Chapter 15 Flashcards
Glucose is metabolized to pyruvate in 10 linked reactions. Under anaerobic conditions, pyruvate is metabolized to what? Under aerobic conditions?
Anaerobic: lactate
Aerobic: acetyl CoA (which ate oxidized to CO2)
Living organisms require a continual input of free energy for what three major purposes?
1) the performance of mechanical work in muscle contractions and cellular movements 2) the active transport of molecules and ions 3) the synthesis of macromolecules and other biomolecules from simple precursors
Metabolism?
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
Which enzymes are predominant in metabolism?
Allosteric
Metabolic pathways can be divided into what two broad classes?
1) those that convert energy from fuels into biologically useful forms 2) those that require inputs of energy to proceed
Catabolic reactions (catabolism)?
Those reactions that transform fuels into cellular energy
Anabolic reactions (anabolism)?
Those reactions that require energy such as synthesis of glucose, fats, or DNA
Why are the useful forms of energy produced in catabolism employed in anabolism?
To generate complex structures from simple ones or energy-rich states from energy-poor ones
Amphibolic pathways?
Pathways that are either anabolic or catabolic
Formula for catabolism?
Fuel (carbohydrates, fats) -> CO2 + H2O + useful energy
Formula for anabolism?
Useful energy + simple precursors –> complex molecules
What two criteria must a pathway satisfy?
1) the individual reactions must be specific 2) the entire set of reactions that constitute the pathway must be thermodynamically favored
What does the free energy of a reaction depend on?
The nature of the reactants and products and in their concentrations
What is the overall free energy change for a chemically coupled series of reactions equal to?
The sum of the free energy changes of the individual steps
How can a thermodynamically unfavorable reaction be made favorable?
When it is coupled to a thermodynamically favorable reaction.
How are metabolic pathways formed?
By the coupling of enzyme-catalyzed reactions such that the overall free energy of the pathway is negative
How is metabolism facilitated?
ATP
Function of ATP?
The free-energy donor in most energy-requiring processes such as motion, active transport, or biosynthesis.
How is ATP made?
By catabolic reactions that extract energy from fuels such as carbohydrates and fats which are converted to ATP
Components of ATP?
Adenosine, ribose, triphopshate
The active form of ATP is usually a complex of ATP with ___.
Mg 2+ or Mn 2+
Why is ATP an energy-rich molecule?
Because its triphosphate unit contains two phosphoanhydride bonds.
Role of ATP?
Energy carrier
A large amount of free energy is liberated when ATP is hydrolyzed to what two forms?
1) ADP and orthophosphate (Pi) 2) AMP and pyrophosphate (PPi)
What is the universal energy currency in biological systems?
ATP
ATP hydrolysis is exergonic or endergonic?
Exergonic
What is the fundamental mode of energy exchange in biological systems?
The ATP-ADP cycle
What three triphosphates are analogous to ATP?
GTP, UTP, CTP
___ catalyze the transfer of the terminal phosphoryl group from one
nucleotide to another.
Enzymes
What is the phosphorylation of nucleoside mono phosphates catalyzed by?
Nucleoside monophosphate kinases
What is the phosphorylation of nucleoside diphosphates catalyzed by?
Nucleoside diphosphate kinase
What two electron carriers are derivatives of ATP?
NAD+ and FAD
How does ATP hydrolysis drive metabolism?
By shifting the equilibrium of coupled reactions
What are the three purposes of phosphoryl transfer?
1) used to derive otherwise endergonic reactions 2) alter the energy conformation of a protein 3) serve as a signal to alter the activity of a protein
What is the phosphoryl group donor in phosphoryl transfer?
ATP
Why is ATP an activated carrier of phosphoryl groups?
Because phosphoryl group transfer from ATP is an exergonic process
What are the three ways activated carriers function as coenzymes?
1) activated carriers of electrons for fuel oxidation 2) an activated carrier of electrons for reductive biosynthesis 3) an activated carrier of two-carbon fragments
In aerobic organisms, what is the ultimate electron acceptor in the oxidation of fuel molecules?
O2
What is the major electron carrier in the oxidation of fuel molecules?
Nicotinamide adenine dinucleotide (NAD+)
What is the active part of NAD+?
Its Nicotinamide ring, a pyridine derivative synthesized from the vitamin niacin
Which is the oxidized/reduced form: NAD+ or NADH?
Oxidized: NAD+
Reduced: NADH
What is the reactive part of FAD?
The isoalloxazine ring, a derivative of the vitamin riboflavin
Difference/similarities between NAD+ and FAD?
Similar: they can accept two electrons
Differences: FAD can take up 2 protons unlike NAD+
Why is reducing power needed in addition to ATP?
High potential electrons are required in most biosynthesis because the precursors are more oxidized than the products.
What is the electron donor in most reductive biosyntheses?
NADPH
What is the difference between NADPH and NADH?
NADPH is used almost exclusively for reductive biosyntheses. NADH is used primarily for the generation of ATP.
What tag on NADPH enables enzymes to distinguish between high-potential electrons to be used in anabolism and those to be used in catabolism?
The extra phosphoryl group
What is the reactive site of CoA?
The terminal sulfhydryl group
What bonds are acyl groups linked to CoA by?
Thioester bonds
Why does acetyl CoA have a high acetyl-group transfer potential?
Because transfer of the acetyl group is exergonic and acetyl CoA carries an activated acetyl group just as ATP carries an activated phosphoryl group
Why is the kinetic stability of these molecules in the absence of specific catalysts essential for their biological function?
Because it enables enzymes to control the flow of free energy and reducing power
What are almost all the activated carriers that act as coenzymes derived from?
vitamins
Vitamins?
organic molecules that are needed in small amounts in the diets of some higher animals
What is the coenzyme of vitamin B1 (thiamine)?
thiamine pyrophosphate
What is the typical reaction type of Vitamin B1?
aldehyde transfer
What is the coenzyme of B2 (riboflavin)?
flavin adenine dinucleotide (FAD)
What is the typical reaction type of B2?
oxidation-reduction
What is the coenzyme of B6 (pyridoxine)?
pyridoxal phosphate
What is the typical reaction type of B6?
group transfer to or from amino acids
What is the coenzyme of niacin?
NAD+
What is the typical reaction type of niacin?
oxidation-reduction
What is the coenzyme of pantothenic acid?
coenzyme A
What is the typical reaction type of pantothentic acid?
carboxylation and carboxyl-group transfer
What is the coenzyme of biotin?
biotin-lysine adducts (biocytin)
What it the typical reaction type of biotin?
ATP-dependent carboxylation and carboxyl-group transfer
What is the coenzyme of folic acid?
tetrahydrofolate
What is the typical reaction type of folic acid?
transfer of one-carbon components; thymine synthesis
What is the coenzyme of B12?
5’-deoxyadenosyl cobalamin
What is the typical reaction type of B12?
transfer of methyl groups; intramolecular rearrangements
What are the noncoenzyme vitamins?
A,C,D,E,K
What are the 6 reaction types of metabolism?
1) oxidation-reduction 2) ligation 3) isomerization 4) group-transfer 5) hydrolytic 6) functional groups may be added to double bonds to form single bonds or removed from single bonds to form double bonds
What two oxidation-reduction reactions are components of the citric acid cycle?
Succinate + FAD –> Fumarate + FADH2; Malate + NAD+ –> Oxaloacetate + NADH + H+
Ligation bonds?
form by using free energy from ATP cleavage
What is oxaloacetate formed from?
pyruvate and CO2 by ligation reactions (pyruvate + CO2 + ATP + H2O –> oxaloacetate + ADP + Pi + H+)
Isomerization reactions?
rearrange atoms within a molecule
Example RxN of isomerization?
citrate –> isocitrate
Group transfer reactions?
used to synthesize ATP by transferring a phosphoryl group from the activated phosphoryl-group carrier, ATP, to glucose (Glucose + ATP –> Glucose-6-Phosphate + ADP)
Hydrolyic reactions?
cleave bonds by the addition of water; a means to break down large molecules in order to facilitate further metabolism or reuse some of the components for biosynthetic purposes
Functional group reactions?
dehydration reactions (Fructose-1,6-Bisphospate –> DHAP + GAP
How is metabolism regulated?
1) amounts of enzymes 2) catalytic activities 3) accessibility of substrates
How do you control the amount of enzymes?
adjusted by the change in the rate of transcription of the genes encoding them
How do you control the catalytic activity?
reversible allosteric control (feedback ihibition); reversible covalent modification; hormones; energy charge
What is reversible allosteric control?
instantaneous (inhibition of aspartate transcarbamoylase by citidine triphosphate)
Feedback inhibition?
two pathways with a common initial step may each be inhibited by its own product and activated by the product of the other pathway
Reversible covalent modification?
glycogen phosphorylase, the enzyme catalyzing the breakdown of glycogen, a storage form of sugar, is activated by the phosphorylation of a particular serine residue when glucose is scarce
How do hormones coordinate metabolic relations?
by regulating the reversible modification of key enzymes (ex. epinephrine triggers a signal-transduction cascade in muscle resulting in the phosphorylation and activation of key enzymes and leading to the rapid degradation of glycogen to glucose, which is then used to supply ATP for muscle-contractions
What are ATP-generating (catabolic) pathways inhibited by?
a high energy charge
What are ATP-utilizing (anabolic) pathways stimulated by?
a high energy charge
Energy charge = ?
([ATP] + 1/2[ADP]) / ([ATP] +[ADP]+ [AMP])
Phosphorylation Potential?
[ATP]/ ([ADP] + [Pi])
How do you control the accessibility of substrates?
compartmentalization (segregates opposed reactions)
Oxidation-reduction?
electron transfer
Ligation requiring ATP cleavage?
formation of covalent bonds
Isomerization?
rearrangement of atoms to form isomers
Group transfer?
transfer of a functional group from one molecule to another
Hydrolytic?
cleavage of bonds by the addition of water
Addition or removal of functional groups?
addition of functional groups do double bonds or their removal to form double bonds
