Glycolysis is a key metabolic pathway for organisms. In it, glucose is converted into two pyruvate molecules. The process includes ten enzymes, described in further detail on the linked pages.
Step 1: Hexokinase:
The first phase of glycolysis is sometimes referred to as the "investment phase", where ATP is consumed to set up later, energy generating steps.
The first step of the pathway is the conversion of to by either hexokinase or glucokinase. Hexokinases should not be confused with glucokinase, which is a specific isoform of hexokinase. All hexokinases are capable of phosphorylating several hexoses but glucokinase acts with a 50-fold lower substrate affinity and its main hexose substrate is glucose.[1]
Step 2: Phosphoglucose isomerase
isomerizes to ; this reaction is catalyzed by phosphoglucoisomerase. This isomerization allows for the creation of two, three carbon sugars as a product.
Rabbit [1]. Water molecules are shown as red spheres.
Step 3: Phosphofructokinase
Phosphofructokinase catalyzes the second phosphorylation reaction, and is the most highly regulated step of the pathway. Phosphofructokinase-1 (PFK-1) is a glycolytic enzyme that catalyzes the transfer of a phosphoryl group from to to yield and . Mg2+ is also important in this reaction (). Phosphofructokinase-2 (PFK-2) acts on the same substrates to yield ADP and . . PFK reaction is strongly exergonic (irreversible) under physiological conditions and hence is one of the glycolytic pathway's rate-determining steps. In most organisms/tissues, PFK is the glycolytic pathway's major flux-regulating enzyme; its activity is controlled by the concentrations of an unusually large number of metabolites including ATP, ADP, AMP, PEP and fructose-2,6-bisphosphate.
Step 4: Aldolase
Aldolase catalyzes the retro-aldol cleavage of into two three-carbon phosphosugars, and .
The reaction is an aldol cleavage, or otherwise termed, retro aldo condensation. Catalysis occurs first when the nucleophilic ε-amine group of Lys229 attacks the carbonyl carbon of the substrate (FBP) in its open-ring state, pushing an electron pair to the oxygen of the carbonyl. The oxygen is protonated and leaves as water as a protonated is produced (an imine resulting from a ketone and amine) with the open-ring form of FBP
Step 5: Triose Phosphate Isomerase
Triose Phosphate Isomerase (TPI or TIM) is a ubiquitous dimeric enzyme with a molecular weight of ~54 kD (27 kD per subunit) which catalyzes the reversible interconversion of the triose phosphate isomers and , an essential process in the glycolytic pathway. More simply, the enzyme catalyzes the , also referred to as PGAL.
The next five reactions are the "payoff" phase of glycolysis, where energy in the forms of ATP and NADH is generated. All of the subsequent reactions happen twice (once for each of the two glyceraldehyde 3 phosphate molecules generated from glucose).
Step 6: Glyceraldehyde-3-phosphate Dehydrogenase
First, glyceraldehyde-3-phosphate dehydrogenase oxidizes glyceraldehyde-3-phosphate, transferring a hydride to NAD+, generating NADH and H+. A phosphate ion is used instead of a water molecule, leading to the formation of 1,3-bisphosphoglycerate, a high energy compound.
Step 7: Phosphoglycerate kinase
Phosphoglycerate Kinase catalyzes the transfer of a phosphate from the 1 position of 1,3-bisphosphoglycerate to ADP. This is the "break even" point of glycolysis: the two ATPs that were consumed in preparing for the cleavage have been now been regenerated, in addition to two molecules of NADH, which can be used to generate ATP through electron transport and oxidative phosphorylation. Phosphoglycerate kinase is the seventh enzyme in the cycle which catalyzes the reaction of 1,3-Biphosphoglycerate and ADP to produce and . This method for ATP production is known as substrate-level phosphorylation because it produces energy storing ATP molecules without the use of oxygen, NADH, or an ATPase. The reaction is highly exergonic allowing it to be coupled with the less thermodynamically favored GADPH reaction of the cycle so both reactions occur spontaneously.
Glycerate 3-phosphate (formed from 3-phosphoglycerate) is also a precursor for serine, which, in turn, can create cysteine and glycine through the homocysteine cycle.
Step 8: Phosphoglycerate mutase
The resultant 3-phosphoglycerate isomerizes to 2-phosphoglycerate in a reaction catalyzed by phosphoglycerate mutase.
Step 9: Phosphopyruvate hydratase (enolase)
A second high energy intermediate, phosphoenolpyruvate, is formed by Enolase.
Step 10: Pyruvate kinase
The final reaction of the pathway is catalyzed by pyruvate kinase, which converts phosphoenol pyruvate to pyruvate, while generating ATP from ADP.
The fates of pyruvate
Under oxidative conditions, pyruvate continues to be metabolized through the tricarboxylic acid cycle. While energy can be obtained under anaerobic conditions from glycolysis alone, the accumulation of pyruvate and NADH limits this. There are two main strategies for dealing with this problem. In most cells, lactate dehydrogenase converts the pyruvate and NADH to lactate, dealing with both problems at once and regenerating NAD+ so glycolysis can continue. Fortunately for us, some yeast cells do something else--leading to the generation of ethanol. First, pyruvate decarboxylase catalyzes the converstion from pyruvate to acetaldehyde, releasing carbon dioxide. Next, aldehyde dehydogenase reduces the acetaldehyde to ethanol, converting NADH to NAD+ in the process.
Additional Resources
For additional information, see:
