the preparatory phase yields 2 molecules of glyceraldehyde 3 phosphate
TRANSCRIPT
The preparatory phase yields 2 molecules of glyceraldehyde 3 phosphate
Step 6 of glycolysis involves oxidation-reduction
• The aldehyde group of G3P is dehydrogenated, not to a free carboxyl group but to a carboxylic acid anhydride with phosphoric acid
Glyceraldehyde 3 Phosphate dehydrogenase reaction mechanism
• This reaction is a source of NADH and protons for the cell
Iodoacetate is a potent (suicide) inhibitor of G3P dehydrogenase
Reaction 7, getting some ATP from glycolysis
Another example of energy coupling in metabolic pathways
• Note the standard free energy change in reaction 6 was positive.
G = G’o + RT ln [products]/[reactants]
By reducing the [1,3 bisphosphoglycerate] through reaction 7, reaction 6 becomes favorable
Substrate-level phosphorylation
• Formation of ATP by phosphoryl group transfer from a substrate is referred to as substrate-level phosphorylation
• This term distinguishes this process from respiration-linked phosphorylation (ATP synthase-mediated)
Step 8 is about phosphate group movement
• Mutases are a subclass of isomerases that transfer a group from one position to another on the same molecule
Phosphoglycerate mutase works through a phosphorylated intermediate
Step 9: Formation of PEP
• Condensation catalyzed by enolase
Step 10: Getting more ATP from glycolysis
Pyruvate switches from enol to keto form
Glycolysis accounting
• Glucose + 2NAD+ + 2ADP + 2 Pi 2 pyruvate + 2 NADH + 2 ATP + 2 H2O
• Chemical transformations that occur during glycolysis include 1) degradation of glucose to pyruvate; 2) phosphorylation of ADP to ATP and 3) transfer of hydride ion with its electrons to NAD to form NADH
Fate of pyruvate
• In animal cells, pyruvate can go to mitochondria and be metabolized by the TCA, citric acid, or Kreb’s cycle (same cycle)
• However, when oxygen is limiting, cells ferment pyruvate to lactic acid or ethanol– Fermentation allows the oxidation of NADH to NAD+
(Protons are conserved among metabolites during fermentation)
– Pyruvate acts or supplies a terminal electron acceptor for fermentative processes
– In addition to ethanol and lactate, some microbes make useful solvents or products through fermentation.
Lactic acid production
• The resulting
NAD+ can then
be used for
glycolysis
Also used in
yogurt production
Other cells (i.e. yeast) ferment pyruvate to ethanol
• Note, in all fermentations
The C:H ratio of reactants
And products remain the same.
Glucose H:C = 12/6 = 2
2 ethanol and 2 CO2
H:C = 12/6 = 2
Pyruvate decarboxylase has a vitamin-like cofactor, TPP
• Thiamine pyrophosphate (TPP) is a derivative of vitamin B1
• TPP plays an important role in the cleavage of bonds adjacent to a carbonyl gorup
Cells tightly regulated levels of ATP
• This regulation is achieved by the regulation of key enzymes in catabolism.
• For glycolysis, these include– Glycogen phosphorylase– Hexokinase– Phosphofructokinase– Pyruvate kinase
Regulating glycogen?• Glycogen, and other saccharides feed
glycolysis
Other monosaccharides enter glycolysis through phosphorylation
• Fructose can be phosphorylated either at C1 or C6 depending on tissue (Fructose-1-phosphate is cleaved by an aldolase enzyme to produce DHAP and glyceraldehyde,both of which are converted to glyceraldehyde-3-phosphate
• Mannose is phosphorylated at C6 carbon by hexokinase, then phosphomannose isomerase makes fructose-6-phosphate from mannose-6-phosphate
Galactose entry into glycolysis is seemingly more complex
• Galactose is first phosphorylated at the C1 position by galactokinase
• Galactose-1-phosphate is then converted to it’s epimer glucose-1-phosphate through a set of reactions utilizing uridine diphosphate, which acts as a coenzyme
Back to: Glycogen and Starch are degraded by phosphorolysis
• Glycogen phosphorylase attacks the non-reducing end of glycogen, breaking the 1-4 glycosidic bond using inorganic phosphate to generate glucose-1-phosphate.
• You have seen before that glycogen phosphorylase is regulated by post-translational modification – phosphorylation, but more complex
Glycogen breakdown
Connecting glycogen to glycolysis
• In addition to glycogen phosphorylase, a debranching enzyme is necessary for glycogen breakdown (cleaves 1-6 bonds)
• Glucose-1-phosphate enters glycolysis through a phosphoglucomutase-mediated reaction which isomerizes this molecule to Glucose-6-phosphate.
More regulation of glycogen phosphorylase
• In addition to covalent modification, there are two allosteric control mechanisms that regulate this enzyme. – Calcium binds and activates transformation of the
inactive form of this enzyme (phosphorylase b) to active (phosphorylase a)
– AMP (adenosine mononucleotide) activates phosphorylase (High ATP levels outcompete AMP for this binding site; competitive inhibition)
Hormonal regulation
• The hormone glucagon also regulates glycogen phosphorylase activity
• When blood glucose level is too low, glucagon activates phosphorylase b kinase which converts inactive phosphorylase b to its active a form
• When blood glucose levels get high enough, glucose binds to phosphorylase a, leading to dephosphorylation via a phosphatase
Next time
• More regulation
• Metabolic flux
• And the pentose phosphate pathway