Hormones binding to receptors

Some water-soluble hormones bind to receptors with intrinsic protein kinase activity (often tyrosine kinases). In this case, no second messenger is required for protein kinase activation. The insulin receptor is an example of a tyrosine kinase receptor. 

Neurotransmitter or hormone bind to receptors on endothelial cells lining the artery, which in response releases nitric oxide (NO). 2. NO molecules from the endothelium travel into smooth muscle cells, where they activate the enzyme, guanylyl cyclase (GC). 3. 

Peptide or amine hormone binds to receptor on the outside of the cell acts through receptor without entering the cell. 

The binding of small molecules to larger ones is basic to most biological phenomena. Substrates bind to enzymes and hormones bind to receptors. Metal ions bind to ATP, to other small molecules, and to metalloproteins. Hydrogen ions bind to amino acids, peptides, nucleotides, and most macromolecules. In this section we will consider ways of describing mathematically the equilibria involved. 

Why do we need vitamins Early clues came in 1935 when nicotinamide was found in NAD+ by H. von Euler and associates and in NADP+ by Warburg and Christian. Two years later, K. Lohman and P. Schuster isolated pure cocarboxylase, a dialyz-able material required for decarboxylation of pyruvate by an enzyme from yeast. It was shown to be thiamin diphosphate (Fig. 15-3). Most of the water-soluble vitamins are converted into coenzymes or are covalently bound into active sites of enzymes. Some lipid-soluble vitamins have similar functions but others, such as vitamin D and some metabolites of vitamin A, act more like hormones, binding to receptors that control gene expression or other aspects of metabolism. 

D. Voet and J. Voet, Biochemistry, 2nd ed. (1995), John Wiley Sons (New York), pp. 82-85 1274-1276. Introduction to gel filtration and hormone binding to receptors. 

Many hormones bind to receptors that act through an intermediary protein to either activate or inhibit adenylate cyclase (table 24.4). Cyclic AMP is formed from ATP by adenylate cyclase, which is bound to the inside of the cell membrane (fig. 24.15). 

FIGURE 28-2 Primary cellular locations of hormone receptors. Peptide hormones tend to bind to surface membrane receptors (site 0 steroid hormones bind to cytosolic receptors [site 10 and thyroid hormones bind to receptors in the cell nucleus [site III). 

Figure 17.4 Intracellular steroid receptor activation by hormone ligands, (a) Steroid hormones diffuse across the cell membrane into the cell. (b) Steroid hormone receptors in the basal state bound to accessory proteins, (c) Steroid hormones bind to receptors and accessory proteins are dissociated from the receptors. (d) Hormone receptor complexes dimerize. (e) Dimer complexes enter the nucleus and initiate transcription of responsive genes.

Correct answer = E. All steroid hormones bind to receptors in the nucleus or the cytosol. 

B. Epinephrine is synthesized from tyrosine. It functions like a polypeptide hormone, binding to receptors on the cell membrane. cAMP levels rise in response to epinephrine. 

C. Steroid hormones bind to receptors that subsequently bind to DNA, as do 1,25-DHC, thyroid hormone, and retinoic acid (from vitamin A). 

Hormones bind to receptors, which can either be on the cell surface or, as in the case of steroid and similar hormones, within the cell. 

Adenylate cyclase The enzyme that, when activated by hormones binding to receptors, catalyzes the cyclization of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) with the release of pyrophosphate. 

IGF-1 Insulin-like growth factor-1, also called somatomedin C a 70-amino acid peptide that shares structural homology with proinsulin. IGF-1 is synthesized and released by the hver in response to growth hormone binding to receptors on the hepatocyte. 

Roth, J., Peptide hormone binding to receptors A review of direct studies in vitro, Metab. Clin. Exp., 22[8], 1059, 1973. 

FIGURE 11-40 Examples of hormones that bind to nuclear receptors. These and related lipid-soluble hormones bind to receptors located in the cytosol or nucleus. The ligand-receptor complex functions as a transcription activator. 

Thyroid hormone binds to receptors in the nucleus that control the expression of genes responsible for many metabolic processes. The T3 receptor exists in two monomeric forms alpha and beta. When activated by T3, the a and (3 monomers combine to form ao, PP, or aP dimers. These Tj-activated dimers bind to DNA response elements and control the synthesis of RNA, which codes for specific proteins that mediate the actions of thyroid hormones. 

After hormones bind to receptors in the cell nucleus, they activate appropriate genes, which bring about sdective transcription and expression of the hormones. Steroid hormones regulate growth and development, as well as homeostatic processes. Trophic hormones from the pituitary regulate hormonal secretions by the thyroid, adrenals, and gonads. 

The thyroid releasing hormone binds to receptors in the pituitary and like many other hormones activates adenyl cyclase and increases the intracellular levels of cyclic AMP. 

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