Question

Regulation of the Calvin Cycle Iodoacetate reacts irreversibly with the free - SH groups of Cys residues in proteins. Predict which Calvin cycle enzyme(s) would be inhibited by iodoacetate, and explain why.

Short answer

Iodoacetate inhibits glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in the Calvin cycle.

Step-by-step solution

Understanding the Question

The question asks us to predict which enzyme(s) in the Calvin cycle would be inhibited by iodoacetate. Iodoacetate can react with free -SH groups in proteins, which are typically found in cysteine (Cys) residues. Therefore, our task is to identify which enzymes in the Calvin cycle have active sites with Cys residues that are crucial for their function.

Identifying Enzymes with Cysteine

During the Calvin cycle, particular attention should be given to the enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). This enzyme uses a cysteine residue in its active site to form a thiohemiacetal intermediate, crucial for its catalytic activity. Any modification to this site will inhibit its function.

Predicting Enzyme Inhibition

Based on GAPDH's requirement for Cys residues in its activity, iodoacetate can irreversibly react with these -SH groups, blocking the active site and thus inhibiting the enzyme. This suggests that GAPDH is the enzyme in the Calvin cycle likely to be inhibited by iodoacetate.

Key concepts

Glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase, abbreviated as GAPDH, plays a significant role in the Calvin Cycle. It acts as a catalyst in the process of converting glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate. This step is crucial for the conversion of carbon dioxide and water into carbohydrates during photosynthesis.

GAPDH is notable for its dependency on specific amino acids in its active site. One such essential amino acid is cysteine, which facilitates the formation of a thiohemiacetal intermediate. This intermediate is necessary for the enzyme's catalytic action. Hence, anything affecting the cysteine residue could impede the enzyme's functionality in the Calvin Cycle.

Enzyme inhibition

Enzyme inhibition occurs when the activity of an enzyme is reduced or stopped. This happens when a molecule binds to the enzyme and blocks its function. Inhibition can either be reversible or irreversible, affecting enzymes differently.

In the context of the Calvin Cycle, enzyme inhibition is specifically discussed with regards to irreversible inhibitors like iodoacetate. Iodoacetate's mode of action involves forming a covalent bond with the cysteine residue in enzymes. By binding to the sulfur atom in the -SH group of cysteine, iodoacetate permanently inactivates the enzyme. This type of inhibition prevents the enzyme from performing its normal catalytic role, thereby disrupting the Calvin Cycle.

Cysteine residues

Cysteine residues are amino acids present in many proteins, including enzymes. They contain a functional group known as a thiol or sulfhydryl group (-SH). This group is highly reactive, enabling it to participate in various biochemical reactions.

In enzymes, cysteine residues play a crucial part in forming active sites. These active sites are where substrates bind and reactions occur. The thiol group in cysteine can form covalent bonds, making it essential for enzyme activity. If the thiol group of a cysteine residue is altered or blocked, the enzyme may lose its ability to function.

Because of its importance, the reactive nature of the thiol group in cysteine residues often makes them targets for inhibitors like iodoacetate that aim to inactivate enzymes by modifying these groups.

Iodoacetate reaction

The iodoacetate reaction specifically targets and modifies cysteine residues within proteins. Iodoacetate, a chemical compound, reacts with the free thiol groups in cysteine. This reaction results in the formation of a thioether bond, attaching iodoacetate covalently to the sulfur atom of cysteine.

Once attached, this modification is irreversible. Such alteration of the cysteine residue by iodoacetate renders the enzyme inactive because it changes the structure and the chemical properties of the active site.

In the Calvin Cycle, glyceraldehyde-3-phosphate dehydrogenase is a primary target for this iodoacetate reaction. Any interference with cysteine residues in GAPDH, such as through iodoacetate, would therefore lead to the enzyme's permanent inactivation, halting the process of carbon fixation in photosynthesis.