Biochemistry Signaling Pathways Quiz

Illustrated Notebook of Biochemistry - Metabolism

• The empiric formula for simple sugars, or monosaccharides is (CH₂O)ₙ, hence the name "hydrate of carbon."
• Monosaccharides cannot be hydrolyzed into simpler carbohydrates (e.g., glucose, fructose).
• Disaccharides are condensation products of two monosaccharide units (e.g., maltose, sucrose).
• Oligosaccharides are condensation products of 3-10 monosaccharides.
• Polysaccharides are condensation products of more than 10 monosaccharide units (e.g., starches, dextrins, inulin, cellulose).
• Carbohydrate derivatives can contain nitrogens, phosphates, and sulfur compounds.
• Carbohydrates can also combine with lipids to form glycolipids or with proteins to form glycoproteins.
• The suffix "-ose" is used for naming carbohydrates.
• The predominant carbohydrates in the body are structurally related to glyceraldehyde and dihydroxyacetone.
• All carbohydrates contain at least one chiral carbon and are optically active.
• Carbohydrates exist in either D- or L- conformations with the D-form being more commonly seen in humans.

Carbohydrate Classifications

• Monosaccharides are classified by the number of carbons (triose, tetrose, pentose, hexose, heptose).
• Relevant examples of 3-carbon monosaccharides include glyceraldehyde and dihydroxyacetone.
• Relevant examples of 4-carbon monosaccharides include erythrose.
• Relevant examples of 5-carbon monosaccharides include ribose, ribulose, and xylulose.
• Relevant examples of 6-carbon monosaccharides include glucose, galactose, mannose, and fructose
• Relevant examples of 7-carbon monosaccharides include sedoheptulose.
• Relevant examples of 9-carbon monosaccharides include neuraminic acid (also called sialic acid).

Cyclization of Aldoses and Ketoses

• Intermolecular cyclization of D-glucose creates a new chiral center(C-1) becoming a 5 membered ring (furanose) or a 6 membered ring (pyranose).
• Unlike pyran and furan, the rings of CHets do not contain double bonds.

Monosaccharides Classification

• The monosaccharides found in humans are classified according to the number of carbons they contain in their backbone structures. The major monosaccharides contain four to six carbon atoms.
• Triose (3 carbons)
• Tetrose (4 carbons)
• Pentose (5 carbons)
• Hexose (6 carbons)
• Heptose (7 carbons)
• Nonose (9 carbons)

Nomenclature

• The predominant carbohydrates encountered in the body are structurally related to the aldotriose glyceraldehyde and to the ketotriose dihydroxyacetone.
• All carbohydrates contain at least one asymmetrical (chiral) carbon and are optically active.
• Carbohydrates exist in two conformations (D or L) based on the orientation of the hydroxyl group farthest from the carbonyl.

Sugar Phosphates

• Monosaccharides in metabolic pathways are frequently converted to phosphate esters.
• Examples of these include dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate.

Deoxy Sugars

• A hydrogen atom replaces a hydroxyl group on the parent monosaccharide (e.g., 2-deoxy-D-ribose).
• 2-Deoxy-D-ribose is a crucial structural component of DNA.

Amino Sugars

• An amino group replaces a hydroxyl group. The amino group might be acetylated.
• Important examples of amino sugars include a-L-fucose (6-deoxy-L-galactose) and β-2-deoxy-D-ribose.

Sugar Alcohols

• The carbonyl oxygen of the parent monosaccharide is reduced resulting in a polyhydroxy alcohol (e.g., glucitol and sorbitol).

Sugar Acids

• Aldonic acids result from oxidation of the first carbon atom of the parent monosaccharide
• Alduronic acids result from oxidation of the highest numbered carbon atom of the parent monosaccharide.

Disaccharides and Other Glycosides

• The anomeric hydroxyl group and a hydroxyl group of another sugar or compound combine, splitting out water to form a glycosidic bond.
• Examples include maltose and sucrose.

Reducing and Non-reducing Sugars

• Some sugars (e.g., glucose, maltose, cellobiose, and lactose) are reducing sugars as they can be readily oxidized.
• Others (e.g., sucrose) are non-reducing as their anomeric carbon atoms are involved in glycosidic linkages.

Polysaccharides

• Homoglycans contain residues from only one type of monosaccharide (e.g., starch and glycogen).
• Heteroglycans contain residues from more than one type of monosaccharide.

Glycoconjugates

• Proteoglycans are protein complexes with glycosaminoglycans
• Glycosaminoglycans frequently contain acidic sugars
• Peptidoglycans are polysaccharides linked to short peptides, found in bacterial cell walls.
• Glycoproteins are proteins with covalently attached oligosaccharides (e.g., enzymes, hormones).

Introduction to Metabolism

• Most metabolic pathways can be classified as catabolic (degradative) or anabolic (synthetic).
• Catabolic reactions break down complex molecules into simple molecules like CO2, NH3 ,and H2O.
• Anabolic reactions combine simple molecules into complex molecules like proteins, polysaccharides.
• Catabolic reactions are typically oxidative while anabolic reactions are mostly reductive.
• Coenzymes (e.g., NAD+, FAD) are necessary for catabolic reactions.
• Catabolic hormones (e.g., glucagon, adrenaline) stimulate catabolic reactions.
• Anabolic hormones (e.g., insulin) stimulate anabolic reactions.

Regulation of Metabolism

• The production of energy must be coordinated with synthesis of end products in cells
• Intracellular signals (e.g., substrate availability, product inhibition)
• Intercellular signals (blood-borne hormones, neurotransmitters)
• Second Messenger Systems (e.g., Calcium/Phosphatidylinositol and Adenylyl cyclase).
• Adenylyl cyclase is responsible for converting ATP to cAMP

Transport of Glucose in Cells

• Glucose enters cells via facilitated diffusion (GLUT family) or Na⁺-monosaccharide co-transport.
• GLUT-1–14 are different glucose transporters located in cell membranes
• GLUT-2 is abundant in erythrocytes, pancreatic cells, etc.

Glycolysis (Embden-Meyerhof Pathway)

• Glycolysis breaks down glucose to pyruvate.
• It occurs in the cytoplasm
• It is an anaerobic process, resulting in a net production of two ATP molecules.
• Under anaerobic conditions, pyruvate can be converted to lactate (e.g., in erythrocytes)
• Glycolysis is used to generate energy in many tissues, especially in the absence of oxygen.

Introduction to Metabolism: Stages

• The first stage of catabolism involves the digestion of macromolecules like carbohydrates, proteins, and fats, followed by absorption and transport
• The second stage involves the degradation of the products into smaller intermediates within cells, in the cytosol. This stage of catabolism usually produces high energy molecules such as ATP.

Regulation of Glycolysis

• Key rate-limiting steps: Hexokinase, phosphofructokinase-1, and pyruvate kinase
• Allosteric regulators: ATP, citrate, F-2,6-BP (regulation of PFK-2 and FBP-2 are via phosphorylation/dephosphorylation).

Glycolysis: Reactions

• A series of enzyme-catalyzed reactions that convert glucose to pyruvate, with the production of ATP and NADH.
• Contains 10 distinct reactions, grouped into two phases: energy investment and energy payoff.

Oxidative Phosphorylation

• Involves a multi-enzyme system for ATP production, using the electrons from NADH and FADH2 created during catabolism to create a proton gradient that results in a flow of protons through ATP synthase, the ultimate energy generator.

Gluconeogenesis

• Biosynthesis of glucose from non-carbohydrate precursors such as pyruvate, lactic acid, glycerol, and amino acids
• The primary location is the liver, however, the kidney plays a role as well.
• This metabolic pathway is essentially the reversal of glycolysis, bypassing the irreversible steps.
• Key regulatory enzymes differ from those in glycolysis.

The Cori Cycle

• The metabolic pathway involving the exchange of lactate between muscle tissue and the liver.

Oxidation of Fatty Acids in Mitochondria

• Fatty acids are activated in the cytoplasm before entering the mitochondria.
• Transport to mitochondria is mediated by a carnitine shuttle system.
• Fatty acyl coA is oxidized via the b-oxidation pathway, which produces acetyl-CoA, NADH, and FADH2.

The Citric Acid Cycle (TCA)

• The central pathway for oxidizing acetyl-CoA to CO2.
• Important for the production of NADH, FADH2, and GTP (converted to ATP)
• Also important in providing intermediates for certain biosynthetic pathways.

Metabolism of Acetyl-CoA

• Oxidation products of carbohydrate, lipid and protein oxidation end as Acetyl CoA.
• Acetyl coenzyme A is an important intermediate carrying two-carbon units.

Synthesis of Fatty Acids

• Occurs in the cytosol
• Acetyl-CoA is transported from the mitochondrion by the citrate shuttle
• A series of reactions involves the sequential addition of two-carbon units from malonyl-CoA
• Fatty acids are made via the fatty acid synthase complex.

Lipid Transport and Storage

• Lipids are transported in the blood as lipoproteins.
• Different types of lipoproteins (e.g. chylomicrons, VLDL, LDL, HDL) have variable lipid compositions and transport different lipids to different tissues.

Lipoprotein Metabolism

• Lipoproteins transport lipids between tissues.
• Chylomicrons transport dietary lipids.
• VLDL transport lipids from liver to other tissues.
• LDL carry cholesterol to peripheral tissues.
• HDL remove cholesterol from peripheral tissues and transport it back to the liver
• The liver is involved in the final metabolism of many lipids transported by lipoproteins.

Steroid Hormones

• Derivatives of cholesterol.
• Have diverse functions in the body.
• Include sex hormones, cortisol, and other steroid hormones.

Protein Digestion and Absorption

• Proteins are digested to amino acids in the stomach and small intestine
• The resulting amino acids are absorbed by the mucosal cells in the small intestine
• Secretion of digestive enzymes from the stomach, pancreas, and small intestine is essential
• The enzymes are usually produced as inactive zymogens

Amino Acid Metabolism

• Amino acids undergo transamination to transfer their a-amino groups to α-ketoglutarate, forming glutamate, an α-amino group carrier.
• Glutamate undergoes oxidative deamination to release ammonia
• The resulting alpha-keto acids can be used for energy production with different pathways.
• Excess amino acids are catabolized to provide energy or used to synthesize other components.

Urea Cycle

• Urea is the primary nitrogen-containing waste product in humans.
• The liver synthesizes urea primarily from ammonia and aspartate.
• The urea cycle is a series of reactions that converts ammonia into urea, an excreted waste product.

Regulation of the Urea Cycle

• The rate limiting step in the urea cycle is the formation of carbamoyl phosphate.
• Nitrogen-containing molecules can cause a buildup of ammonia in the body.

Clinical Significance of Urea

• Deficiencies within the urea cycle can result in ammonia buildup with adverse health consequences.
• Dietary proteins can either be glucogenic or ketogenic depending on breakdown products.

Test your knowledge on the intricate roles of G-proteins and cAMP in biochemical signaling pathways. This quiz covers key concepts such as the activation of protein kinases, the role of fatty acids, and the enzymatic processes involved. Dive deep into the functionalities and implications of these essential components in cellular communication.