Biochemistry Chen Yonggang Dept. of Biochemistry Zhejiang Univ. School of Medicine

1. Biochemistry Chen Yonggang Dept. of Biochemistry Zhejiang Univ. School of Medicine E-mail: chenyg9@163.com • .

2. Integration of Metabolism Functions of Hormones

3. Overview Of Metabolism • Adult animals reach a steady state where anabolism and catabolism are approximately equal. • Intercellular communication is responsible for this steady state. • The nervous and endocrine systems are responsible for coordinating metabolism.

4. Overview of Metabolism-2 • Neurons emit neurotransmitters that evoke specific responses from nearby cells. • Endocrine system releases hormones into the bloodstream which act either directly on a cell or via a second messenger. • Overview of metabolism is on next slide.

6. Division of Labor • Small Intestine • Digestion of nutrients • Into molecules small enough to be absorbed by enterocytes • To blood and lymph systems • Energy via glutamine

7. Division of Labor-2 • Liver • Key role in carbohydrate, lipid, and AA metabolism • Regulates composition of blood • Regulates nutrients availability • Adipose Tissue • Storage of energy in form of TAGs

8. Division of Labor-3 • Brain • Directs most metabolic activity • Uses energy (usually glucose) • Kidney (energy via fatty acids and glucose) • Filtration of blood plasma • Reabsorb electrolytes, sugars and amino acids from filtrate • Regulate body pH • Regulate body’s water content

9. Feeding-Fasting Cycle • Mammals are able to consume food intermittently because of elaborate mechanisms for storing and mobilizing energy-rich molecules derived from food. • Postprandial-directly after a meal when blood nutrient levels are elevated over the fasting state. • Post absorptive- blood nutrient levels are low, e. g. overnight.

10. The Feeding Phase • Food propelled along gastrointestinal tract by muscular contraction (nervous system). • Products are: sugars, fatty acids, glycerol, and amino acids. • Sugars and amino acids absorbed and transported by the portal blood to the liver. • Lipids (as chylomicrons) to muscle and adipose tissue. Chylomicron remnants to liver.

11. The Feeding Phase-2 • Glucose moves to the liver. High concentration in blood triggers insulin release in pancreas. • Insulin triggers: glucose uptake by muscle and adipose tissue, fat synthesis in liver and adipocytes, and gluconeogenesis (liver with three C sources. Using excess amino acids and lactate) • Allosteric effectors ensure that competing pathways do not occur simultaneously.

12. The Fasting Phase • Nutrient flow from intestine decreases. • Blood glucose and insulin levels fall and glucagon promotes glycogenolysis and gluconeogenesis in the liver. • Low insulin levels promotes lipolysis and release of AA from muscle. • Prolonged fasting (overnight) results in FAs from adipose tissue providing glucose for muscle.

13. The Fasting Phase-2 • Starvation • FAs (adipocytes) and ketone bodies (liver) are used for energy. • Large amounts of AAs from muscle are used for gluconeogenesis. • After several weeks, the brain uses ketone bodies for fuel.

14. Intercellular Communication • Endocrine hormones are chemicals secreted by special cells that exert some biochemical effect on a distant target cell. • Growth hormone (pituitary), testosterone (gonads), and insulin (pancreas) are common examples. • Some hormones target specific cells and some have effects on a variety of different cell types.

15. Hormones • Thyroid-stimulating hormone (TSH) stimulates follicular cells in the thyroid gland to release T3 and T4. • T3 and T4 stimulate a variety of cellular responses in numerous cell types. • Thyroid hormones stimulate glycogenolysis in liver but glucose absorption in the small intestine.

16. Hormone Examples • Source Hormone Function • Hypothalamu gonadotropin-RH stimulate • s LH and FSH • Pituitary growth hormone growth • oxytocin uterine contr • Gonads estrogens female repr • androgens male repr • Pancreas insulin glucose uptake

17. Hormone Examples-2 • Hormone molecules can be: • polypeptides: ACTH, FSH, GnRH, oxytocin • steroids: estrogens, androgens, glucocorticoids • AA derivatives: epinephrine, thyroxine, norepinephrine

18. Cascade System-1 • Many hormones synthesis and release are regulated by a “cascade mechanism” ultimately controlled by the central nervous system (CNS). • The hypothalamus directs the anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis).

19. Cascade System-2 CRH GnRH GHRH TRH inhibit GH TSH ACTH LH/FSH thyroid adrenal cortex testes ovaries many tissues reproductive organs bone hypothalamus anterior pituitary T3, T4 sex hormones corticoids

20. Cascade System-3 oxytocin vassopressin smooth muscle mammary glands kidney tubules arterioles hypothalamus posterior pituitary

21. Growth Factors • A variety of hormonelike polypeptides and proteins. Called growth factors or cytokines, are thought to regulate the growth, differentiation, and proliferation of cells. • Epidermal growth factor (EGF) • A mitogen (a stimulator of cell division) for many kinds of epithelial cells. • Platelet-derived growth factor (PDGF) • Stimulates mitosis in fibroblasts.

22. Growth Factors-2 • Somatomedins (insulinlike growth factors I and II, IGF-1 and IGF-2 in humans) • Mediate actions of growth hormone (GH). • Secreted in the liver (and other tissue cells)

23. Growth Factors-3 • Interleukin-2 (IL-2) • Also regulate the immune system. • Secreted by T cells after they have been activated by binding to a specific antigen-binding cell. • Numerous identical T cells are produced.

24. Growth Factors-4 • Interferons • Type I protect cells from viral infection by stimulating phosphorylation and inactivation of a protein factor required for protein synthesis. • Type II (from T lymphocytes) inhibit growth of cancerous cells. • Tumor necrosis factors (TNF) are toxic to tumor cells.

25. Mechanisms of Hormone Action • Steroid receptors are usually within the cell as the steroid molecules pass through the cell membrane. • Other hormones bind to specific sites on the cell membrane and release within the cell second messengers which actually cause the hormonal response. • This signal transduction also allows for a tremendous amplification of the hormone molecule’s impact on the cell.

26. Second Messengers • Common second messengers are cyclic AMP (cAMP), cGMP, diacylglycerol (DAG), inositol-1,4,5-triphosphate (IP3), and Ca2+. • We will examine cAMP synthesis and action in more detail.

27. cAMP • cAMP is produced when hormone binds to a membrane receptor. This binding activates a G protein (binds guanine nucleotide) as shown on the next slide. • The stimulated G protein replaces GDP with GTP. This activates the a subunit to bind to and activate adenylate cyclase which then produces cAMP. • cAMP then activates a cAMP-dependent protein kinase.

28. cAMP-2

29. cAMP-3 • Adenylate cyclase stays activated until the GTP on the a subunit of the G protein is hydrolyzed back to GDP. • Thus a single hormone molecule (eg. glucagon) can stimulate synthesis of many cAMP molecules which in turn can turn on phosphorylation and activate, for example, multiple phosphorylase kinase molecules to hydrolyze glycogen to G-1-P.

30. cAMP-4 • Adenylate cyclase is deactivated when the a subunit of the G protein uses its built in hydrolysis capability to return GTP to GDP. • Before this occurs, however, many protein molecules have been activated. This is the “cascade” process referred to earlier.

31. cGMP • From GTP by guanylate cyclase • Two types of guanylate cyclase are involved in signal transduction, one is membrane bound and the other cytoplasmic. • Membrane bound activated by: • Atrial natriuretic factor (ANF) peptide • Bacterial enterotoxin

32. cGMP-2 • ANF is released by atrial heart cells responding to increased blood volume. • Lowering blood pressure and diuresis seem to be mediated by cGMP. • Binding of enterotoxin to intestinal cells causes diarrhea via excessive secretion of electrolytes and water into the lumen of the small intestine. • Cytoplasmic guanylate cyclase may bind NO to a heme group to activate the enzyme.

33. Phosphatidyl Inositol and Ca2+ • The phosphatidyl inositol cycle (slide 36) mediates the action of hormones and growth factors • Phosphatidyl inositol-4,5-bisphosphate (PIP2) is cleaved by phospholipase C to form second messengers DAG and IP3 (inositol-1,4,5-triphosphate) • DAG activates protein kinase which activates or deactivates an enzyme.

34. Phosphatidyl Inositol and Ca2+ -2 • IP3 diffuses to the calcisome (SER, smooth endoplasmic reticulum) where it binds to a receptor, a calcium channel. • Calcium flows in to the cytoplasm regulating calcium-binding proteins. • Calmodulin mediates many calcium-regulated reactions. It is a regulatory subunit for some enzymes (e. g. phosphorylase kinase important in glycogen metabolism).

35. Fig 16.12

36. Steroid Hormone Mechanism • Signal transduction by hydrophobic hormones (e. g. steroids) result in changes in gene expression. • i. e. protein mix changes in the cell. • Transport into the cell is via binding to a protein. • E. g. : transcortin • androgen-binding protein • sex hormone-binding protein • albumin

37. Steroid Hormone Mechanism-2 • In the cell, hormones bind to intracellular receptors. The complex moves to the nucleus. (Slide 39) • In the nucleus, each complex binds to specific DNA segments, called hormone response elements (HRE), via a zinc finger domain. • The receptor enhances or diminishes transcription of a specific gene. • Each HRE may influence 50-100 genes.

39. The Insulin Receptor • The insulin receptor (Slide 41) is a trans- membrane glycoprotein with two subunits connected by disulfide bridges. • Insulin binding activates receptor tyrosine kinase activity and causes a phosphorylation cascade that modulates intracellular proteins. • Binding also induces transfer of some proteins to the cell surface.

41. The End Integration of Metabolism