Clinical Chemistry 1: Proteins & Non-Protein Compounds

Characteristics of Proteins

• Proteins are macromolecules made of amino acids, linked via peptide bonds.
• Proteins consist of 16% nitrogen, differentiating them from carbohydrates and lipids.
• N-terminal: End of protein structure with a free amino group.
• C-terminal: End of protein structure with a free carboxyl group.

Protein Structure

• Primary structure: Amino acids linked through covalent peptide bonding in a specific sequence.
• Secondary structure: Polypeptide chain winds to form alpha helixes and beta sheets through hydrogen bonds.
• Tertiary structure: Coiled polypeptide chain folds to form a 3D structure through R group interactions.
• Quaternary structure: Two or more folded polypeptide chains bind to form a functional protein.

Types of Proteins

• Antibodies: Proteins that protect the body against "foreign" invaders.
• Cellular proteins: Function as receptors for hormones.
• Structural proteins: Collagen is the fibrous component that maintains body structure.
• Enzymes: Catalysts that accelerate chemical reactions.

Plasma Total Protein Regulation

• The liver synthesizes most plasma proteins.
• Plasma cells synthesize immunoglobulins.
• Proteins are synthesized from amino acids, with one amino acid linked to another through peptide bond formation.
• When proteins degrade, their constituent amino acids undergo deamination with ammonia formation, converted to urea for excretion in urine.

Clinical Significance of Major Proteins

• Prealbumin (Transthyretin): Indicator of nutritional status, transports thyroid hormones.
• Albumin: Synthesized in the liver, binds many analytes for transport, contributes to plasma osmotic pressure.
• α1-Acid glycoprotein (orosomucoid): Acute-phase reactant, binds to basic drugs.
• Haptoglobin: α2-globulin that binds free hemoglobin, acute-phase reactant.
• Ceruloplasmin: α2-globulin, copper-containing protein with enzymatic activity, acute-phase reactant.
• α2-Macroglobulin: Proteolytic enzyme inhibitor, increased in nephrotic syndrome, contraceptive use, pregnancy, and estrogen therapy.
• Transferrin: β-globulin that transports iron.
• C-Reactive Protein (CRP): β-globulin, acute-phase reactant, increased in tissue necrosis, infections, and inflammation.
• Immunoglobulins: Antibodies, synthesized in plasma cells as an immune response.

Methodology for Serum Total Protein, Albumin, and Protein Fractionation

• Refractometry: Based on the change in velocity of light, proportional to the concentration of solutes (proteins) present in serum.
• Biuret method: Based on cupric ions complexing with peptide bonds in an alkaline medium, producing a purple-colored complex.
• Dye binding: Proteins bind to a dye, forming a protein-dye complex that results in a shift of the maximum absorbance of the dye.
• Kjeldahl: Based on the quantification of the nitrogen content of protein, considered the reference method of choice.
• Electrophoresis: Serum protein electrophoresis, serum is applied in the cathode region of an agarose gel or cellulose acetate plate saturated with a buffer of pH 8.6.