Properties

Understanding the physical and chemical properties of proteins and other nitrogen-containing compounds is crucial for their accurate measurement and interpretation in the clinical laboratory

• Key Compounds
  • Proteins
  • Amino Acids
  • Urea
  • Uric Acid
  • Creatinine
  • Ammonia
  • Tumor Markers (CEA, PSA, AFP, CA 125)
  • Cardiac Markers (Troponin, CK-MB, Myoglobin)

Proteins

• Definition: Large molecules composed of amino acids linked by peptide bonds. They are essential for structure, function, and regulation in living organisms
• Structure
  • Primary: The linear sequence of amino acids
  • Secondary: Local folding patterns (alpha-helices, beta-sheets) stabilized by hydrogen bonds
  • Tertiary: The overall three-dimensional structure, determined by interactions between amino acid side chains
  • Quaternary: The arrangement of multiple polypeptide chains in a multi-subunit protein
• Physical Properties
  • Molecular Weight: Varies widely depending on the number and type of amino acids
  • Solubility: Varies depending on the amino acid composition and three-dimensional structure
  • Amphoteric Nature: Proteins have both acidic and basic groups and can act as zwitterions
  • Optical Activity: Proteins are chiral and exhibit optical activity
  • Colloidal Properties: Proteins form colloidal solutions in water
• Chemical Properties
  • Peptide Bond Formation: Amino acids are linked by peptide bonds
  • Denaturation: Proteins can be denatured by heat, pH changes, organic solvents, or detergents, leading to loss of structure and function
  • Hydrolysis: Proteins can be hydrolyzed by acids, bases, or enzymes, breaking peptide bonds and releasing amino acids
  • Amphoteric Nature: Proteins can act as both acids and bases due to the presence of ionizable groups
  • Color Reactions: Proteins react with various reagents to produce colored products, used in protein quantification assays

Amino Acids

• Definition: Organic molecules containing an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (-H), and a unique side chain (R group) attached to a central carbon atom
• Structure
  • Alpha Carbon: The central carbon atom to which the amino and carboxyl groups are attached
  • Amino Group: A basic group (-NH2)
  • Carboxyl Group: An acidic group (-COOH)
  • Side Chain (R Group): A unique group that determines the identity and properties of the amino acid
• Physical Properties
  • Solid at Room Temperature: Amino acids are crystalline solids at room temperature
  • Solubility: Soluble in water
  • Amphoteric Nature: Amino acids have both acidic and basic groups and can act as zwitterions
  • Optical Activity: All amino acids except glycine are chiral and exhibit optical activity
• Chemical Properties
  • Peptide Bond Formation: Amino acids are linked by peptide bonds to form proteins
  • Amphoteric Nature: Amino acids can act as both acids and bases due to the presence of ionizable groups
  • Reactions of Side Chains: The side chains of amino acids undergo various chemical reactions, contributing to protein function and modification
  • Reactions with Ninhydrin: Amino acids react with ninhydrin to form a colored product, used in amino acid quantification assays

Urea

• Definition: A nitrogen-containing compound (NH2-CO-NH2) that is the major end product of protein metabolism in mammals. It is synthesized in the liver and excreted in the urine
• Structure
  • Molecular Formula: CH4N2O
  • Amide Groups: Two amide groups (-NH2) attached to a carbonyl group (C=O)
• Physical Properties
  • Solid at Room Temperature: Urea is a crystalline solid at room temperature
  • Solubility: Highly soluble in water and alcohol
  • Odorless: Pure urea is odorless, but it can decompose to form ammonia, which has a pungent odor
• Chemical Properties
  • Neutral pH: Urea has a neutral pH in solution
  • Hydrolysis: Urea can be hydrolyzed by urease, an enzyme, to produce ammonia and carbon dioxide
  • Biuret Reaction: Urea reacts with biuret reagent to form a colored product, used in urea quantification assays
  • Decomposition: Urea decomposes upon heating to form ammonia and other products

Uric Acid

• Definition: A nitrogen-containing compound (C5H4N4O3) that is the major end product of purine metabolism in humans. It is excreted in the urine
• Structure
  • Heterocyclic Compound: Contains fused pyrimidine and imidazole rings
  • Keto-Enol Tautomerism: Exists in keto and enol forms
• Physical Properties
  • Solid at Room Temperature: Uric acid is a crystalline solid at room temperature
  • Solubility: Sparingly soluble in water
  • pH: Weakly acidic
• Chemical Properties
  • Salt Formation: Forms salts with metals
  • Oxidation: Can be oxidized to allantoin by uricase
  • Reduction: Can be reduced to xanthine
  • Reaction with Phosphotungstic Acid: Reacts with phosphotungstic acid to form a colored product, used in uric acid quantification assays

Creatinine

• Definition: A nitrogen-containing compound (C4H7N3O) that is a waste product of muscle metabolism. It is formed from creatine and phosphocreatine and is excreted in the urine
• Structure
  • Cyclic Amide: Contains a cyclic amide group
• Physical Properties
  • Solid at Room Temperature: Creatinine is a crystalline solid at room temperature
  • Solubility: Soluble in water
• Chemical Properties
  • Tautomerism: Exists in different tautomeric forms
  • Picrate Reaction (Jaffe Reaction): Reacts with picric acid in alkaline solution to form a colored product, used in creatinine quantification assays
  • Enzymatic Methods: Can be measured using enzymatic assays that convert creatinine to measurable products

Ammonia

• Definition: A nitrogen-containing compound (NH3) that is a product of amino acid deamination and other metabolic processes. It is toxic and must be converted to urea for excretion
• Structure
  • Molecular Formula: NH3
  • Trigonal Pyramidal Shape: The nitrogen atom has a lone pair of electrons
• Physical Properties
  • Gas at Room Temperature: Ammonia is a colorless gas at room temperature
  • Pungent Odor: Has a characteristic pungent odor
  • Solubility: Highly soluble in water
• Chemical Properties
  • Basic: Acts as a base and can accept a proton (H+) to form ammonium ion (NH4+)
  • Toxicity: Toxic to the central nervous system
  • Conversion to Urea: Converted to urea in the liver through the urea cycle
  • Reaction with Nessler’s Reagent: Reacts with Nessler’s reagent to form a colored product, used in ammonia quantification assays

Tumor Markers

• Definition: Substances produced by cancer cells or other cells of the body in response to cancer or certain benign (noncancerous) conditions. They are often proteins or glycoproteins
• CEA (Carcinoembryonic Antigen)
  • Glycoprotein involved in cell adhesion
  • Elevated in colorectal cancer, lung cancer, breast cancer, and other cancers
• PSA (Prostate-Specific Antigen)
  • Serine protease produced by prostate gland cells
  • Elevated in prostate cancer and benign prostatic hyperplasia (BPH)
• AFP (Alpha-Fetoprotein)
  • Glycoprotein produced by the fetal liver and yolk sac
  • Elevated in hepatocellular carcinoma, germ cell tumors, and certain other cancers
• CA 125 (Cancer Antigen 125)
  • Glycoprotein expressed on the surface of ovarian cancer cells
  • Elevated in ovarian cancer and certain other conditions
• Physical and Chemical Properties
  • Proteins or Glycoproteins: Varies depending on the specific marker
  • Solubility: Soluble in blood and other body fluids
  • Immunogenicity: Can be detected and quantified using immunoassays
• Detection
  • Typically measured using immunoassays (e.g., ELISA, chemiluminescence)

Cardiac Markers

• Definition: Substances released into the blood when the heart is damaged or stressed. They are typically proteins
• Troponin
  • Proteins involved in muscle contraction
  • Troponin I (cTnI) and Troponin T (cTnT) are cardiac-specific isoforms
  • Elevated in myocardial infarction
• CK-MB (Creatine Kinase-MB)
  • Isoenzyme of creatine kinase found primarily in heart muscle
  • Elevated in myocardial infarction
• Myoglobin
  • Oxygen-binding protein found in muscle tissue
  • Elevated in myocardial infarction and other muscle injuries
• Physical and Chemical Properties
  • Proteins: Varies depending on the specific marker
  • Solubility: Soluble in blood
  • Immunogenicity: Can be detected and quantified using immunoassays
• Detection
  • Typically measured using immunoassays (e.g., ELISA, chemiluminescence)

Key Terms

• Protein: A large molecule composed of amino acids linked by peptide bonds
• Amino Acid: The building block of proteins
• Peptide Bond: The covalent bond linking amino acids together
• Urea: The major end product of protein metabolism
• Uric Acid: The major end product of purine metabolism
• Creatinine: A waste product of muscle metabolism
• Ammonia: A toxic nitrogen-containing compound
• Tumor Marker: A substance produced by cancer cells or other cells in response to cancer
• Cardiac Marker: A substance released into the blood when the heart is damaged
• Isoelectric Point: The pH at which a molecule has no net electrical charge
• Zwitterion: A molecule that has both a positive and a negative electrical charge
• Heterocyclic Compound: A cyclic compound containing atoms of at least two different elements as members of its ring(s)
• Tautomerism: The ability of certain chemical compounds to exist as a mixture of two interconvertible isomers