Classification of Hormones

 

Classification of Hormones

•    Hormones can be classified by a number of different ways, according to 

– Chemical nature

•    Steroidal, proteinous, amino acid derivatives etc

–  Solubility properties (lipophilicity or hydrophilicity)

• Group I and Group II hormones

–  Mechanism of action

•     Location of receptors and nature of signal used to mediate its action within cell

•     Intracellular receptor binding or cell surface receptor binding and second messenger based mechanism etc.

 

•     Most commonly used system of classification for hormones is according to their chemical nature

•     Chemically hormones can be classified into:

i)     Protein hormones (peptide hormones):

–  Either large proteins or small peptide chains

–  E.g. insulin, glucagon, parathormone & pituitary hormones etc.

ii)   Amino acid derivatives:

–  Mostly these are tyrosine derivatives

–  E.g. Epinephrine, norepinephrine & thyroid hormone iii) Steroid hormones:

–  Contain steroidal nucleus in their structure

–  E.g. Adrenocorticosteroids, endrogens, estrogens & progesteron etc.

• Classification based on mechanism of action can be described as:

i) Hormones that bind to the intracellular

receptors:

•    Also called group-I hormones; lipophilic in nature • Transported through carrier proteins via blood

– Bound hormone is inactive while free hormone is active

•    Readily crosses plasma membrane

•    Bind to the receptors in cytosol or nucleus of target cells

•    Long plasma half-life

•    The ligand-receptor complex is the intracellular messenger in this group

 

 

ii) Hormones that bind to cell surface receptors:

•    Water soluble hormones

–  Including proteinous and aminoacid derivatives

•    Binds to the specific receptors on plasma membrane of target cells

–  Communicate with intracellular metabolic processes through intermediate molecules called second messengers

•    As their synthesis is triggered by primary hormone

– Second messengers

•   Activated by binding of first messengers (hormones) with the receptors

•   Starts a cascade of events that ultimately results in the response from a cell

•   E.g. cAMP, Ca²⁺ ions, protein kinase, proteins phosphatase