These amino acids have aromatic side chains, which contribute to protein structure, UV absorbance, and biochemical signaling.

1. Phenylalanine (Phe, F)

• Structure: Contains a benzyl side chain (a phenyl group attached to a CH₂).

• Properties: Non-polar, hydrophobic.

• Function: Precursor to tyrosine, dopamine, norepinephrine, and epinephrine.

2. Tyrosine (Tyr, Y)

• Structure: Similar to phenylalanine but with a hydroxyl group (-OH) on the aromatic ring.

• Properties: Polar, can participate in hydrogen bonding.

• Function: Precursor to catecholamines and thyroid hormones; involved in signal transduction via phosphorylation.

3. Tryptophan (Trp, W)

• Structure: Contains an indole ring (a fused double ring with nitrogen).

• Properties: Slightly polar, bulky.

• Function: Precursor to serotonin, melatonin, and niacin; absorbs UV light strongly at 280 nm.

4. Histidine (His, H) (sometimes included due to its aromatic-like imidazole ring)

• Structure: Has an imidazole ring (five-membered ring with two nitrogen atoms).

• Properties: Polar, positively charged at physiological pH.

• Function: Key role in enzyme active sites; acts as a proton donor/acceptor.

Bonus Insight: UV Absorbance

• Tryptophan and Tyrosine absorb UV light at ~280 nm, which is useful for measuring protein concentration.

• Phenylalanine absorbs weakly at ~260 nm.

Functions of  lipids

1. They are the concentrated fuel reserve of the body  (triacylglycerols).

2. Lipids are the constituents of membrane structure and regulate the membrane permeability (phospholipids  and cholesterol).

3. They serve as a source of fat soluble vitamins (A, D, E and K).

4. Lipids are important as cellular metabolic regulators (steroid  hormones and prostaglandins).

5. Lipids protect the internal organs, serve as insulating materials and give shape and smooth appearance to the body.

 Troponins remain elevated for up to 10 days—ideal for late diagnosis.

Proteins are complex macromolecules composed of amino acids that perform diverse biological functions. Understanding their structure-function relationships is crucial for medical applications and biochemistry.

Levels of Protein Structure

Primary Structure

• Linear sequence of amino acids connected by peptide bonds
• Determines all higher levels of organization
• Coded by DNA sequence

Secondary Structure

• Local folding patterns stabilized by hydrogen bonds
• Alpha helix: Right-handed spiral structure
• Beta sheet: Extended polypeptide chains arranged side by side
• Beta turn: Connects different secondary structural elements

Tertiary Structure

• Three-dimensional folding of entire polypeptide chain
• Stabilized by:
  • Hydrogen bonds
  • Disulfide bridges
  • Van der Waals forces
  • Electrostatic interactions
  • Hydrophobic interactions

Quaternary Structure

• Assembly of multiple polypeptide subunits
• Present only in proteins with more than one polypeptide chain
• Examples: Hemoglobin (4 subunits), antibodies

Protein Classification

Based on Structure

1. Fibrous Proteins

   • Elongated, insoluble
   • Structural functions
   • Examples: Collagen, keratin, elastin

2. Globular Proteins

   • Compact, soluble
   • Functional proteins
   • Examples: Enzymes, antibodies, hormones

Based on Composition

1. Simple Proteins

   • Composed only of amino acids
   • Examples: Albumin, globulins

2. Conjugated Proteins

   • Contain non-protein prosthetic groups
   • Glycoproteins: Contain carbohydrates
   • Lipoproteins: Contain lipids
   • Nucleoproteins: Contain nucleic acids
   • Phosphoproteins: Contain phosphate groups
   • Metalloproteins: Contain metal ions

Growth hormone

Growth hormone (GH or HGH), also known as somatotropin or somatropin, is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans.

Growth hormone is a single-chain polypeptide that is synthesized, stored, and secreted by somatotropic cells within the lateral wings of the anterior pituitary gland.

Regulation of growth hormone secretion

Secretion of growth hormone (GH) in the pituitary is regulated by the neurosecretory nuclei of the hypothalamus. These cells release the peptides Growth hormone-releasing hormone (GHRH or somatocrinin) and Growth hormone-inhibiting hormone (GHIH or somatostatin) into the hypophyseal portal venous blood surrounding the pituitary.

GH release in the pituitary is primarily determined by the balance of these two peptides, which in turn is affected by many physiological stimulators (e.g., exercise, nutrition, sleep) and inhibitors (e.g., free fatty acids) of GH secretion.

Regulation

Stimulators of growth hormone (GH) secretion include peptide hormones, ghrelin, sex hormones, hypoglycemia, deep sleep, niacin, fasting, and vigorous exercise.

Inhibitors of GH secretion include somatostatin, circulating concentrations of GH and IGF-1 (negative feedback on the pituitary and hypothalamus), hyperglycemia, glucocorticoids, and dihydrotestosterone.

Clinical significance

The most common disease of GH excess is a pituitary tumor composed of somatotroph cells of the anterior pituitary. These somatotroph adenomas are benign and grow slowly, gradually producing more and more GH excess. The adenoma may become large enough to cause headaches, impair vision by pressure on the optic nerves, or cause deficiency of other pituitary hormones by displacement.

1. Kwashiorkor

• Cause: Severe protein deficiency with adequate or near-adequate calorie intake.

• Typical Age: 1–3 years (post-weaning)

• Clinical Features:

  • Edema (due to hypoalbuminemia)

  • Moon face and swollen abdomen

  • Dermatosis and skin depigmentation

  • Fatty liver

  • Apathy and irritability

• Key Insight: Protein is lacking, but energy (carbohydrates) may be sufficient.

2. Marasmus

• Cause: Deficiency of both protein and calories

• Typical Age: Infants under 1 year

• Clinical Features:

  • Severe wasting and muscle loss

  • Prominent ribs and sunken eyes

  • No edema

  • Alert but irritable

• Key Insight: Total energy deficit leads to extreme emaciation.