# Structure and mechanism of mammalian stearoyl-CoA desaturases

> **NIH NIH R01** · BAYLOR COLLEGE OF MEDICINE · 2020 · $632,360

## Abstract

Mammalian stearoyl-CoA desaturase (SCD) is a member of a super family of redox enzymes that have
four transmembrane helices and a diiron center composed of nine conserved histidine residues. This
superfamily has over 20,000 members from all kingdoms of life and yet very little is known about the function
and mechanism of these proteins. The overall goal of the proposal is to understand the mechanism of SCD at
the atomic resolution. SCD resides on endoplasmic reticulum membrane and catalyzes the formation of a
double bond on saturated fatty acyl-CoAs. Mice with reduced SCD activity do not become obese or diabetic
when fed with a high-fat diet, illustrating the significance of SCD activity in energy homeostasis. SCD
expression level is upregulated in cancer cells because of a higher demand for membrane biosynthesis, and
inhibition of SCD activity thwarts cancer cell growth. These results led to intense efforts on targeting SCDs for
the treatment of obesity, diabetes, and cancer, but the efforts are hampered by a lack of 3-dimenional
structures of SCD and incomplete understanding of the catalytic mechanism.
 It remains unclear how SCDs recognize their substrates and achieve specificity in double bond
formation. The all-histidine coordinated diiron center is different from other diiron centers of known structures
and its mechanism of catalysis has not been examined. In addition, sustained SCD activity requires two
additional membrane proteins, cytochrome b5 (b5) and cytochrome b5 reductase (b5R), which mediate electron
transfer from NADH to the diiron center. Yet how the diiron center is redox-cycled through dynamic interactions
with b5 and b5R remains a mystery.
 We expressed and purified functional mouse SCD1 and solved its crystal structure to 2.6 Å resolution.
We also expressed and purified full length b5 and b5R, and assembled the enzymatic reactions in vitro. In
addition, we produced stable binary complexes between b5R and b5 and between b5 and SCD1, and we
established assays to measure electron transfer between the binding partners. These preliminary results
allowed us to propose the following three Specific Aims:
Aim 1: To characterize the structure and function of SCD1.
Aim 2: To understand the oxidative and reductive pathways of the SCD1 diiron center.
Aim 3: To understand the structural basis of electron transfer between b5 and SCD1 and between b5R
and b5.

## Key facts

- **NIH application ID:** 9959410
- **Project number:** 5R01DK122784-02
- **Recipient organization:** BAYLOR COLLEGE OF MEDICINE
- **Principal Investigator:** AH-LIM TSAI
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $632,360
- **Award type:** 5
- **Project period:** 2019-07-01 → 2024-05-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9959410

## Citation

> US National Institutes of Health, RePORTER application 9959410, Structure and mechanism of mammalian stearoyl-CoA desaturases (5R01DK122784-02). Retrieved via AI Analytics 2026-05-28 from https://api.ai-analytics.org/grant/nih/9959410. Licensed CC0.

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