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