Hormones: Cell Signaling, Definition and Chemical Nature of Hormones

Living organisms control and coordinate their activities through complex chemical signals.

In multicellular organisms adjacent cells communicate through plasmodesmata (in plants) and gap junctions (in animals).

But, a cell can communicate distant cells by releasing variety of signal molecules such as paracrine signals (neurohormones), endocrine signals (hormones), synaptic signals (neurotransmitters) and gaseous signals (e.g. NO, Nitric Oxide).

Many organisms release Pheromones that alter the behaviour of other organisms of the same species. For example, some algae and animals release pheromones to attract opposite sex. Some strains of Streptococcus faecalis, Gram-positive bacteria secrete pheromones that induce conjugation. The signal molecules bind to specific receptor proteins on or within the cell surface which induce or suppress gene expression, differentiation and metabolism.

What are Hormones?

Hormones are organic compounds, naturally secreted at low concentration by endocrine cells that exert an influence on physiological processes in multi-cellular organisms. Hormones are also called chemical messengers, information molecules or endocrine signals. The hormones, in general, are stimulatory in action. The hormones having inhibitory actions are called chalones. Bayliss and Straling (1904) coined the term ‘Hormone’ to describe the nature of secretion, the first animal hormone discovered. Went (1928) discovered the first plant hormone i.e. auxin.

Chemical Nature of Hormones:

On the basis of chemical composition, hormones are classified into following types:

(a) Amine hormones:

These are derived from amino acids tyrosine and tryptophan and have amino group (-NH₂). E.g. thyroxine, epinephrine, nor-epinephrine, histamine, etc.

(b) Steroid hormones:

These are derived from cholesterol, e.g.. Androgens, estrogens, progesterone etc.

(c) Polypeptide hormones:

They composed of less than 100 amino acids, For example, short peptide hormones are oxytocin, ADH (antidiuretic hormone), MSH (melanocyte stimulating hormone). The long peptide hormones are insulin, glucagons, ACTH, parthormone etc.

(d) Proteinous hormones:

Generally composed of more than 100 amino acids, e.g., LH, FSH, STH, The LH and FSH are glycoproteins hormones.

(e) Ecosanoids (Gr. ecosn = 20):

These are derived from arachidonic acid, a C-20 fatty acid with 4 double bonds, e.g., prostaglandins, thrombaxanes and leukotriene’s. These are called local hormones because they are short lived and have autocrine and paracrine effect.

On the basis of solubility, hormones may be either lipophilic or Hpophobic. The lipophilic hormones are fat soluble, e.g., steroid hormones, thyroxin and renenoids. All other hormones are hydrophilic that are water-soluble and bind to cell surface receptor for action.

On the basis of distance over which they act, hormones are of following types:

(i) Autocrine hormones:

They act on same cells from which they release, e.g., Interleukins (1L- 1,2) which stimulate T-cell proliferation.

(ii) Paracrine hormones:

They act on adjacent cells of their secretion, e.g., prostaglandins.

(iii) Endocrine hormones:

They act on distant cells from the side of their release, e.g., insulin.

(f) Brassinosteroide (= Brassins, BRs):

They are natural growth promoters synthesized from campesterol. They are structurally similar to animal steroid hormones. They were first isolated by Grove et al (1979) in pollens of Brassica napus (rape seed). To date 40 free BRs and 4 BR-conjugates have been discovered. They found in wide range of plant species, in algae, pteridophytes, gymnosperms, and angiosperms. They are synthesized in all parts of plants and mostly in immature seeds, roots and leaves. Brassinosteroides promote cell division, cell elongation, bending of stem, development of vascular tissues and reproductive organs.

(g) Oligosaccharins:

They are oligosaccharides of hormonal properties released from plant cell walls. They elicit defense responses against fungal attack. They are known to inhibit the auxin stimulated apical dominance of pea stems, root formation in tobacco etc.

(h) Polyamines:

They are the compounds having more than one amine groups, synthesized from amino acids lysine, arginine, e.g. putrescine and spermidine etc. They have some effect in cell division growth and development. Putrescine H₂N-(CH₂) NH₂ level increase in stress response.

Organic Compounds:

Organic compounds or bio-molecules are the universal occurrence of all living organisms. For analysis of the types of organic compounds found in living organisms, take any living tissue (a piece of vegetable or liver etc.) and grind with trichloroacetic acid to get thick slurry or homogenate. The process of grinding to disrupt the cells is called homogenization, which is usually done in a high speed blender or using mortar and pestle.

When the homogenate is filtered we would get 2 fractions i.e. a filtrate called acid-soluble pool, and the retentate called acid-insoluble pool. The biomolecules found in both the pools (fractions). The acid soluble pool usually contains the cytoplasmic composition, while the acid insoluble fraction contains macromolecules from cytosol and organelles.

On the basis of molecular weight (MW) and solubility, biomolecules are of 2 types, micro-molecules and macromolecules. Micro-molecules are low MW organic compounds (less than 100 Dalton) usually found in acid soluble pool. For example, sugars, amino acids, nucleotides vitamins etc.

Micro-molecules are often called building block molecules or monomers which covalently linked to each other to form macromolecules or polymers. Macromolecules are high MW (about 1000 Dalton or above) organic compounds usually found in acid -insoluble fraction or macromolecular fraction e.g., proteins, nucleic acids, polysaccharides and lipids.

Though lipids found in macromolecular fraction, they are actually micro-molecules, whose molecular weights d or A exceeds 800 Da. The nucleic acids and proteins are called informational molecules. The large size and 3D-shape of macromolecules enables them to function as structural components, enzymes, nutrient .stores, molecular messenger and sources of genetic information.