Protein Structure & Sequencing

The foundation of all proteins. Primary structure is simply the order (sequence) of amino acids in the chain, held together by peptide bonds.

Think of it like beads on a string—the sequence matters because each amino acid has unique chemistry that affects everything that comes after.

Secondary structure is how the chain folds locally into two main shapes:

α-Helix (coiled spring): The chain spirals tightly. Hydrogen bonds form between the N-H of one residue and the C=O of the amino acid 4 residues earlier.

Amino acids that FAVOR helices: Met, Glu, Cys, His, Lys

β-Pleated Sheet (folded paper): Adjacent rows of the chain lie next to each other, with lateral hydrogen bonds connecting them.

Amino acids that FAVOR sheets: Ile, Tyr, Trp, Val

BOARD TRAP: Proline causes KINKS and TURNS in the chain, disrupting normal secondary structure. Rich in collagen, GI tract proteins, and blood vessels—tissues that need to twist.

Tertiary structure is the 3D shape of a single protein chain. Here's the driving rule: Hydrophobic (water-hating) amino acids bury themselves INSIDE the protein, away from water. Hydrophilic (water-loving) amino acids cluster on the OUTSIDE, exposed to water.

This is driven by thermodynamics—it's more stable that way. Covalent disulfide bonds (between cysteine residues) also stabilize tertiary structure and are harder to break than hydrogen bonds.

Quaternary structure is when multiple separate protein chains assemble together. This allows for COOPERATIVITY—the chains talk to each other and change behavior based on what neighboring chains are doing.

In enzymes, this is called ALLOSTERISM: An allosteric enzyme is typically a rate-limiting enzyme—the slowest step in a metabolic pathway. Since it's the bottleneck, it controls everything downstream.

Genetic disease consequence: Most genetic diseases involve deficiency of a rate-limiting enzyme. If you can't make the slow enzyme, the whole pathway backs up or falls apart.

A peptide bond forms when the carboxyl group (C=O) on the LEFT amino acid attacks the amino group (N-H) on the RIGHT amino acid.

Directionality (critical for the COMLEX & USMLE):

• N-terminus (amino terminal) is ALWAYS on the LEFT
• C-terminus (carboxyl terminal) is ALWAYS on the RIGHT
• When we write sequences, we always write LEFT-to-RIGHT (N→C)

1. Planar (flat): The peptide bond doesn't rotate easily. The atoms lay flat in a plane.

2. Restricted mobility: The bond is tighter than a normal single bond but not as tight as a double bond. Limited rotation around the bond axis.

3. R-groups in trans configuration: The side chains (R-groups) of adjacent amino acids point in opposite directions, minimizing steric clash.

What it does: Strong acid denatures (unfolds) the protein and breaks all peptide bonds, leaving you with a pool of individual amino acids.

Result: You know the TYPES and COUNTS of each amino acid, but NOT the order.

BOARD TRAP: Asparagine (Asn) converts to Aspartate (Asp), and Glutamine (Gln) converts to Glutamate (Glu). So you can't accurately count Asn or Gln—only the total of each pair.

Clinical connection: Alcohol is acidic → causes hepatorenal syndrome (liver and kidney damage). Alcoholics can't count their Asn/Gln either—well, their bodies certainly struggle with amino acid metabolism.

What it does: Proteins migrate through a gel matrix under an electric field. Separation is primarily by SIZE (smaller proteins migrate faster, travel further). Can also add electrodes to separate by charge.

Result: Each protein has a unique electrophoretic pattern (fingerprint). Does NOT sequence the protein.

Clinical use: Sickle cell disease diagnosis. Sickle hemoglobin (HbS) has a different charge than normal hemoglobin (HbA), so it migrates differently on gel. Used to identify the disease and distinguish HbS from HbA.

What it does: Ninhydrin reacts with the free amino terminal (N-terminus, the left side). All amino acids turn PURPLE except one.

Proline turns YELLOW because its structure is unusual (it's a cyclic amino acid—the N is part of a ring).

Result: You can COUNT the number of proline residues (useful for collagen and other proline-rich proteins), but that's about it.

What it does: Uses phenyl isothiocyanate (PITC) to react with ANY amino acid starting from the N-terminus. The reacted amino acid is then identified using TLC (thin-layer chromatography) or HPLC (high-performance liquid chromatography).

After identification, that amino acid is cleaved off, and the process repeats on the next residue in line.

Result: Step-by-step sequencing of the N-terminal region. Accurate up to ~100 amino acids.

Limitation: For longer proteins, it gets errors and fades out. You'd need other methods for the rest.

What it does: Different enzymes cut the protein at specific amino acids. By cutting with different enzymes and analyzing the fragments, you can deduce the order.

Core principle: You must know exactly which amino acid each enzyme recognizes.

This is what the interactive builder below teaches you! We'll use restriction enzymes to cut a sequence you build.

Bone marrow makes: Immunoglobulins (antibodies) — these form β-sheets for structure.

Liver makes: ~90% of all proteins in the body. Key ones:

• Albumin — main plasma protein, maintains osmotic pressure
• Fibrinogen — blood clotting (α-helix rich)
• Alpha-1 antitrypsin — protects lungs from enzymes (deficiency → early emphysema)

What are they? Non-specific proteins made during inflammation, triggered by IL-6 (made by macrophages and T-helper cells).

ESR (Erythrocyte Sedimentation Rate): Measures how fast RBCs sediment in plasma in 1 hour.

• ELEVATED: Inflammation present (Takayasu arteritis, temporal arteritis, SLE, Wegener's granulomatosis)
• FALSELY HIGH: Anemia — RBCs fall faster when there are fewer of them
• FALSELY LOW: Sickle cell disease, polycythemia — cells don't sediment normally
• Used to: Monitor treatment response over time

CRP (C-Reactive Protein): An opsonin—it binds dead/damaged cells and activates complement to clear them. Non-specific inflammation marker.

Procalcitonin: Newer test, more SPECIFIC for infections (bacterial/fungal) than viral. Reflects excess protein production during infection.