Biochemistry-II - Integrated Metabolism - Quick Guides | Biochemistry

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Study GuideBiochemistry-II–Integrated Metabolism1.Exercise and MetabolismExercise and fasting place special demands on the body’s metabolism. The type of activity—short andintense or long and sustained—determineswhich energy systems are usedandhow nucleotideslike ATP are regenerated.1.1Immediate Energy for Exercise: Creatine PhosphateRole of Creatine PhosphateSprinting requires avery rapid supply of energy. The first energy source used by muscles iscreatine phosphate (Cr~P).•Creatine contains aguanidino group, similar to arginine•It acts as astorage system for high-energy phosphate bondsin muscle•The free energy of hydrolysis of creatine phosphate is about10.3 kcal/mol, which ishigherthan ATPThis makes creatine phosphate an excellenttemporary energy reservoir.Creatine Kinase ReactionCreatine phosphate transfers its phosphate group to ADP to regenerate ATP.Although this reaction favors the left side under standard conditions,real muscle cells are not instandard conditions.•ATP : ADP ratio in active muscle can be50–100 : 1•This drives the formation and use of creatine phosphate•Creatine phosphate functions like acharged battery or water reservoir, storing energy forrapid release

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Study GuideAnaerobic Exercise: SprintingDuring short, intense exercise (such as sprinting), the following sequence occurs:1.ATP is rapidly consumedduring muscle contraction2.ADP levels increase3.Creatine phosphate transfers phosphate to ADP, regenerating ATP4.Creatine phosphate storesseveral times more energy than ATP5.This system supplies energy for onlya few seconds6.Anaerobic glycolysisthen becomes the primary ATP sourceGlycolysis and Lactate Formation•Glucose comes frommuscle glycogen•It is converted tolactate, which enters the bloodstream•ATP levels still fall as demand continuesAt this point, the enzymemyokinase (adenylate kinase)helps salvage ATP:This reaction allows one ATP molecule to be formed from two ADP molecules.Fatigue During SprintingEventually:•ATP levels fall too low to support myosin binding•Protons (acid) from metabolism accumulate•Oxygen is released more easily from hemoglobin (Bohr effect)•Metabolism begins shifting towardaerobic pathwaysLactate and acidity contribute tomuscle fatigue, limiting speed and power.

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Study GuideIn most people, this occurs after about400 meters, which explains why this distance feels especiallyexhausting.1.2Aerobic Metabolism: Prolonged ExerciseLimits of GlycogenHumans store only a limited amount of glycogen:•Total glucose + glycogen ≈600–700 kcal•This supply lasts1.5–2 hoursduring aerobic exerciseTo continue exercising, the body must rely on:•Fat oxidation•Sometimesmuscle protein breakdownStrategies Used by Endurance AthletesTo improve performance during long-distance exercise, athletes use several strategies:•Fat mobilizationearly in exercise•Caffeine, which:oPrevents cAMP breakdownoActivates glycogen phosphorylaseoMimics epinephrine and glucagonoPromotes lipid mobilization•Carnitine supplementation, which helps transport fatty acids into mitochondria•Carbohydrate loading, a two-step process:1.Low-carbohydrate diet + heavy exercise to deplete glycogen2.High-carbohydrate intake to overfill glycogen stores beyond normal levels

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Study GuideNutritional State and MetabolismHumans normally alternate between two metabolic states:•Well-fed state•Fasting stateThe key difference between these states is thesource of glucose, especially for the brain.1.3The Well-Fed State (0–4 Hours After a Meal)During this state:•Glucose and amino acids are absorbed from the intestine•Excess energy is converted tofat in the liver•Fat and triglycerides are stored inadipose tissue•The pancreas releasesinsulinInsulin promotes:•Glucose → glycogen•Amino acids → protein•Fatty acids → triglyceridesA lack of insulin leads todiabetes mellitus, where glucose remains elevated in the blood.The Fasting StateAs fasting begins:•Glycogen breakdownsupplies glucose•Gluconeogenesisstarts using amino acids, lactate, and pyruvate•Fatty acids fuel peripheral tissues•Glycerol is converted to glucose in the liver

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Study GuideAfter about16 hours:•Gluconeogenesis becomes more important than glycogen breakdownStarvation and Ketone BodiesAfter2 days without food:•Fat breakdown producesketone bodies•Ketones become a major fuel for the brain•Glucose use by the brain decreasesThis marks the beginning ofstarvation, which may last6–10 weeks.Metabolic Consequences of StarvationDuring starvation:•The body entersnegative nitrogen balance•Protein breakdown exceeds protein intake•Limited glucose is reserved for:oBrainoKidneysoRed blood cells•Fat and ketone bodies supply most energyOnce fat stores are exhausted:•Muscle protein becomes the only energy source•Nitrogen is excreted•Organ failure eventually occurs

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Study GuideKey Takeaways•Creatine phosphateprovides immediate energy for sprinting•Anaerobic glycolysissustains short-term high-intensity activity•Aerobic metabolism and fat oxidationsupport long-term exercise•Nutritional state strongly affects metabolic pathways•Starvation leads to ketone body use and protein breakdown2.Hormonal RegulationWhat Are Hormones?Hormones arechemical messengersthat regulate metabolism and coordinate activities betweendifferent tissues.A hormone can be defined as:•Asignaling molecule•Released by one cell into thebloodstream or extracellular fluid•Thatchanges the metabolismof the same cell or a different cellHormones differ from:•Gap junction signaling, which requires direct cell-to-cell contact•Neurotransmitters, although this distinction is not strict(Some neurotransmitters act like hormones, and some hormones act like neurotransmitters.)2.1How Hormones Work: ReceptorsHormones act by binding toreceptors, which are usuallyproteins.Each receptor hastwo major functions:1.Bindingthe hormone2.Transducing the signal, meaning converting the hormone signal into a metabolic or geneticresponse inside the cell

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Study GuideWhat Determines a Cell’s Response to a Hormone?A cell’s response depends on two properties of its receptors:1. Receptor Number•How many receptors are present on the cell surface or inside the cell2. Receptor Affinity•How tightly the receptor binds the hormone•Measured by thedissociation constant (K₍d₎)Theactual responsedepends on thenumber of occupied receptors, not simply how muchhormone is present.2.2Hormone–Receptor Binding EquilibriumThe binding of a hormone (H) to a receptor (R) forms a hormone–receptor complex (RH).•Alow Kdmeanshigh affinity•Ahigh Kdmeanslow affinityRearranging the EquationWe can rearrange the equation to calculate hormone concentration.This equation shows how hormone concentration, receptor number, and affinity work together.

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Study GuideReceptor Number vs Receptor Affinity: An ExampleSuppose:•50 occupied receptorsare required to trigger a metabolic responseThis can happen in two ways:•100 receptors with50% occupancy•55 receptors with90% occupancyBoth situations give the same response becausethe number of occupied receptors is the same.If the second receptor hastenfold higher affinity, the same hormone concentration will bind morereceptors.Two Receptor Types Responding to the Same HormoneIf two receptor types (R₁and R₂) bind the same hormone:If both receptor types have50 occupied receptors:•R₁requires50 unoccupied receptors•R₂requires only5 unoccupied receptorsThis meansR₂has higher affinity.Responsiveness to Hormone ChangesIf hormone concentration increases by 50%:•R₁responds strongly•R₂is nearly saturatedand shows little additional response

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Study GuideKey concept:A receptor system ismost sensitiveto changes in hormone concentration when:•The receptor isabout half occupied•The receptor’s Kd is close to thephysiological hormone concentrationAgonists and AntagonistsNot all molecules that bind receptors are hormones.Agonists•Bind to receptors•Mimic the hormone•Activate the receptorAntagonists•Bind to receptors•Do not activatethem•Block hormone actionExamples:•Isoproterenol: agonist that increases blood pressure•Propranolol: antagonist used to lower blood pressureBoth are structurally related toepinephrine.Down-Regulation of ReceptorsWhen receptors areconstantly occupied, cells protect themselves byreducing receptor numbers.This process is calleddown-regulation.Diabetes as an ExampleType I Diabetes•Pancreas cannot produce insulin•Caused by insulin deficiency

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Study GuideType II Diabetes•More common•Associated with obesity•High insulin levels persist in blood•Cells down-regulate insulin receptorsAs a result:•Fewer occupied receptors•Poor glucose regulation•High blood glucose levelsThis leads to complications such as:•Vision problems•Poor circulation2.3Types of Hormone ReceptorsHormone receptors fall intotwo main categories.1. Cell-Surface Receptors (Water-Soluble Hormones)Examples:•Epinephrine•Growth hormone•InsulinMechanism:•Hormone binds to amembrane receptor•Asecond messengeris produced•This triggers metabolic changes inside the cell

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