Question

In the oxidative deamination of glutamate, is the hydride ion transferred to the Re face or the \(S\) face of \(\mathrm{NAD}^{+} ?\) (Review Section \(5.11 .\) )

Short answer

The hydride ion is transferred to the Si face of NAD⁺.

Step-by-step solution

Understanding Oxidative Deamination

Oxidative deamination involves the removal of an amino group from a molecule, releasing ammonia and replacing it with a ketone. In this specific reaction, the conversion of glutamate to α-ketoglutarate is coupled with the reduction of the coenzyme NAD⁺ to NADH.

Structure of NAD⁺ and Nomenclature

In the structure of NAD⁺, the nicotinamide ring is planar, and the two faces are termed Re and Si. These terms are used to describe the stereochemistry of reactions involving such molecules. Determining if the hydride ion is transferred to the Re or Si face involves examining the orientation of the molecule and the chemical reaction path.

Hydride Transfer Analysis

In this reaction, a hydride ion (H⁻) is transferred from the substrate, glutamate, to the nicotinamide ring of NAD⁺. The conformation and three-dimensional orientation of NAD⁺ during the reaction complex will determine which face the hydride ion is added. According to the stereochemistry involved in this enzyme-catalyzed process, the hydride ion transfer occurs to the Si face of NAD⁺ in the reaction with glutamate.

Conclusion

The transfer of the hydride ion to the Si face of NAD⁺ is consistent with the enzyme-specific activity of glutamate dehydrogenase catalyzing this chemical reaction. This specific stereochemical preference contributes to the formation of a defined product, ensuring that the reaction produces only one of the two possible stereoisomeric forms of NADH.

Key concepts

Glutamate

Glutamate is an important amino acid that plays a critical role in various metabolic processes. It is prominently found in the central nervous system as a key neurotransmitter. One of its significant biochemical reactions is oxidative deamination, where it is converted into α-ketoglutarate.

• This conversion helps in the release of ammonia—a vital process for nitrogen metabolism.
• In this reaction, glutamate serves as a substrate, undergoing changes that contribute to cellular energy production and biosynthesis.

Understanding glutamate's role gives insights into cellular energy metabolism and its crucial function in supporting neurotransmission.

NAD⁺

NAD⁺, or Nicotinamide Adenine Dinucleotide, is a coenzyme found in all living cells. It acts as a pivotal electron carrier in various biochemical reactions, including oxidative deamination.

• NAD⁺ facilitates the transfer of electrons and hydride ions, hence switching between oxidized (NAD⁺) and reduced (NADH) forms.
• This redox property is essential in metabolic pathways like glycolysis, the citric acid cycle, and oxidative phosphorylation.

In the process involving glutamate, NAD⁺ accepts electrons and a hydride ion, thus being reduced to NADH. This transformation is critical for maintaining a balance in cellular redox states.

Hydride Transfer

Hydride transfer is a fundamental chemical reaction where a hydride ion ( H⁻ ) is moved from one molecule to another. In oxidative deamination, this process is crucial for the conversion of glutamate to α-ketoglutarate.

• During this reaction, glutamate transfers a hydride ion to NAD⁺, facilitating its reduction to NADH.
• This step is enzyme-mediated, ensuring that the transfer process is efficient and follows the correct stereochemistry.

Understanding hydride transfer helps explain how electrons and protons are managed in cells, driving energy and metabolic pathways.

Stereochemistry

Stereochemistry refers to the spatial arrangement of atoms in molecules, influencing the outcome of biochemical reactions. In the example of oxidative deamination, stereochemistry determines how NAD⁺ is reduced.

• The nicotinamide ring in NAD⁺ has two sides—Re and Si—key to understanding the transfer process.
• The enzyme glutamate dehydrogenase facilitates the correct positioning, ensuring the hydride adds to the Si face of NAD⁺.

The specificity provided by stereochemistry is vital for creating the correct NADH form, avoiding unwanted byproducts and ensuring metabolic efficiency.