Comprehensive Study Notes on Proteins and Heme Derivatives

Composition and Structural Foundations of Amino Acids

Amino acids serve as the fundamental building blocks of proteins. Their basic chemical structure consists of at least one amino group and one carboxyl functional group attached to a central carbon atom. These molecules are utilized by the human body to synthesize several critical biological components, including enzymes, hormones, antibodies, muscle proteins, and transport proteins such as albumin and hemoglobin. The body relies on a specific set of amino acids for protein synthesis, which are categorized based on the body's ability to produce them.

Essential and Nonessential Amino Acids

The primary distinction between essential and nonessential amino acids lies in whether the human body can synthesize them internally. Essential amino acids cannot be synthesized in sufficient amounts by the body and must be obtained through the diet. There are nine essential amino acids: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine. Nutritional sources providing all essential amino acids are termed complete proteins. These include food items such as eggs, fish, poultry, beef, milk, cheese, yogurt, and soy products.

Conversely, nonessential amino acids can be synthesized by the body and do not necessarily have to be obtained from dietary intake. The body typically produces these from metabolic intermediates and other amino acids. Common nonessential amino acids include Alanine, Asparagine, Aspartate, Glutamate, and Serine. In some classifications, Selenocysteine is also listed as a nonessential amino acid utilized in human protein synthesis.

Conditionally Essential Amino Acids and Aminoacidopathies

Conditionally essential amino acids represent a group of amino acids that are normally synthesized by the body but may become essential during specific physiological states or medical conditions. These conditions include severe illness, trauma, burns, prematurity, and periods of rapid growth. Examples of conditionally essential amino acids are Arginine, Cysteine, Glutamine, Glycine, Proline, and Tyrosine. A notable clinical example involves Tyrosine, which is synthesized from Phenylalanine. In individuals with Phenylketonuria (PKU), Phenylalanine cannot be metabolized properly, which causes Tyrosine to become an essential amino acid.

Aminoacidopathies are a class of inborn errors of metabolism characterized by enzyme defects that inhibit the body's ability to metabolize specific amino acids. These disorders can lead to severe medical complications, including brain damage, due to the accumulation of toxic amino acids or their metabolic by-products in the blood and tissues.

Protein Structure and Levels of Organization

Proteins are macromolecules consisting of carbon, oxygen, hydrogen, nitrogen, and sulfur. They are polymers constructed from one or more unbranched chains of amino acids. The structural complexity of proteins is divided into four distinct levels. The primary structure is the specific sequence of amino acids linked together by peptide bonds. The secondary structure refers to local interactions between amino acids near each other, primarily through hydrogen bonds, forming structures such as the $\alpha$ helix and $\beta$ pleated sheet. The tertiary structure is the three-dimensional folding of the protein driven by the hydrophobic effect, ionic attraction, hydrogen bonds, and disulfide bonds. Finally, the quaternary structure involves the molecular association of more than one peptide or protein chain to form complexes such as dimers, trimers, or tetramers through noncovalent bonds.

Protein Synthesis and Metabolism

Proteins are predominantly synthesized in the liver through the processes of transcription and translation before being secreted into the circulation. In the body, proteins undergo a continuous cycle of synthesis (anabolism) and degradation (catabolism), which facilitates the efficient recycling of amino acids. When amino acids are present in excess of the body's requirements, they can be converted into urea for excretion by the kidneys. Additionally, they may be transformed into glucose or ketones to serve as an alternative energy source.

Proteins are chemically characterized by containing many ionizable groups on their amino acid side chains as well as on their N- and C-terminal ends. The specific pH at which an amino acid or protein possesses no net charge is defined as its isoelectric point (pI). Proteins are classified based on their structure and composition into simple proteins and complex proteins. Simple proteins contain peptide chains composed only of amino acids and can be globular or fibrous. Complex (conjugated) proteins consist of a protein and a non-amino part, which is referred to as the prosthetic group.

Major Plasma Proteins: Prealbumin and Albumin

Prealbumin, also known as transthyretin, is a transport protein for thyroid hormones, specifically thyroxine ($T_4$) and triiodothyronine ($T_3$). It is so named because it migrates before albumin in classic serum protein electrophoresis (SPE). Low concentrations of prealbumin are often utilized as an indicator of poor nutritional status.

Albumin is the most abundant protein in plasma and serves as a negative acute-phase reactant, meaning its concentration decreases during acute disease states such as inflammation, infection, or injury. Albumin is responsible for approximately $80\%$ of the colloid osmotic pressure in the blood. Pathologically, increased renal loss of albumin, known as albuminuria, is a common occurrence in renal disease. In serum protein electrophoresis, albumin typically accounts for $53\% - 65\%$ of the total protein, with a reference range of $3.5 - 5.5\,g/dL$.

Globulins of Clinical Significance

Globulins are classified into four major fractions based on their electrophoretic mobility: $\alpha_1$, $\alpha_2$, $\beta$, and $\gamma$. Alpha-1 Antitrypsin (AAT) is a protease inhibitor synthesized by the liver that protects lung and connective tissues from destruction by proteolytic enzymes, specifically neutrophil elastase released during inflammation. An inherited deficiency in AAT can lead to early-onset emphysema and COPD in non-smokers.

Haptoglobin is an acute-phase protein produced by the liver that binds free hemoglobin released during red blood cell destruction. This binding prevents iron loss and kidney damage caused by free hemoglobin. A clinical hallmark is that low haptoglobin levels indicate intravascular hemolysis until proven otherwise. Transferrin is a $\beta$-globulin responsible for transporting ferric iron ($Fe^{3+}+$) to the bone marrow, liver, and spleen. In iron deficiency anemia, transferrin and Total Iron Binding Capacity (TIBC) increase, while serum iron and ferritin decrease. In the anemia of chronic disease, iron and transferrin decrease, whereas ferritin increases. Finally, immunoglobulins (such as IgG and IgM) reside in the $\gamma$ fraction and serve as antibodies for immune defense.

Cardiac Markers and Specific Proteins

Troponin is a regulatory protein complex found in cardiac and skeletal muscle that regulates the interaction between actin and myosin. It consists of three subunits: Troponin C (TnC), which binds calcium; Troponin I (TnI), which inhibits actin-myosin interaction; and Troponin T (TnT), which anchors the complex to tropomyosin. Cardiac troponin (cTn) is the gold-standard biomarker for myocardial infarction (MI) due to its high sensitivity and specificity. Troponin I (cTnI) values below $0.04\,ng/mL$ and Troponin T (cTnT) values between $0.01 - 0.03\,ng/mL$ are typical reference ranges. Following an MI, troponin levels rise within $3 - 6$ hours; TnI peaks at $12 - 24$ hours and returns to normal in $5 - 10$ days, while TnT peaks at $12 - 48$ hours and returns to normal in $10 - 14$ days.

Natriuretic peptides, including BNP and NT-proBNP, are hormones found in high concentrations in the left ventricular myocardium and serve as markers for heart failure. Other significant proteins include Fibronectin, where fetal fibronectin (fFN) is used to predict the risk of premature delivery, and Cystatin C, which is used to evaluate glomerular function when creatinine measurements are misleading.

Clinical Proteomics and Methodology

Total protein in serum typically reaches adult levels of $6.5 - 8.3\,g/dL$ by age 3. Hypoproteinemia, or low plasma protein, can be caused by excessive renal loss, decreased synthesis (malnutrition), or increased catabolism. Hyperproteinemia is often the result of dehydration (vomiting, diarrhea, diabetic acidosis) or excessive $\gamma$-globulin production. The Biuret reaction is the standard method for total protein analysis; in an alkaline medium, cupric ions ($Cu^{2+}+$) complex with at least two peptide bonds. Albumin is commonly measured using dye-binding procedures like Bromocresol Green (BCG) or Bromocresol Purple (BCP).

Serum Protein Electrophoresis (SPEP) separates proteins based on size, shape, and charge into five bands: Albumin, $\alpha_1$, $\alpha_2$, $\beta$, and $\gamma$. Specific fractions and relative percentages include:

• Albumin: $53\% - 65\%$ ($3.5 - 5.5\,g/dL$)
• $\alpha_1$-Globulins: $2.5\% - 5.0\%$ ($0.1 - 0.3\,g/dL$)
• $\alpha_2$-Globulins: $7.0\% - 13.0\%$ ($0.6 - 1.0\,g/dL$)
• $\beta$-Globulins: $8.0\% - 14.0\%$ ($0.7 - 1.1\,g/dL$)
• $\gamma$-Globulins: $12.0\% - 22.0\%$ ($0.8 - 1.6\,g/dL$)

Heme Derivatives and Hemoglobin

Hemoglobin is a transport protein that moves oxygen from the lungs to tissues and carries carbon dioxide back to the lungs. It is synthesized in immature red blood cells within the bone marrow and exists as a tetramer. Myoglobin is a monomeric oxygen-carrying protein found in striated skeletal and cardiac muscle. Heme itself is not a protein but a prosthetic group found in hemoglobin, myoglobin, and cytochromes. Heme biosynthesis disorders, known as porphyrias, are categorized as either erythropoietic or hepatic and can be further classified as acute or cutaneous based on symptoms.