Chapter 17, Problem 29DAP

(a)

Summary Introduction

To determine: The reason of using CoA derivatives rather than free fatty acids in experiments by scientists.

Introduction:

Trans fats are those unsaturated fats that have trans double bonds. The β-oxidation of fatty acid is a catabolic process in which fatty acid molecules are broken down to release acetyl-CoA. To investigate the effect of trans fats on health, scientists presented a model trans fatty acid whose cis isomer was processed differently.

Explanation of Solution

Explanation:

To explore the difference between same sized isomers of cis and trans beta oxidation, scientists used related fatty acids of 18 carbons. They incubated derivatives of coenzyme A of each acid along with rat liver mitochondria and separated remaining derivatives of coenzyme A in each mixture through high-performance liquid chromatography (HPLC).

In cytoplasm, free fatty acids are usually transformed into their coenzyme A derivatives before being transported into mitochondria in the form of $\text{acyl-CoAs}$ for oxidation. It is given that researchers were using isolated lat liver mitochondria, thus they needed to use coenzyme derivatives rather than free fatty acids.

(b)

Summary Introduction

To determine: The reason why no lower molecular weight CoA derivatives were found in the reaction with stearoyl-CoA.

Introduction:

To investigate the effect of trans fats on health, scientists presented a model trans fatty acid whose cis isomer was processed differently. To explore the difference between same sized isomers of cis and tarns beta oxidation, they used three related fatty acids of 18 carbons: stearoyl-CoA, oleoyl-CoA, andelaidoyl-CoA.

Explanation of Solution

Explanation:

When scientists incubated derivatives of coenzyme A of each acid along with rat liver mitochondria and separated remaining derivatives of coenzyme A in each mixture through HPLC, they found that no CoA derivatives of low molecular weight was obtained by the reaction with stearoyl-CoA .

Through beta oxidation, stearoyl-CoA is converted into nine molecules of acetyl-CoA. All the intermediate molecules reacted immediately and thus are not observed at detectable levels.

(c)

Summary Introduction

To determine: The round of beta oxidation which would be required to convert the oleoyl-CoA and elaidoyl-CoA to cis-∆⁵-tetradecenoyl-CoA and trans-∆⁵-tetradecenoyl-CoA, respectively.

Introduction:

The effect of trans fats on health was investigated by some scientists. They presented a model of trans fatty acid whose cis isomer was processed differently. To explore the difference between same sized isomers of cis and tarns beta oxidation, they used three related fatty acids of 18 carbons: stearoyl-CoA, oleoyl-CoA, and elaidoyl-CoA.

Explanation of Solution

Explanation:

Total two rounds of beta oxidation would be required to convert the 18 carbon oleoyl-CoA and elaidoyl-CoA to 14 carbon compound cis-∆⁵-tetradecenoyl-CoA andtrans-∆⁵-tetradecenoyl-CoA, respectively.

These 18 carbon fatty acids are converted to 14 carbon fatty acids through two rounds of beta oxidation. Each round removes two atoms of carbon from them that leads to release of two molecules of acetyl-CoA.

(d)

Summary Introduction

To explain: The possible explanation for the observation that “for LCAD, the K_m differs dramatically for the cis and trans substrates” in terms of structures of the substrate molecules.

Introduction:

The kinetic parameters of two forms of acetyl-CoA dehydrogenase: LCAD and VLCAD, were measured by Yu and his coworkers. They used three derivatives of coenzyme A of fatty acids: tetradecenoyl-CoA, cis-∆⁵-tetradecenoyl-CoAand trans-∆⁵-tetradecenoyl-CoA.

Explanation of Solution

Explanation:

Scientists measured the kinetic parameters of long-chain acyl-CoA dehydrogenase (LCAD) and very long-chain acyl-CoA dehydrogenase (VLCAD). They observed that for LCAD there is difference between the K_m for cis and trans substrates. K_m indicates the substrate concentration at which the enzyme achieves half V_max (maximum rate of reaction).

The K_m for trans isomers were higher as compared to cis isomers. This is because the structure of cis isomers allows them to bind better to the enzyme as compared to trans isomers. Thus high concentration of trans isomer would be required for similar rate of breakdown.

(e)

Summary Introduction

To determine: The evidence that support the given assumption “the kinetic parameter of the two enzymes are relevant to the differential processing of these fatty acids only if LCAD or VLCAD reaction is the rate-limiting step in the pathway”.

Introduction:

The kinetic parameters of two forms of acyl-CoA dehydrogenase: LCAD and VLCAD, were measured by scientists. They used three derivatives of coenzyme A of fatty acids: tetradecenoyl-CoA, cis-∆⁵-tetradecenoyl-CoAandtrans-∆⁵-tetradecenoyl-CoA.

Explanation of Solution

Explanation:

When scientists measured the kinetic parameters of LCAD and VLCAD, they made an assumption stating that: “the kinetic parameter of the two enzymes are relevant to the differential processing of these fatty acids only if LCAD or VLCAD reaction is the rate-limiting step in the pathway”.

The evidence that supports this assumption is observed by the different build up of substrates of LCAD and VLCAD. The accumulation of a particular LCAD or VLCAD substrates provides evidence that they are the rate limiting steps in pathway.

(f)

Summary Introduction

To determine: The way in which the different kinetic parameters explain the different levels of the coenzyme A derivatives found after incubation of rat liver mitochondria with stearoyl-CoA, oleoyl-CoA ,  and elaidoyl-CoA

Introduction:

To explore the difference between same sized isomers of cis and tarns beta oxidation, scientists used related fatty acids of 18 carbons. They incubated derivatives of coenzyme A of each acid along with rat liver mitochondria and separated remaining derivatives of coenzyme A in each mixture through HPLC.

Explanation of Solution

Explanation:

The kinetic parameter indicates that trans isomer is not a well substrate for LCAD as compared to cis form. This is because the structure of cis isomers allows them to bind better to the enzyme as compared to trans isomers. The kinetic parameters for LCAD do not show much difference. As trans isomer is not a well substrate it will be accumulated in high levels as compared to cis forms.

(g)

Summary Introduction

To describe: The pathway that led to extra mitochondrial trans-∆⁵-tetradecenoic acid and also indicate where in the cell the various transformations take place along with the enzymes that catalyzes the transformations.

Introduction:

Yu and his coworkers determined the substrate specificity of rat’s mitochondrial thioesterase enzyme. This enzymes hydrolyzes acyl-CoA into free fatty acids andCoA. Other researchers suggested that free fatty acids can cross membranes. The scientist’s observed trans-∆⁵-tetradecenoic acid outside the mitochondria, which was incubated with elaidoyl-CoA.

Explanation of Solution

Explanation:

The one possible pathway that led to extra mitochondrial trans-∆⁵-tetradecenoic acid is as follows:

$\begin{array}{l}\text{Elaidoyl-CoA}\stackrel{\text{carnitine acyltransferase I}}{\to }\text{Elaidoyl carnitine}\stackrel{\text{transport}}{\to }\\ \text{Elaidoyl carnitine}\stackrel{\text{carnitine acyltransferase II}}{\to }\text{Elaidoyl-CoA}\stackrel{\text{2 rounds of β-oxidation}}{\to }\\ \text{5-trans-tetradecenyl-CoA}\stackrel{\text{thioesterase}}{\to }\text{5-trans-tetradecenoic acid}\stackrel{\text{diffusion}}{\to }\\ \text{5-trans-tetradecenoic acid}\end{array}$

1. Elaidoyl-CoA is found outside the mitochondria and is converted into elaidoyl carnitine by carnitine acyltransferase I enzyme. Later elaidoyl carnitine is transported inside the mitochondria.

2. Elaidoyl carnitine which is now inside mitochondria is converted to elaidoyl-CoA inside the mitochondria by carnitine acyltransferase II enzyme.

3. Elaidoyl-CoA after undergoing two rounds of beta oxidation is converted into 5-trans-tetradecenyl-CoA inside the mitochondria.

4. 5-trans-tetradecenyl-CoA (inside) is converted into 5-trans-tetradecenoic acid (inside mitochondria) through thioesterase enzyme. Finally, 5-trans-tetradecenoic acid which is present inside the mitochondria diffuses out from the mitochondria.

(h)

Summary Introduction

To determine: The sense in which the statement “trans fats are not broken down by your cells and instead accumulate in your body” is correct and in what sense is it an oversimplification.

Introduction:

Trans fats are those unsaturated fats that have trans double bonds. The β-oxidation of fatty acid is a catabolic process in which fatty acid molecules are broken down to release acetyl-CoA. To investigate the effect of trans fats on health, scientists presented a model trans fatty acid whose cis isomer was processed differently.

Explanation of Solution

Explanation:

It is correct to say that trans fats are accumulated in the body as the efficiency of trans fat breakdown is very low. Due to which they could leak out of the mitochondria and could accumulate in the body.

It is incorrect to say that cells cannot breakdown trans fats. The rate of their breakdown is much slower than those of cis fats but they are broken down by the cells.