Protein Basics and Plasma Proteins

Functional Roles of Proteins in the Human Body

• Foundational Composition: Proteins are fundamentally composed of amino acids, which serve as their building blocks.
• Transport Mechanisms: Proteins are essential for the movement of substances throughout the body. A primary example provided is Albumin, which serves as a major transport protein in the blood.
• Oxygen Transport: Specific proteins are dedicated to carrying oxygen; Hemoglobin is the specific example cited for this function.
• Immune Defense: Proteins play a critical role in the immune system to fight infection, specifically in the form of Antibodies.
• Biocatalysis: Proteins function as Enzymes, which are necessary to catalyze biochemical reactions, allowing them to occur at rates necessary for life.
• Hormonal Regulation: Some proteins act as Hormones to communicate and perform specific jobs within the body; Insulin is a notable example.
• Mechanical Movement: In the muscular system, proteins facilitate muscle contraction. The two primary proteins involved are Actin and Myosin.

Levels of Protein Structure

• Primary Structure: This is defined as the linear amino acid sequence of the protein.
• Secondary Structure: This level involves localized folding patterns, specifically the $\alpha\text{-helix}$ and the $\beta\text{-sheet}$.
• Tertiary Structure: This represents the full, three-dimensional ($3D$) folding of a single protein molecule.
• Quaternary Structure: This highest level of structure occurs when multiple protein chains (subunits) come together to form a functional complex. Hemoglobin is an example of a protein with quaternary structure.

Plasma Proteins: Albumin and Prealbumin

• Albumin:
  • It is the most abundant plasma protein found in the body.
  • Oncotic Pressure: Albumin is the primary protein responsible for maintaining oncotic pressure (colloid osmotic pressure), which keeps fluid within the vasculature.
  • Transport Functions: It serves as a carrier for various substances, including drugs, hormones, and calcium ($Ca^{2+}$).
  • Hypoalbuminemia (Low Albumin): Caused by liver disease (impaired synthesis), kidney disease (loss through filtration), malnutrition (lack of precursors), and inflammation.
  • Hyperalbuminemia (High Albumin): Usually results from dehydration, causing a relative increase in concentration.
• Prealbumin:
  • Also known as transthyretin.
  • Transport: Specifically transports the thyroid hormones $T3$ (triiodothyronine) and $T4$ (thyroxine).
  • Clinical Significance: It serves as a sensitive marker of nutritional status. Low prealbumin levels indicate poor nutrition.

Globulins: Classification and Clinical Significance

• The globulins are categorized into four distinct groups:
• $\alpha_1\text{-Globulins}$ (Alpha-1):
  • Alpha-1 Antitrypsin: Its primary job is to inhibit neutrophil elastase, preventing it from destroying lung tissue.
  • Pathology: A deficiency in Alpha-1 antitrypsin leads to early-onset emphysema or Chronic Obstructive Pulmonary Disease (COPD).
• $\alpha_2\text{-Globulins}$ (Alpha-2):
  • Haptoglobin: This protein binds to free hemoglobin in the plasma.
  • Pathology: Low haptoglobin levels are a clinical indicator of intravascular hemolysis (the destruction of red blood cells within the blood vessels).
• $\beta\text{-Globulins}$ (Beta):
  • Transferrin: Known as the "Iron Taxi," its function is to transport iron throughout the body.
  • Iron Deficiency Lab Profile: Characterized by low Iron, low Ferritin, high Transferrin, and high Total Iron Binding Capacity (TIBC).
  • Chronic Disease Lab Profile: Characterized by low Iron, high Ferritin, and low Transferrin.
• $\gamma\text{-Globulins}$ (Gamma):
  • Immunoglobulins: These are the antibodies of the immune system.
  • Elevations: Levels increase during infection and in plasma cell dyscrasias such as Multiple Myeloma.
  • Decreases: Low levels indicate an immunodeficiency state.

Serum Protein Electrophoresis (SPEP)

• Purpose: SPEP is a laboratory technique used to separate proteins based on their physical properties.
• Order of Separation: When proteins are separated via electrophoresis, they generally appear in the following order:
  1. Albumin
  2. $\alpha_1$
  3. $\alpha_2$
  4. $\beta$
  5. $\gamma$

Total Protein Measurements and Clinical Correlation

• Normal Reference Range: The standard range for total protein is $6.5-8.3\,g/dL$.
• Hyperproteinemia (High Total Protein):
  • The most common cause is dehydration (hemoconcentration).
• Hypoproteinemia (Low Total Protein):
  • Primary causes include liver disease, kidney disease, and malnutrition.