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

Clinical significance

Primary hyperparathyroidism is due to autonomous, abnormal hypersecretion of PTH in the parathyroid gland

Secondary hyperparathyroidism is an appropriately high PTH level seen as a physiological response to hypocalcemia.

A low level of PTH in the blood is known as hypoparathyroidism and is most commonly due to damage to or removal of parathyroid glands during thyroid surgery.

Cori Cycle

The Cori Cycle operates during exercise, when aerobic metabolism in muscle cannot keep up with energy needs.

For a brief burst of ATP utilization, muscle cells utilize ~P stored as phosphocreatine. For more extended exercise, ATP is mainly provided by Glycolysis.

Lactate, produced from pyruvate, passes via the blood to the liver where it is converted to glucose. The glucose may travel back to the muscle to fuel Glycolysis.

The Cori Cycle costs 6 P in liver for every 2P made available in muscle. The net cost is 4 P Although costly in terms of "high energy" bonds, the Cori Cycle allows the organism to accommodate to large fluctuations in energy needs of skeletal muscle between rest and exercise.

Niacin: Vitamin B3, Nicotinamide, Nicotinic Acid Niacin, or vitamin B3,

 is involved in energy production, normal enzyme function, digestion, promoting normal appetite, healthy skin, and nerves.

RDA Males: 16 mg/day; Females: 14 mg/day

Niacin Deficiency : Pellagra is the disease state that occurs as a result of severe niacin deficiency. Symptoms include cramps, nausea, mental confusion, and skin problems.

Vitamin B12: Cobalamin

Vitamin B12, also known as cobalamin, aids in the building of genetic material, production of normal red blood cells, and maintenance of the nervous system.

RDA The Recommended Dietary Allowance (RDA) for vitamin B12 is 2.4 mcg/day for adult males and females

Vitamin B12 Deficiency

Vitamin B12 deficiency most commonly affects strict vegetarians (those who eat no animal products), infants of vegan mothers, and the elderly. Symptoms of deficiency include anemia, fatigue, neurological disorders, and degeneration of nerves resulting in numbness and tingling.