ECDA October 2009

METABOLISM Metabolism is essentially a linked series of chemical reactions that begins with a particular molecule and converts it into some other molecule or molecules in a carefully defined fashion

METABOLISM  There are many such defined pathways in the cell  The pathways are interdependent, and their activity is coordinated by exquisitely sensitive means of communication in which allosteric enzymes are predominant

METABOLISM  Metabolic pathways can be divided into two broad classes: (1) those that convert energy into biologically useful forms (CATABOLISM) (2) those that require inputs of energy to proceed (ANABOLISM)

METABOLISM  Those reactions that transform fuels into cellular energy are called catabolic reactions or, more generally, catabolism. Fuels (CHO, Fats) CO2 + H2O + useful energy

METABOLISM  Those reactions that require energy—such as the synthesis of glucose, fats, or DNA— are called anabolic reactions or anabolism. Energy + small molecules complex molecules  The useful forms of energy that are produced in catabolism are employed in anabolism to generate complex structures from simple ones, or energy-rich states from energy-poor ones.

METABOLISM  A metabolic pathway is constructed from individual reactions and it satisfies two criteria at the least: 1. The individual reactions must be specific. - that is, it will yield only one particular product or set of products from its reactants 2. The entire set of reactions that constitute the pathway must be thermodynamically favored. - A reaction can occur spontaneously only if ΔG, the change in free energy, is negative.

Gibb’s Free Energy  In thermodynamics, the Gibbs free energy measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system.  ΔG < O means favored reaction (Spontaneous)  ΔG = O means neither the forward nor the reverse reaction prevails (equilibrium)  ΔG > O means disfavored reaction (Nonspontaneous)

Gibb’s Free Energy  An important thermodynamic fact is that the overall free-energy change for a chemically coupled series of reactions is equal to the sum of the free energy changes of the individual steps. Consider the following reactions: A B + CΔG = +5 kcal/mol B DΔG = - 8 kcal/mol A C + DΔG = - 3 kcal/mol

Gibb’s Free Energy  Under standard conditions, A cannot be spontaneously converted into B and C, because ΔG is positive.  However, the conversion of B into D under standard conditions is thermodynamically feasible. (ΔG is negative)  Because free- energy changes are additive, the conversion of A into C and D has a ΔG° ′ of -3 kcal/ mol, which means that it can occur spontaneously under standard conditions.  Thus, a thermodynamically unfavorable reaction can be driven by a thermodynamically favorable reaction to which it is coupled.

METABOLISM  Just as commerce is facilitated by the use of a common currency, the commerce of the cell—metabolism—is facilitated by the use of a common energy currency, adenosine triphosphate (ATP).  ATP, a highly accessible molecule, acts as the free-energy donor in most energy-requiring processes such as motion, active transport, or biosynthesis.

ATP  ATP is a nucleotide consisting of an adenine, a ribose, and a triphosphate unit  The active form of ATP is usually a complex of ATP with Mg 2+ or Mn 2+  In considering the role of ATP as an energy carrier, focus on its triphosphate moiety.

ATP  ATP is an energy-rich molecule because its triphosphate unit contains two phosphoanhydride bonds, which are referred to as high- energy bonds.

ATP  A large amount of free energy is liberated when ATP is hydrolyzed to adenosine diphosphate (ADP) and orthophosphate (P i ) or when ATP is hydrolyzed to adenosine monophosphate (AMP) and pyrophosphate (PP i ).  ATP + H2O AMP + PPi  ΔG = -10 kcal/mol  ATP + H2O ADP + Pi  ΔG = -7.3 kcal/mol

ATP  The free energy liberated in the hydrolysis of ATP is harnessed to drive reactions that require an input of free energy, such as muscle contraction  In turn, ATP is formed from ADP and P i when fuel molecules are oxidized in chemotrophs or when light is trapped by phototrophs.  This ATP—ADP cycle is the fundamental mode of energy exchange in biological systems.

ATP  Some biosynthetic reactions are driven by hydrolysis of nucleoside triphosphates that are analogous to ATP—namely, guanosine triphosphate (GTP), uridine triphosphate (UTP), and cytidine triphosphate (CTP).  Although all of the nucleotide triphosphates are energetically equivalent, ATP is nonetheless the primary cellular energy carrier.

ATP  ATP hydrolysis makes possible an otherwise unfavorable reaction  Consider a chemical reaction that is thermodynamically unfavorable without an input of free energy, a situation common to many biosynthetic reactions: A B ΔG = kcal/mol ATP ADP + PPi ΔG = kcal/mol A + ATP B + ADP + Ppi ΔG = -3.3

ATP  ATP acts as an energy-coupling agent.  Thus, a thermodynamically unfavorable reaction sequence can be converted into a favorable one by coupling it to the hydrolysis of a sufficient number of ATP molecules in a new reaction.  The active transport of Na + and K + across membranes is driven by the phosphorylation of the sodium-potassium pump by ATP and its subsequent dephosphorylation

METABOLISM  Phosphoryl transfer is a common means of energy coupling. Furthermore, it is also widely used in the intracellular transmission of information.  ATP and many prosphoryl containing molecule can be a phosphoryl donor, and thus, good energy producers upon hydrolysis

METABOLISM  Consider the hydrolysis of glycerol-3-phosphate: Glycerol-3-PO4 + H2O glycerol + Pi ΔG = -2.2  Compare with hydrolysis of ATP: ATP + H2OADP + PPiΔG = -7.3

METABOLISM  The magnitude of ΔG° ′ for the hydrolysis of glycerol 3-phosphate is much smaller than that of ATP, which means that ATP has a stronger tendency to transfer its terminal phosphoryl group to water than does glycerol 3-phosphate.  In other words, ATP has a higher phosphoryl transfer potential (phosphoryl-group transfer potential) than does glycerol 3-phosphate.

METABOLISM  The standard free energies of hydrolysis, ΔG, provide a convenient means of comparing the phosphoryl transfer potential of phosphorylated compounds.  ATP is not the only compound with a high phosphoryl transfer potential. In fact, some compounds in biological systems have a higher phosphoryl transfer potential than that of ATP.

METABOLISM  Some compounds in biological systems have a higher phosphoryl transfer potential than that of ATP:  phosphoenolpyruvate (PEP)  1,3-bisphosphoglycerate (1,3- BPG)  creatine phosphate

METABOLISM  With the information given, it can be deduced that:  PEP can transfer its phosphoryl group to ADP to form ATP  ATP has a phosphoryl transfer potential that is intermediate among the biologically important phosphorylated molecules  This intermediate position enables ATP to function efficiently as a carrier of phosphoryl groups

METABOLISM  Creatine phosphate in vertebrate muscle serves as a reservoir of high-potential phosphoryl groups that can be readily transferred to ATP.  Creatine phosphate is used to regenerate ATP from ADP every time we exercise strenuously. This reaction is catalyzed by creatine kinase. Creatinine-PO4 + ADP + H ATP + creatine

METABOLISM REMEMBER!  Creatine phosphate and ATP are abundant in muscle cells, Crea-Po4 > ATP in amount  Crea-PO4 is the major source of phosphoryl groups for ATP regeneration for a runner during the first 4 seconds of a 100-meter sprint.  After that, ATP must be generated through metabolism of CHO and fats

SUBSTRATE-LEVEL PHOSPHORYLATION  The phosphorylation of ADP to form ATP, a good energy storage molecule, is a type of substrate-level phosphorylation  It is the direct transfer of phosphate group to ADP from a higher energy containing molecule (e.g. Crea-PO4, PEP)  This type of phosphorylation is present in Glycolysis and Kreb’s cycle.

SUBSTRATE-LEVEL PHOSPHORYLATION  Substrate-level phosphorylation is also seen in working skeletal muscles and the brain.  Phosphocreatine is stored as a readily available high-energy phosphate supply Phosphocreatine  The enzyme creatine phosphokinase transfers a phosphate from phosphocreatine to ADP to produce ATP.creatine phosphokinase  Then the ATP releases giving chemical energy.

 SUBSTRATE-LEVEL PHOSPHORYLATION