Lec 1

Lecture 1: Introduction to Metabolism, Fuel Selection & PCr

Metabolism

• Metabolism = greek for change

• The sum of chemical changes that convert nutrients into energy and the chemically complex finished products of the cell

  • series of interconnected pathways that involve the production of energy or the synthesis of molecules using energy

• Pathway = consecutive reactions

  • linear (glycolysis)

  • cyclic (citric acid cycle)

  • branched (amino acid metabolism)

• Macronutrients: fat, carbs, protein

• Micronutrients: vitamins, minerals

• Energy: ATP, GTP

• Pathways start with an initial substrate (A) and end with a final product (E)

  • many reactions are required in between before the initial substrate (A) can be converted to the final product (E)

  • Each of these reactions is catalyzed by an enzyme (b,c,d,e)

  • Various intermediates (B,C,D) are produced along this pathway

  • All of these molecules (A,B,C,D,E) are metabolites

• Anabolism: synthesis of carbs, protein, lipids (ex. gluconeogenesis)

• Catabolism: degradation (oxidation) of carbs, protein, lipids (ex. glycolysis)

• Must have distinct enzymes which catalyze key, regulated, irreversible reactions to drive the reaction and pathway forward.

  • Both cannot operate simultaneously  ‘futile cycling’ or ‘substrate cycling’

• Pathways are controlled by allosteric regulation and/or covalent regulation by phosphorylation

• By localizing enzymes responsible for an individual pathway in one location, compartmentalization maintains separate pools of enzymes and metabolites/cofactors for these separate processes

  • Compartmentalization of pathways into organelles facilitates effective functioning of various pathways requires specific transport proteins  offers control/regulation

  • By localizing all enzymes responsible for glycolysis or for citric acid cycle in one location, diffusion distance for metabolites between consecutive enzymes in a pathway is effectively reduced efficiency of assembly line

• Types of experimental analysis

  •  Identify and measure concentration of reactants and products (metabolites), cofactors, hormones

  • Enzyme activity

  • Regulation/control mechanisms

  • Physiological relevance

•  In vivo vs in vitro

  • Complex system in vivo, no complete control over conditions

  • Difficult to extrapolate from controlled experiment in vitro to intact cell, oversimplification or addition of experimental artifacts, not usually under physiological conditions

• Organizational levels of study

  • Whole organism

  • Isolated organism

  • Tissue

  • Whole cells

  • Cell extracts

  • Purified components (isolated mitochondria or enzymes)

• Tools such as metabolic probes and radioisotope labels can be used to study metabolism at any of these levels.

Why do we care about studying metabolism

• For energy

  • mechanical work (eg. exercise)

  • active transport of ions across membranes against a concentration gradient

  • for biosynthesis of molecules (eg. fats), transfer of genetic information

• To understand the etiology of inborn errors of metabolism:

  • PKU

  • Maple syrup urine disease

  • McArdle’s disease

• Diet:

  • for weight loss

  • for prevention of diabetes

  • for treatment of inborn errors of metabolism (eg. low Phe for PKU

• Manufacturing food products

• Studying hibernation/estivation

Fuel Selection

• BMI used to rank obesity (but not the most accurate, but the most common)

  • 20 = average

  • 25 = overweight

  • 30 = obese

• Different kinds of diets

  • Atkins – low carbohydrate

  • South Beach – replaces "bad carbs" and "bad fats" with "good carbs" and "good fats.“

  • Dukan – also known as the princess diet, is a diet plan originating in France. It is a protein-based diet designed by French doctor Pierre Dukan.

  • Paleo – aka “caveman” diet, the "Paleolithic diet" consists mainly of fish, veggies, nuts

• Macronutrients give body energy

• muscle accounts for ~1/2 of total body mass

  • major consumer of glucose in the body.

• Muscle contraction is caused by cycling of the actin-myosin cross-bridges in the cytoplasm

• For muscle contraction to occur, ATP is hydrolyzed by the actomyosin ATPase: ATP D ADP + Pi

• The requirement for ATP (rate of ATP turnover) varies w/ tissue and its metabolic activity

• Our ATP stores are finite:

  • From rest to maximal exercise, the rate of ATP turnover in muscle cells can increase by 100-fold (15 mmol ATP/kg dw/s)

  • Muscle cells only have enough ATP (15-20 mmol ATP/kg dw) to maintain ~1-2s of maximal sprint exercise

  • Brain cells also have only a few seconds supply of ATP

• ATP must continuously be resynthesized and Replenished

• brain is one of the highest energy demanding tissues of the human body.

• inverse relationship between exercise intensity and duration of exercise

• Ranking: PCr > carbs > fat

  • PCr = phosphocreatine

  • As energy intensity increases, more glucose/carbs are used

  • When energy intensity is less, more fats are used

PCr

• Generation of ATP from fat and carbohydrate takes time

• body has a ‘safeguard’ to protect its ATP levels – to maintain ATP homeostasis during transition to exercise

• Phosphagen system

  • PCr is a small ‘reservoir’ of energy that can rapidly replenish ATP

  • ATP is replenished at the site of contraction by substrate phosphorylation of ADP by PCr:
    PCr + ADP → ATP + Cr

  • Creatine Kinase (CK) catalyzes the ~P transfer 

  • PCr can only support an additional ~10s of maximal exercise in muscle

• PCr shuttle

  • no efficient mechanism for ATP to diffuse rapidly enough through the cell

  • “PCr shuttle” shuttles energy from the site where ATP is made (mitochondria) to where it is required in the cell

  • Clusters of CK are scattered throughout the cell to ensure efficient transfer of PCr from the mitochondria

• Creatine supplementation are ergogenic aid

  • ergogenic aids: agents ingested to enhance capacity for sports exercise and physical performance

  • ATP stores can last ~1-2 s during maximal exercise

  • PCr stores can offer energy for another ~10 s

  • Ingestion of exogenous creatine is used in ATP production

• Sources of creatine

  • Endogenous: Amino acid derivative synthesized in body (glycine, arginine, methionine)

  • Exogenous: dietary intake ~1-2 g/day

• Body stores: 

  • Skeletal muscle (~ 95 % of creatine)

  •  ~60% stored as Phosphocreatine (PCr)

  • Remaining 5% distributed in brain, liver, kidney, and testes

• Role of PCr during exercise

  • Important during high-intensity exercise and first few seconds of exercise

  • – Provides 50-70% energy in first 2-10s

  • PCr able to replenish in between exercise bouts

  • Halved over exercise boute

• Creatine loading protocols:

• Cr supplementation:

  • Increases total muscle creatine and PCr 20-50%

  • Increases short-term power output (<30s) by increasing PCr/Cr energy stores

    •  increase intensity of contraction

  • Enhances resynthesis of PCr during recovery (lower fatigue?)

    •  increase volume of training

  • Reduces reliance on glycolysis (less lactate production)

  • May delay breakdown of muscle glycogen

  • Consumes H +, and thus reduces lactic acidosis

• Significance of findings

  • Results are variable and often conflicting due to

    • different experimental exercise protocols

    •  inter-individual variations in response to supplementation

  • Most beneficial for sports involving

    • short bouts of high intensity activity

    • repetitive bouts of high intensity activity (sprinting/cycling, weight lifting)

• Side effects

  • Anecdotal reports of

    • Gastrointestinal upset

    • Muscle cramps

    • Renal dysfunction

    • Hepatic dysfunction

  •  No short-term studies have demonstrated these side effects

*demonstrates inter-induvidual variation