploits stereochemistry by alternating D- and L-amino acids within a single chain, producing unusual structures like the hollow tubular helix found in the antibiotic Gramicidin A, which behaves fundamentally differently from the solid cylinder of a conventional α-helix. Finally, computational de novo design tools such as Rosetta allow structural biologists to mathematically explore novel hydrogen-bonding networks and backbone torsion angles, effectively engineering stable helical architectures that nature never arrived at through evolution. Together, these approaches reveal that the helices found in natural proteins represent only a small fraction of what is geometrically and chemically possible.
Q8 Why are most molecular helices right-handed?
Most molecular α-helices are right-handed because biological systems exclusively use L-amino acids. In a right-handed helix made of L-amino acids, the side chains (R-groups) point outward and downward, which minimizes steric hindrance (spatial crowding) with the carbonyl oxygen atoms of the polypeptide backbone. If L-amino acids were forced into a left-handed α-helix, the side chains would structurally clash with the backbone, creating a high-energy, thermodynamically unstable state.
Q9 Why do β-sheets tend to aggregate? What is the driving force?
The tendency of β-sheets to aggregate is driven by two primary forces that act in concert to pull individual sheets together into larger, often insoluble structures. The first is the presence of unsatisfied hydrogen bonds along the exposed edges of each β-sheet, where unpaired amide donors and carbonyl acceptors remain highly reactive and eagerly seek out complementary partners on neighboring sheets, effectively stitching them together through intermolecular hydrogen bonding. The second driving force is the hydrophobic effect, which arises from the amphipathic nature of many β-sheets — one face is hydrophilic while the opposing face is hydrophobic. In an aqueous environment, the hydrophobic faces are thermodynamically compelled to shield themselves from surrounding water molecules, driving them to pack tightly against one another and stack into larger fibrillar assemblies. Together, these two forces create a powerful thermodynamic pull toward aggregation, which has significant biological consequences — this very mechanism underlies the formation of amyloid fibrils, the insoluble protein aggregates associated with devastating neurodegenerative diseases such as Alzheimer’s and Parkinson’s, where normally soluble proteins misfold and their exposed β-sheet edges and hydrophobic faces drive runaway aggregation in the cell.
Q10 Why do many amyloid diseases form β-sheets? Can you use amyloid β-sheets as materials?
The prevalence of β-sheet misfolding in diseases like Alzheimer’s is rooted in the extraordinary thermodynamic stability of a structure known as the cross-β spine, in which tightly interdigitated β-strands form a rigid, highly ordered fibril that represents a deep low-energy state. Once a protein misfolds into this conformation, it does not simply remain an isolated aberration — it acts as a template, recruiting and converting neighboring normal proteins into the same misfolded state in a self-propagating chain reaction that progressively builds insoluble amyloid plaques. This prion-like mechanism makes amyloid formation particularly insidious, as the thermodynamic favorability of the fibril structure means the process is essentially irreversible under physiological conditions. However, the same properties that make amyloid fibrils so destructive in disease also make them remarkably attractive as engineering materials. Their mechanical strength is exceptional, rivaling that of spider silk and steel at the nanoscale, and their resistance to biological degradation gives them a durability that few natural materials can match. Recognizing this potential, engineers and materials scientists have begun harnessing modified, non-toxic amyloid sequences to design a wide range of advanced materials, including nanomaterials, hydrogels, biosensors, and scaffolds for tissue engineering — effectively repurposing one of biology’s most feared structural motifs into a platform for cutting-edge biotechnology.