Bioenergetics in the Cytosol Flashcards
high free energy of hydrolysis are sometimes depicted as
a squiggle (~).The chemical group attached to the squiggle is readily transferred to an acceptor molecule. ATP is a good example
bonds that have a standard free energy of hydrolysis at pH of 7 equal to greater than -29KJ/mol
high energy bonds
H2O + ATP ADP Pi
G’= -35 KJ/mol
Glucose + Pi Glucose-6-phosphate + H2O
G’ = +14 KJ/mol
ATP + Glucose Glucose-6-phosphate + ADP
G’ = -21 KJ/mol
ATP has a standard free energy of hydrolysis of
(G’)= -35 KJ/mol
The Phosphate groups are ionized at pH 7, this produces …
electrostatic propulsion between phosphate groups
Reactions during which phosphate is removed will be favored because the electrostatic repulsion is decreased as the result of the
hydrolysis
ATP can phosphorylate many different compounds because
an electronegative group (such as hydroxyl ) can attack the electropositive phosphorus
enzymes that catalyze phosphoryl group transfers-
kinases
PEP
phosphoenolpyruvic acid
PEP is a better donor than
ATP. G’= -62 KJ/mol
The removal of the phosphate allows for
an enol-keto tautomerization, with the keto form being more stable than the enol form.
What is the reason for the high free energy of hydrolysis of Acyl Derivatives of Phosphate and Coenzyme A?
Acyl Derivatives of Phosphate and Coenzyme A resonate poorly.
In a normal ester, electrons shift from oxygen but phosphorus hinders resonance as does the sulfur.
Sulfur does not form double bonds easily and the positive charge in phosphorus does not allow the electrons from oxygen to shift.
Acyl derivatives of Phosphate are used to make
ATP
Acyl derivatives of CoA are used to make ATP as well as
as the transfer of acyl groups
Group transfer reactions are important to the biosynthesis
of proteins, lipids, carbohydrates and nucleic acids. The transfers occur by means of high energy donors of ATP (or other nucleotide derivative), acyl-coenzyme A and acyl-phosphates.
ATP as a donor
ATP can donate phosphate
ATP can donate (pyrophosphate (PPi)
ATP can donate AMP
A series of group transfer reactions are necessary for
peptide bond formation
step A of peptide bond formation
The AMP from ATP is transferred to the carboxyl group of the amino acid. This is driven by the hydrolysis of pyrophosphate
What type of bond is it that connects the amino acid to the AMP?
?
Step B.
The AMP of the acyl-AMP is replaced by tRNA. This is reversible
Step C.
The tRNA is replaced by the amino group of the amino acid docked in the ribosome. This has a large free energy change and is not reversible. The amino acid-tRNA bond is an acyl-phosphate bond
What supplies the energy to form the peptide bond?
breaking of the acyl phophate bond
The amino acid (alanine) is attached to the tRNA by the
3’ end of the “A” nucleotide.
The amino acid (alanine) is attached to the 5’ end of the “A” nucleotide and
the 3’ end links to the tRNA.
ATP is made in the cytosol by the
phosphorylation of ADP by means of high energy phosphoryl donor (these reactions are catalyzed by kinases)
donors for substrate level phosphorylation
1,3 bisphosphoglycerate (1,3 BPGA) and Phosphoenolpyruvate (PEP) are found in glycolysis
Succinyl-CoA is found in the TCA cycle
Acetyl phosphate is derived from acetyl CoA used frequently in fermentation
With the exception of PEP all the high energy donor are produced by oxidation reactions.
PEP is produced by hydrolysis
What provides the energy to add inorganic phosphate to the carbonyl group?
oxidation of phosphoglyceraldehyde. the electron acceptor is NAD
Acetyl phosphate donates phosphate to ADP to produce ATP and is an important source of ATP under
anaerobic conditions
acetyl phosphate is made by
acetate kinase
pyruvic acid + pyruvate ferredoxin oxidoreductase (anaerobic)
acetyl-CoA, CO2, H2
pyruvic acid + pyruvate dehydrogenase (aerobic)
acetyl-CoA, CO2, NADH
pyruvic acid + pyruvate formate lyase (anaerobic)
acetyl CoA, formic acid
Although acetyl phosphate is made from acetyl CoA, the energy was supplied by
the oxidative decarboxylation of pyruvate.
Pyruvate dehydrognease, the electron acceptor is
NAD (NADH is produced).This happens under aerobic conditions
The acetyl CoA that is formed under aerobic conditions does not proceed to
acetyl-phosphate, instead Acetyl-CoA enter the Krebs cycle
Pyruvate dehydrogenase
Aerobic pathway (intermediate step between glycolysis and TCA cycle)
Pyruvate + NAD+ + CoA > Acetyl-CoA + CO2 + NADH + H+
Acetyl-CoA typically enters TCA cycle for complete oxidation
Under anaerobic conditions, what catalyzes acetyl-coA from pyruvate?
pyruvate-ferredoxin oxidoreductase . H2 is produced
Pyruvate-formate hydrogen lyase
Pyruvate-formate hydrogen lyase reaction
Pyruvate + CoA > acetyl-CoA + formic acid (formate)
In this reaction the electrons stay with the carboxyl group (formate
Acetyl-Phosphate from Acetyl-CoA
Acetyl-CoA + Pi > Acetyl-PO4 + CoA
The reaction is (reversibly) catalyzed by phosphotransacetylase (PTA).
This reaction usually follows the acetyl-CoA formation from Pyruvate-ferredoxin oxidoreductase or pyruvate-formate hydrogen lyase
Acetate Kinase (ACK)
Acetyl-Phosphate is used in a substrate level phosphorylation reaction to form ATP.
The route from Acetyl-CoA to ATP is referred to as ACK-PTA because the reations often together
Succinyl-Coenzyme A
Succinyl CoA is made from the oxidative decarboxylation of alpha–keto glutarate.
The enzyme that catalyzes the reaction is alpha–keto glutarate dehydrogenase and involve a similar mechanism as pyruvate dehydrogenase.
This is because pyruvate and –keto glutarate are structurally similar (alpha–keto acids)
Succinate Dehydrogenase
This is an oxidative decarboxylation. This ultimately supplies the energy for the next step which is to make ATP
Succinate Thiokinase (ATP in the TCA cycle)
this is the step that makes ATP
Pyruvate dehydrogenase
PEP + ADP —-> pyruvate + ATP
Enolase (dehydration) reaction
2 pga —-> PEP + H20
