Explain beta-oxidation: its definition, steps, enzymes, and comparison with fatty acid biosynthesis.

Here are the answers to question 5:

a) (i) What is Beta oxidation?
Beta oxidation is a catabolic process that breaks down fatty acids into acetyl-CoA. This occurs in the mitochondria (and peroxisomes) and involves the sequential removal of two-carbon units from the carboxyl end of the fatty acyl chain, generating ATP precursors like NADH and FADH$_{2}$.

b) (ii) Outline the steps involved in Beta oxidation indicating enzymes involved.
Beta oxidation proceeds through four main steps:

1. Dehydrogenation: Acyl-CoA is oxidized by acyl-CoA dehydrogenase, forming a trans-$\Delta^{2}$-enoyl-CoA and FADH$_{2}$.
2. Hydration: Water is added across the double bond of trans-$\Delta^{2}$-enoyl-CoA by enoyl-CoA hydratase, forming L-$\beta$-hydroxyacyl-CoA.
3. Dehydrogenation: L-$\beta$-hydroxyacyl-CoA is oxidized by $\beta$-hydroxyacyl-CoA dehydrogenase, forming $\beta$-ketoacyl-CoA and NADH.
4. Thiolysis: $\beta$-ketoacyl-CoA is cleaved by thiolase (acyl-CoA acetyltransferase), releasing acetyl-CoA and a fatty acyl-CoA shortened by two carbons, which then re-enters the cycle.

c) (iii) Compare and contrast beta oxidation and Fatty acid biosynthesis.
Here's a comparison and contrast:

Feature	Beta Oxidation (Degradation)	Fatty Acid Biosynthesis (Synthesis)
Purpose	Breaks down fatty acids to produce energy.	Synthesizes fatty acids for storage or structural components.
Location	Primarily mitochondrial matrix (also peroxisomes).	Primarily cytoplasm (also endoplasmic reticulum).
Carrier	Coenzyme A (CoA).	Acyl Carrier Protein (ACP).
Redox Cofactors	Produces FADH$_{2}$ and NADH.	Consumes NADPH.
Intermediates	L-$\beta$-hydroxyacyl-CoA.	D-$\beta$-hydroxyacyl-ACP.
Enzymes	Separate, soluble enzymes.	Multifunctional enzyme complex (Fatty Acid Synthase).
Energy	Exergonic; produces ATP (via ETC).	Endergonic; consumes ATP.
Starting/Ending	Fatty acyl-CoA $\rightarrow$ Acetyl-CoA.	Acetyl-CoA $\rightarrow$ Fatty acid.
Regulation	Regulated by substrate availability and energy state.	Regulated by hormones (insulin, glucagon) and citrate levels.

d) (iv) What are the fates of AcetylCoA and NADH during beta oxidation?

• Acetyl-CoA: The acetyl-CoA produced enters the citric acid cycle (Krebs cycle) in the mitochondrial matrix, where it is completely oxidized to carbon dioxide, generating more NADH, FADH$_{2}$, and GTP (which is converted to ATP).
• NADH: The NADH produced enters the electron transport chain (ETC) in the inner mitochondrial membrane, where its electrons are used to drive the synthesis of ATP through oxidative phosphorylation.