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Chapter : Problem 4

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

Expert verified
Iodoacetate inhibits glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by targeting its cysteine thiol groups.

Step by step solution

01

Identifying the Compounds Involved

First, we must understand that iodoacetate targets free sulfhydryl (-SH) groups, which are associated with cysteine residues in proteins. These groups can form disulfide bonds, which are crucial for stabilizing protein structure, affecting enzyme activity.
02

Reviewing Calvin Cycle Enzymes

The Calvin Cycle consists of several enzymes critical for photosynthesis, including ribulose bisphosphate carboxylase/oxygenase (RuBisCO), phosphoribulokinase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and others. We focus on those containing cysteine residues for the potential inhibition by iodoacetate.
03

Exploring Enzyme Structures

Among these enzymes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is known to have cysteine residues in its active site, which are essential for its catalytic function. We need to verify if the activity involves thiol groups that iodoacetate could disrupt.
04

Determining GAPDH Inhibition

GAPDH uses cysteine residues for the catalytic step where it participates in the conversion of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate by nucleophilic attack involving a thiol group. Thus, iodoacetate would inhibit GAPDH by reacting with these thiol groups and disrupting the enzyme's activity.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Iodoacetate Inhibition
Iodoacetate is a chemical compound known for its ability to interfere with specific processes in biological systems. It primarily works by targeting and reacting with free sulfhydryl (-SH) groups in proteins. These -SH groups are commonly found in cysteine residues and play a vital role in various enzymatic functions.
When iodoacetate reacts with these groups, it forms an irreversible bond, effectively inhibiting the enzyme's function. This inhibition is particularly significant in the regulation of metabolic pathways like the Calvin Cycle in plants.
  • Iodoacetate's action prevents proper enzyme function, disrupting crucial biological reactions.
  • The irreversible nature of the bond means that once an enzyme is inhibited, its activity cannot be restored without synthesizing new proteins.
Cysteine Residues
Cysteine residues are molecular components within proteins that contain a thiol group (-SH), which is highly reactive. These residues are crucial for maintaining the structure and function of many enzymes due to their ability to form disulfide bonds.
In enzymatic reactions, cysteine residues often serve as active sites, where they participate directly in catalysis.
  • The presence of cysteine in a protein dictates its potential reactivity with inhibitors like iodoacetate.
  • Cysteine residues are sensitive to oxidation, and their state can significantly alter enzyme activity.
Understanding cysteine's role helps reveal how specific enzymes are regulated or inhibited, such as in the Calvin Cycle.
GAPDH Enzyme
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key enzyme in the Calvin Cycle and glycolysis, playing an essential role in cellular energy processes. In the Calvin Cycle, GAPDH catalyzes the conversion of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate.
The enzyme's activity relies heavily on its cysteine residues located in the active site, which partake in a nucleophilic attack. This step is crucial for its catalytic function and susceptibility to inhibition by iodoacetate.
  • GAPDH is an ideal target for iodoacetate due to its essential cysteine residues.
  • An inhibited GAPDH disrupts not only the Calvin Cycle but has ripple effects on overall plant metabolism.
Photosynthesis
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. The Calvin Cycle is one of the crucial components of photosynthesis, where carbon fixation occurs.
Within the Calvin Cycle, atmospheric CO鈧� is transformed into organic compounds, which later contribute to glucose production. This cycle involves various enzymes, including RuBisCO and GAPDH, all integral to maintaining efficient photosynthesis.
  • Photosynthesis ensures the survival of plants by producing energy-rich compounds.
  • The Calvin Cycle's regulation is critical, as its disruption can affect the global carbon cycle.
Protein Thiol Groups
Protein thiol groups, characterized by the -SH functional group, are essential for various biochemical processes. These groups are primarily found in cysteine residues and determine the protein's reactivity and role in enzymatic activities.
The involvement of thiol groups in protein structure often influences how proteins fold and function, impacting their interaction with inhibitors like iodoacetate.
  • Thiol groups contribute to the redox regulation of enzymes.
  • They are pivotal in maintaining enzyme activity through their ability to form disulfide bonds.
This makes thiol groups crucial attack points for chemical inhibitors in processes such as the Calvin Cycle.

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Most popular questions from this chapter

Thioredoxin in Regulation of Calvin Cycle Enzymes Motohashi and colleagues used thioredoxin as a hook to fish out from plant extracts the proteins that are activated by thioredoxin. To do this, they prepared a mutant thioredoxin in which one of the reactive Cys residues was replaced with a Ser. Explain why this modification was necessary for their experiments. Source: Motohashi, K., Kondoh, A., Stumpp, M.T., \& Hisabori, T. (2001) Comprehensive survey of proteins targeted by chloroplast thioredoxin. Proc. Natl. Acad. Sci. USA \(98,11,224-11,229\)

Inorganic Pyrophosphatase The enzyme inorganic pyrophosphatase contributes to making many biosynthetic reactions that generate inorganic pyrophosphate essentially irreversible in cells. By keeping the concentration of \(\mathrm{PP}_{\mathrm{i}}\) very low, the enzyme "pulls" these reactions in the direction of \(\mathrm{PP}_{1}\) formation. The synthesis of ADP-glucose in chloroplasts is one reaction that is pulled in the forward direction by this mechanism. However, the synthesis of UDP-glucose in the plant cytosol, which produces \(\mathrm{PP}_{\mathrm{i}},\) is readily reversible in vivo. How do you reconcile these two facts?

Regulation of Starch and Sucrose Synthesis Sucrose synthesis occurs in the cytosol and starch synthesis in the chloroplast stroma, yet the two processes are intricately balanced. What factors shift the reactions in favor of (a) starch synthesis and (b) sucrose synthesis?

Phases of Photosynthesis When a suspension of green algae is illuminated in the absence of \(\mathrm{CO}_{2}\) and then incubated with \(^{14} \mathrm{CO}_{2}\) in the dark, \(^{14} \mathrm{CO}_{2}\) is converted to \(\left[^{14} \mathrm{C}\right]\) glucose for a brief time. What is the significance of this observation with regard to the \(\mathrm{CO}_{2}\) -assimilation process, and how is it related to the light reactions of photosynthesis? Why does the conversion of \(^{14} \mathrm{CO}_{2}\) to \(\left[^{14} \mathrm{C}\right]\) glucose stop after a brief time?

Comparison of the Reductive and Oxidative Pentose Phosphate Pathways The reductive pentose phosphate pathway generates a number of intermediates identical to those of the oxidative pentose phosphate pathway (Chapter 14 ). What role does each pathway play in cells where it is active?
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