Monosaccharides: Structure and Isomers

Monosaccharides

• Constitutional isomers share the same molecular formula but have different atomic connectivity.
• Stereoisomers share connectivity but differ in spatial arrangement.
• Enantiomers are non-superimposable mirror images, such as D- and L- forms.
• Diastereomers are stereoisomers that are not mirror images.
• Epimers differ at only one asymmetric center.
• Anomers differ at the anomeric carbon, formed during ring closure, resulting in α and β forms.
• Monosaccharides form cyclic structures through hemiacetal (aldehyde + alcohol) or hemiketal (ketone + alcohol) reactions.
• Glucose forms a six-membered pyranose ring.
• Fructose can form both five-membered furanose and six-membered pyranose rings.
• The anomeric carbon determines if the sugar is in the α (OH below) or β (OH above) configuration.
• Reducing sugars possess a free anomeric carbon capable of reducing oxidizing agents, like glucose reacting with Fehling's solution.
• Non-reducing sugars lack a free anomeric carbon, such as sucrose.
• Glycosidic bond formation links sugars to alcohols (O-glycosidic) or amines (N-glycosidic).
• Phosphorylation is important in metabolic pathways, like glucose-6-phosphate.
• Amino sugars occur, and acetylation is found in glycosaminoglycans.

Disaccharides and Polysaccharides

• α-1,4 linkages are found in starch and glycogen and form linear structures.
• β-1,4 linkages are found in cellulose and create linear, rigid structures.
• α-1,6 linkages create branching in glycogen and amylopectin.
• Glycogen (α-1,4 & α-1,6) is highly branched and compact and functions in energy storage in animals.
• Cellulose (β-1,4) forms straight chains and fibers, plays a structural role in plants, and is indigestible by humans.

Glycoproteins, Proteoglycans, and Mucins

• Glycoproteins mainly consist of proteins involved in cell signaling and membrane functions.
• Proteoglycans are mainly carbohydrate and glycosaminoglycan components, with a structural role in the extracellular matrix.
• Mucins are rich in carbohydrates and act as lubricants in mucus.
• N-linked glycosylation involves carbohydrates attached to asparagine (Asn).
• O-linked glycosylation involves carbohydrates that attach to serine (Ser) or threonine (Thr).
• All N-linked polysaccharides contain a common pentasaccharide core (three mannoses, two N-acetylglucosamines).
• Glycosaminoglycans comprise repeating disaccharide units (one amino sugar + one negatively charged sugar).
• Glycosaminoglycans are important in connective tissue, cartilage, and the extracellular matrix.
• Examples of glycosaminoglycans are chitin (in insects), hyaluronic acid (joints), and heparin (blood anticoagulant).

Fatty Acids

• Carbon numbering starts from the carboxyl terminal carbon.
• The α (2nd) and β (3rd) carbons are adjacent to the carboxyl group.
• The omega (ω) carbon location is last carbon.
• Saturated fatty acids have no double bonds, for example, stearic acid (C18:0).
• Unsaturated fatty acids contain one or more double bonds, for example, oleic acid (C18:1).
• Polyunsaturated fatty acids have more than one double bond, often separated by a methylene (-CH2-) group.
• Cis double bonds create kinked structures and are common in biological systems.
• Trans double bonds create linear structures and are found in processed fats.
• Shorter chains have a lower Tm, that is more fluid. -Cis-double bonds have a lower Tm, that prevents tight packing and increases fluidity.
• Saturated fatty acids have a higher Tm, that packs tightly and are solid at room temperature, for example, butter.
• Unsaturated fatty acids have a lower Tm, that disrupts packing and are liquids at room temperature, for example, olive oil.
• Omega-3 (ω-3) and Omega-6 (ω-6) are essential fatty acids that cannot be synthesized by the body.
• Omega-3 (ω-3) and Omega-6 (ω-6) act as precursors to eicosanoids, such as prostaglandins, which regulate inflammation.
• Omega-3 (ω-3) and Omega-6 (ω-6) have potential cardiovascular benefits.
• Triacylglycerols consist of three fatty acids esterified to a glycerol molecule.
• Triacylglycerols are energy-dense, storing more energy per gram than carbohydrates or proteins.
• Triacylglycerols are hydrophobic, so they are stored without water, reducing weight.
• Triacylglycerols are highly reduced, yielding more ATP per molecule upon oxidation.

Membrane Lipids

• Phospholipids contain two fatty acids, a glycerol or sphingosine backbone, a phosphate group, and an alcohol. -Phospholipids are a major component of biological membranes.
• Glycolipids contain a sugar moiety instead of phosphate.
• Glycolipids are found on the extracellular surface and are involved in cell recognition.
• Cholesterol is steroid-based, contains four fused rings, modulates membrane fluidity, and is a precursor to steroid hormones.
• Hydrophobic tails consisting of fatty acids interact with lipids.
• Hydrophilic head groups consisting of phosphate or sugar interact with water.
• This dual nature allows the formation of bilayers in membranes.
• Lipidated proteins are found in the inner leaflet of the membrane and are covalently attached to fatty acids or prenyl groups.
• GPI-anchored proteins are found in the outer leaflet and are covalently linked to glycosylphosphatidylinositol.
• Membranes form spontaneously due to the hydrophobic effect.
• Hydrophobic tails of phospho- and glycolipids cluster together to avoid water.
• Polar head groups interact with the aqueous environment.
• This arrangement results in a bilayer structure, such that each layer is called a leaflet.
• Longer fatty acid chains decrease fluidity due to stronger van der Waals interactions.
• More unsaturation (cis-double bonds) increases fluidity because it prevents tight packing.
• Cholesterol acts as a buffer to regulate membrane fluidity.
• Cholesterol reduces fluidity at high temperatures by stabilizing lipid packing.
• Cholesterol increases fluidity at low temperatures by preventing tight packing.
• Membranes are not homogeneous and contain lipid rafts, found in specialized domains.

Membrane Proteins

• Integral proteins are embedded within or span the membrane.
• Integral proteins often have hydrophobic transmembrane domains, for example, channels and transporters.
• Peripheral proteins are associated loosely with the membrane.
• Peripheral proteins can be detached with salt/pH changes.
• Peripheral proteins interact with integral proteins or lipid head groups.
• The membrane is a two-dimensional fluid where lipids and proteins move laterally.
• Membranes function both as permeability barriers (for ions and water) and solvents (for lipids and proteins).
• Lateral diffusion is such that lipids move sideways within a leaflet very rapidly (~2 µm/sec).
• Transverse diffusion (flip-flop) is such that lipids move between leaflets very slowly, only once every several hours.
• Transverse diffusion requires specialized proteins called flippases.
• Simple diffusion is the movement along the concentration gradient, for example, steroid hormones.
• Facilitated diffusion (passive transport) uses proteins to transport molecules down their gradient.
• Porters and channels are involved in facilitated diffusion.
• Active transport moves molecules against their concentration gradient, requiring energy.
• Primary active transport uses ATP hydrolysis, for example, the Na+/K+ ATPase pump.
• Secondary active transport uses another ion's gradient, for example, the Na+/glucose symporter.
• The potassium channel specificity is due to size and geometry.
• K+ is larger than Na+, such that the channel's selectivity filter is optimized for K+.
• K+ is stabilized by carbonyl oxygens in the selectivity filter.
• Na+ is too small to interact effectively, preventing its passage.
• Electrostatic repulsion is such that K+ ions repel each other in the channel, driving rapid transport (~10º ions/sec).

Signal Transduction

• The general steps in signal transduction are the release of a primary messenger (hormones, metabolites, etc.) and reception of primary messenger by a receptor.
• Signal transduction involves the conversion of the signal into intracellular chemical forms.
• Secondary messengers are produced.
• Proteins interact, and phosphorylation modifies protein activity.
• Activation of physiological effectors leads to changes in gene expression, metabolism, etc.
• The termination of the signal is necessary to prevent overactivation.
• Transducers are proteins that translate an external signal into an intracellular response.
• Secondary messengers are small molecules that amplify and distribute the signal, like cAMP, Ca2+, DAG, and IP3.
• 7TM (Seven-Transmembrane-Helix) Receptors include the β₂-adrenergic receptor (binds epinephrine).
• Ligand binding causes a conformational change that activates a G-protein.
• In dimeric receptors, ligand binding induces dimerization, which recruits an external kinase (JAK2)..
• In dimeric receptor kinases, ligand binding activates intrinsic tyrosine kinase activity, leading to a phosphorylation cascade, for example, the Epidermal Growth Factor Receptor (EGFR).
• The insulin receptor is a pre-formed dimer that undergoes cross-phosphorylation when insulin binds.

G-Proteins

• Hetero-trimeric G-proteins consists of α, β, and γ subunits.
• Hetero-trimeric G-proteins are inactive when bound to GDP.
• Hetero-trimeric G-proteins are active when bound to GTP, alpha subunit dissociates and activates downstream effectors.
• In G-protein activation by the β₂-adrenergic receptor, adenylate cyclase increases cAMP.
• Monomeric G-proteins (e.g., Ras) function in cell growth & differentiation.
• Monomeric G-proteins are active in the GTP-bound state and inactive in the GDP-bound state.
• Monomeric G-proteins are regulated by GTPase-activating proteins (GAPs) and Guanine-nucleotide exchange factors (GEFs).
• cAMP is produced by adenylate cyclase.
• cAMP activates protein kinase A (PKA), which phosphorylates targets to regulate metabolism.
• Kinases are enzymes that phosphorylate proteins which include JAK2, protein kinase C, and Akt kinase.
• Phosphatases remove phosphate groups to downregulate signaling.
• Calcium (Ca2+) is a secondary messenger in muscle contraction, neurotransmitter release, and metabolism regulation.
• Calcium (Ca2+) is released via IP3-gated channels.
• Calmodulin binds Ca2+ and activates enzymes like CaM kinase, regulating ion permeability, neurotransmitters, and metabolism.
• The example pathways are the β₂-adrenergic receptor, GHR, EFGR, and insulin receptor.

Cell Signaling Pathways

-In the β₂-Adrenergic Receptor (Fight-or-Flight Response), epinephrine activates G-protein, which in turn activates adenylate cyclase, increases cAMP, and activates PKA.

• In the Growth Hormone Receptor (GHR), growth hormone binds, the receptor dimerizes, recruits JAK2 kinase, phosphorylates targets, and changes gene expression.
• In the Epidermal Growth Factor Receptor (EGFR), ligand binds, the receptor dimerizes, autophosphorylates, activates Ras, and causes cell growth & proliferation.
• In the Insulin Receptor, insulin binds the pre-dimerized receptor, activates PI3 kinase, produces PIP3, activates Akt kinase, and increase in glucose uptake via GLUT4 transporter.
• Disruptions in signal transduction can contribute to diseases like Cholera & Whooping Cough.
• Bacterial toxins alter G-protein activity, leading to constant activation of adenylate cyclase and ion loss.
• Cancer (Oncogenes & Tumor Suppressors) can occur due to Ras mutations that prevent GTP hydrolysis, leading to uncontrolled cell division.
• EGFR overexpression promotes excessive growth.
• Chronic Myelogenous Leukemia (CML): Bcr-Abl fusion gene creates a hyperactive kinase that drives leukemia.
• Gleevec (Imatinib) effectively treats CML by inhibiting this kinase.

Digestion

• Proteins Denatured by low pH (stomach acid, pH 1-2) and hydrolyzed by proteases (e.g., pepsin, trypsin) into amino acids & small peptides.
• Carbohydrates are digested via α-amylase cleaves α-1,4 bonds in starch & glycogen.
• Carbohydrates are reduced into mono- & disaccharides via enzymes like maltase for maltose, lactase for lactose.
• Lipids are aided by Bile salts that solubilize lipids.
• Lipases hydrolyze triglycerides that turn into fatty acids & monoacylglycerols.
• Lipids reassemble into triacylglycerols and are packaged into chylomicrons for transport.
• Partially-digested food that exits the stomach triggers the release of enzymes into the intestine to complete digestion.
• Secretin is released in response to low pH, stimulates NaHCO3 secretion from the pancreas to neutralize acid.
• Cholecystokinin (CCK) is stimulated by peptides & fats, promotes pancreatic enzyme release (proteases, lipases, amylase) and bile salt secretion from the gallbladder.
• Glucagon-Like Peptide-1 (GLP-1) enhances insulin secretion & promotes satiety. -Insulin regulates glucose uptake after meals.
• Leptin is active during long-term energy homeostasis and signals fat storage levels.
• Zymogens are inactive enzyme precursors that require cleavage for activation.
• Pepsinogen activates to pepsin by acidic pH.
• Trypsinogen activates to trypsin by enteropeptidase into trypsin, activating other proteases.
• Chymotrypsinogen, proelastase, and procarboxypeptidase activate by trypsin.
• Caloric homeostasis refers to the that body maintains energy balance by adjusting intake & expenditure.
• Short-term signals (CCK, GLP-1) regulate immediate satiety.
• Long-term signals (insulin, leptin) regulate stored energy reserves.

Metabolism

• Metabolic pathways are composed of interconnected reactions.
• Pathways must have a net negative ΔG to proceed.
• Unfavorable reactions are coupled to favorable ATP hydrolysis.
• ATP provides the universal energy currency.
• High-energy phosphoanhydride bonds store energy.
• Hydrolysis releases energy for metabolic work.
• Energy is extracted by the oxidation of carbon-containing molecules (e.g., glucose, fatty acids).
• More reduced carbons (e.g., fats) yield more ATP than oxidized ones (e.g., sugars).
• NAD+ accepts electrons in catabolism (oxidative metabolism).
• FAD accepts two electrons & two protons during oxidation.
• NADPH is used for biosynthetic (anabolic) reactions.
• CoA carries two-carbon units (acetyl groups) for metabolism.
• Enzyme amount is controlled by gene expression & protein degradation.
• Allosteric regulation (feedback inhibition by end products),
• Covalent modification examples are phosphorylation.
• Compartmentalization examples are fatty acid oxidation in mitochondria vs. synthesis in cytoplasm.
• Transport regulation examples are glucose uptake via GLUT transporters.
• In glycolysis, glucose is oxidized into two molecules of pyruvate.
• Two molecules of ATP are generated in glycolysis.
• Two NAD+ molecules are reduced to NADH.

Glycolysis Stages

• Stage 1 (Investment Phase) is such that glucose is phosphorylated & cleaved into two three-carbon intermediates.
• ATP activates glucose during stage 1.
• Key enzymes Hexokinase, Phosphofructokinase (PFK), and Aldolase are active during stage 1.
• Stage 2 (Payoff Phase) is such that oxidation of glyceraldehyde 3-phosphate (GAP) generates ATP & NADH.
• Net gain for glycolysis is 2 ATP, 2 NADH, and 2 pyruvate per glucose.
• ATP synthesis occurs via substrate-level phosphorylation in the phosphoglycerate kinase reaction (1,3-BPG → 3-PG) and the pyruvate kinase reaction (PEP → Pyruvate).
• Oxidation occurs because Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) catalyzes oxidation.
• NAD+ is reduced to NADH during oxidation.
• NAD+ must be regenerated for glycolysis to continue to maintain redox balance.
• Without NAD+, glyceraldehyde 3-phosphate cannot be oxidized, stopping ATP production.

Fermentation

• Alcoholic Fermentation (Yeast & Microbes) reaction from pyruvate to acetaldehyde to ethanol.
• NADH is oxidized back to NAD+ in the final step of alcoholic fermentation.
• Lactic Acid Fermentation (Humans & Bacteria) reaction from pyruvate to lactate via lactate dehydrogenase.
• NADH is oxidized to NAD+ in lactic acid fermentation.
• Fermentation produces no net oxidation of carbon (NADH is recycled, and aerobic oxidation fully breaks pyruvate into CO2 and H2O via TCA cycle & ETC, yielding more ATP.
• Pyruvate is converted to Acetyl-CoA via pyruvate dehydrogenase in preparation for aerobic oxidation.
• Acetyl-CoA enters the TCA cycle, it leads to complete oxidation & ATP production.

Glucose Metabolism

• In muscle & adipose tissue, fructose is phosphorylated by hexokinase to fructose-6P.
• In the liver, fructose enters via the fructose-1P pathway, bypassing PFK.
• Galactose converts to glucose-6P via the Leloir pathway.
• Defects in the Leloir pathway cause galactosemia (toxic buildup of galactitol).
• In muscle (Energy Demand-Based Regulation), PFK is inhibited by ATP and activated by AMP (low energy).
• pH inhibits PFK to prevent lactic acid overproduction.
• In the liver (Metabolic Homeostasis-Based Regulation), PFK is inhibited by citrate (signals abundant biosynthetic precursors).
• PFK is activated by fructose-2,6-bisphosphate (overrides ATP inhibition).
• Glucokinase (liver-specific hexokinase) is active only at high glucose levels.
• GLUT2 (low-affinity glucose transporter) allows glucose into β cells only when blood glucose is high. - Glycolysis increases ATP, which closes ATP-sensitive K+ channels, depolarizes the membrane, and voltage-gated Ca²+ channels open, trigger insulin release
• In gluconeogenesis, glucose is synthesized from non-carbohydrate precursors.

Glycolysis

• Hexokinase: Glucose turns into Glucose-6-phosphate (-33 kJ/mol).
• Phosphofructokinase-1 (PFK-1): Fructose-6-phosphate turns into Fructose-1,6-bisphosphate (-22 kJ/mol).
• Pyruvate Kinase: Phosphoenolpyruvate (PEP) turns into Pyruvate (-17 kJ/mol).
• Pyruvate carboxylase is when pyruvate turns in to oxaloacetate (OAA).
• PEP Carboxykinase (PEPCK) is when OAA turns to PEP.
• Fructose-1,6-bisphosphatase is when Fructose-1,6-bisP turns to Fructose-6P.
• Glucose-6-phosphatase is when Glucose-6P turns to Glucose.
• Pyruvate Carboxylase (Mitochondria) turns to Pyruvate to Oxaloacetate in a reaction that requires biotin & ATP.
• OAA is converted to malate (via NADH), that is transported to the cytoplasm, then converted back to OAA via the Malate Shuttle.
• PEP Carboxykinase turns to (Cytoplasm): OAA PEP with GTP is required. Intermediates are then up to glucose-6P in the cytoplasm.
• Glucose-6-phosphatase turns Glucose-6P to Glucose in the Endoplasmic Reticulum (ER).

Metabolic Regulation

• High in ATP and AMP Ratio inhibits glycolysis while activating gluconeogenesis.
• High in AMP, inhibits gluconeogenesis, activates glycolysis.
• Fructose-2,6-bisphosphate activates PFK-1 to stimulate glycolysis.
• Fructose-2,6-bisphosphate inhibits FBPase-1 to block gluconeogenesis.
• Acetyl-CoA activates Pyruvate Carboxylase to promote gluconeogenesis.
• Acetyl-CoA signals there is sufficient energy to inhibit glycolysis.
• Citrate inhibits PFK-1 to slow glycolysis.
• Citrate activates Gluconeogenesis.
• Glycolysis is active with High glucose and low in ATP
• Gluconeogenesis is Active When: Low glucose and high in ATP.
• High in Glucose activates glycolysis, via insulin, which increases PFK-1 and decreases PEPCK.
• Low Glucose activates gluconeogenesis, via glucagon, which activates PEPCK and inhibits PFK-1. Stimulates PFK-1 Increases Fructose-1,6-bisP production, activating glycolysis and inhibits Fructose-1,6-bisphosphatase inhibiting Gluconeogenesis.
• Glucose is converted to Lactate (to regenerate NAD+) via Anaerobic in the muscle.
• Lactate transfers to the liver via the bloodstream.
• Lactate Liver converts to pyruvate to Glucose (via gluconeogenesis).
• Glucose is transported back to muscle for ATP production , and prevents acid buildup to maintain energy supply.