Biochemistry

Pepsin

• Primary enzyme for protein degradation in stomach
• Optimal pH: 1.5-2.0 (acidic)
• Secreted as inactive pepsinogen
• Cleaves proteins into smaller peptides

Histone

• Protein rich in basic amino acids (lysine and arginine)
• Packages DNA in chromosomes
• Forms nucleosomes with DNA
• Types: H1, H2A, H2B, H3, H4

Casein

• Example of phosphoprotein
• Major protein in milk
• Contains phosphate groups attached to serine residues
• Forms micelles in milk

Antibodies (Immunoglobulins)

• Structure composed of 2 long (heavy) & 2 short (light) peptide chains
• Y-shaped structure
• Connected by disulfide bonds
• Variable regions bind to antigens
• Constant regions determine antibody class

Insulin Receptors

• Tetrameric glycoprotein
• Consists of 2 α and 2 β subunits
• α subunits: Extracellular, insulin-binding
• β subunits: Transmembrane, tyrosine kinase activity
• Essential for glucose homeostasis

Collagen

• Contains triple helix structure
• Most abundant protein in mammals
• Composed of three polypeptide chains (tropocollagen)
• Amino acid sequence: Gly-X-Y (where X is often proline, Y is often hydroxyproline)
• Provides structural support to tissues