Fatty Acid Synthesis Mcat

a) Protein absorption

Fatty Acid Synthesis

Fatty acid synthesis is a biochemical pathway responsible for making fatty acids from excess carbohydrates. Fatty acids primarily serve as the main storage of energy and are essential components in constructing cell membranes and lipoproteins. Key components in this process include acetyl-CoA, malonyl-CoA, insulin, and fatty acid synthase (FAS). Synthesis takes place in the cytosol of adipose tissue, hepatocytes, and lactating mammary glands when there are high levels of glucose and ATP available inside cells.

To begin the synthesis, acetyl-CoA must combine with oxaloacetate to form citrate. This citrate then crosses the mitochondrial membrane and enters the cytosol where it is converted back to acetyl-CoA and oxaloacetate by ATP citrate lyase. The enzyme acetyl-CoA carboxylase irreversibly uses ATP to add a CO2 group to acetyl-CoA, forming malonyl-CoA. Insulin and citrate activate acetyl-CoA carboxylase, while glucagon, epinephrine, and palmitoyl-CoA inhibit it. Finally, fatty acid synthase produces the 16-carbon palmitic acid from acetyl-CoA and malonyl-CoA, which can undergo further modifications in the smooth endoplasmic reticulum.

• Fats are important for various functions such as phospholipids, lipoproteins, and breastmilk.
• Main role of fats: energy storage.
• Fatty acid synthesis: biochemical pathway making fatty acids from excess carbohydrates.
• Types of fats:
  • Saturated fats
  • Unsaturated fats
  • Omega 3 fats (ALA, EPA, DHA)
  • Omega 6 fat (linoleic acid)
  • Glycolysis to make pyruvate
  • Pyruvate enters mitochondrial matrix and becomes acetyl-CoA
  • Acetyl-CoA enters TCA cycle
  • ATP slows down TCA cycle, leading to excess acetyl-CoA
  • Acetyl-CoA cannot cross the membrane alone
  • Citrate synthase makes citrate from acetyl-CoA and oxaloacetate
  • Citrate crosses the membrane
  • Citrate converted back to acetyl-CoA by ATP citrate lyase
  • Oxaloacetate in the cytosol is reduced to malate, which can be transported into the mitochondria and converted back to oxaloacetate.
  • Note: this process produces NADPH, which is needed for fatty acid synthesis
  • Uses ATP to add a CO2 group to acetyl-CoA, making malonyl-CoA
  • Requires vitamin B7 (biotin)
  • Regulation by insulin, citrate, glucagon, epinephrine, and palmitoyl-CoA
  • Large multi-enzyme complex responsible for making palmitic acid
  • Initiation: binding of acetyl-CoA and malonyl-CoA to FAS
  • Series of 4 reactions to add 2 carbons each cycle and grow fatty acyl CoA
  • Final product: 16-carbon palmitic acid
  • Converted to palmitoyl-CoA
  • Desaturases add double bonds

  

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  What is the role of fatty acid synthesis in the production of phospholipids and cell membranes?

  Fatty acid synthesis is crucial for the production of phospholipids, which are a primary component of cell membranes. Phospholipids consist of a hydrophilic head containing a phosphate group and two hydrophobic tails derived from fatty acids. The fatty acids can be either saturated or unsaturated, affecting the fluidity and function of cell membranes. Fatty acid synthesis provides the building blocks necessary to create these essential components in cells.

  What are the differences between saturated and unsaturated fats, and how do they affect fatty acid synthesis?

  Saturated fats contain only single bonds between their carbon atoms, leading to a straight and compact structure. Unsaturated fats contain at least one double bond between carbon atoms, which creates a kinked structure. This structural difference affects the fluidity and properties of cell membranes. In fatty acid synthesis, the enzyme stearoyl-CoA desaturase introduces double bonds into the fatty acid chain, creating unsaturated fats. Unsaturated fats, such as omega-3 and omega-6 fats, help to regulate important functions, including inflammation, blood clotting, and cholesterol levels.

  How are omega-3 and omega-6 fats synthesized, and what is their significance in the body?

  Omega-3 and omega-6 fats are polyunsaturated fatty acids (PUFAs) that are essential for our health. The body cannot synthesize these fats, so they must be obtained through dietary intake. Omega-3 fats, such as alpha-linolenic acid (ALA), are primarily found in fatty fish, nuts, and seeds. Omega-6 fats, like linoleic acid, are typically found in vegetable oils and seeds. These fatty acids play integral roles in cell membrane structure, inflammation regulation, mood, and cognitive function. An appropriate balance between omega-3 and omega-6 fats is needed to maintain optimal health.

  What is the relationship between fatty acid synthesis and the formation of triglycerides?

  Triglycerides are the primary storage form of fat in the body and are generated through the process of fatty acid synthesis. They consist of a glycerol backbone joined to three fatty acid molecules through ester bonds. Fatty acid synthesis generates the necessary fatty acids that make up triglycerides. Once synthesized, triglycerides can be stored in adipose tissue for later use as an energy source, or they can be incorporated into lipoprotein particles for transport and metabolism.

  How does adipose tissue function in fatty acid synthesis and energy storage?

  Adipose tissue, or fat tissue, is composed of adipocytes, which are specialized cells that store lipids in the form of triglycerides synthesized from fatty acids. Adipose tissue plays a crucial role in energy metabolism because it stores excess energy from consumed food in the form of lipids. During periods of fasting or increased energy demand, triglycerides are broken down into fatty acids and glycerol by lipolysis, releasing energy for use by other cells. In addition to energy storage, adipose tissue maintains lipid homeostasis, provides insulation, and produces hormones that regulate energy balance, appetite, and inflammation.

  Lipid and Amino Acid Metabolism for the MCAT: Everything You Need to Know

  Learn key MCAT concepts about lipid and amino acid metabolism, plus practice questions and answers

  

  (Note: This guide is part of our MCAT Biochemistry series.)

  Part 1: Introduction

  Part 2: Fatty acid synthesis

  a) The citrate shuttle

  b) The oxaloacetate shuttle

  c) Palmitic acid synthesis

  Part 3: Beta oxidation

  a) Lipid absorption

  b) Activation

  c) Oxidation of saturated fatty acids

  d) Unsaturated fatty acid metabolism

  e) Ketogenesis

  Part 4: Amino acid metabolism

  a) Protein absorption

  b) Protein catabolism

  c) Urea cycle

  Part 5: Metabolic overview and high-yield terms

  Part 6: Passage-based questions and answers

  Part 7: Standalone questions and answers

  Part 1: Introduction

  Whether you are running a marathon or sleeping in on a Sunday morning, your body is carrying out a plethora of chemical reactions. These reactions all contribute to maintaining homeostasis and using energy. While you may already be familiar with carbohydrate metabolism, your body, the ever-so versatile machine, has additional metabolic pathways to acquire and store energy.

  While the MCAT will only rarely test you on details of each metabolic pathway, you will need to understand the big picture behind metabolism by identifying patterns and making connections. Knowing the underlying rationale behind the topics you review is what will ultimately allow you to demonstrate mastery on test day. Make sure to complement your studying with extensive practice, including the practice passage and questions we’ve included at the end of this guide.

  Let’s get started!

  Part 2: Fatty acid synthesis

  Fatty acids are long hydrocarbon chains that serve as great sources of energy for the body. The only fatty acid that the human body can synthesize by itself is palmitic acid, a 16-carbon fatty acid.

  

  Its synthesis occurs primarily in the cytoplasm of hepatocytes and follows the net reaction:

  7 ATP + 8 Acetyl-CoA + 14 NADPH → Palmitic Acid + 7 ADP + 7 Pi + 8 CoA + 14 NADP⁺ + 6H₂O

  The synthesis of palmitic acid is fairly lengthy and is composed of several different components. We’ll walk through each of the stages of this synthesis that you will need to know for the MCAT.

  a) The citrate shuttle

  After the consumption of excess carbohydrates, acetyl-CoA begins to accumulate in the mitochondrial matrix. Recall that glycolysis produces pyruvate, which is converted into acetyl-CoA via the pyruvate dehydrogenase complex. Citrate synthase then catalyzes the formation of citrate from acetyl-CoA and oxaloacetate.

  Typically, this is how acetyl-CoA enters the tricarboxylic acid cycle (TCA). However, since an excess of carbohydrates has been consumed, regulatory measures are taken to slow the TCA cycle, and citrate begins to accumulate. Recall that the TCA cycle’s rate-limiting step is isocitrate dehydrogenase, which acts downstream of citrate synthase—hence causing a build-up of citrate.

  To remedy this, citrate is shuttled to the cytoplasm via a citrate shuttle. An enzyme in the cytoplasm catalyzes the reverse reaction of citrate synthase, by splitting citrate into acetyl-CoA and oxaloacetate.

  

  b) The oxaloacetate shuttle

  The oxaloacetate that is now present in the cytoplasm then re-enters the mitochondrial matrix in a series of steps:

  1. Oxaloacetate is converted to malate.
  2. Malic enzyme catalyzes the conversion of malate into pyruvate and produces NADPH as a byproduct. This NADPH will be critical in later steps of synthesis.
  3. Pyruvate enters the mitochondrion and is converted into oxaloacetate by pyruvate carboxylase.

  Here, oxaloacetate can again be paired with acetyl-CoA to form citrate via citrate synthase.

  Why go through the trouble of shuttling citrate and pyruvate back and forth? Note that the oxaloacetate shuttle results in the production of NADPH. This NADPH is a crucial electron carrier that will be needed later in the synthesis. Without the oxaloacetate shuttle, these electrons would not be able to move from the inner mitochondrial membrane into the cytoplasm.

  c) Palmitic acid synthesis

  In the cytoplasm, acetyl-CoA is converted into malonyl-CoA via the addition of a carbon dioxide molecule. This reaction is catalyzed by acetyl-CoA carboxylase, the rate-limiting step of fatty acid synthesis. Finally, fatty acid synthase, a multienzyme complex, catalyzes the polymerization of palmitic acid. This requires NADPH and produces NADP⁺, carbon dioxide, and water as byproducts.

  

  Note that the synthesis of fatty acids does not require the presence of any precursor or template molecules. This is in contrast to the synthesis of DNA or RNA, which requires the presence of a template molecule to form “copies” of itself.