BIOCHEM 4.4 - OXIDATION OF LIPIDS (BETA-OX. OF FAs); KETONES & KETONE SYNTHESIS

Fatty Acids

• Act as an immediate energy source, excluding the brain and red blood cells
• Fatty acids aren't in free form at significant concentrations in the body
• They are transported in plasma after they are released from adipose stores
• Albumin binds to fatty acids in plasma; each molecule carries 6-8 fatty acids
• Fatty acids are bound by fatty acid-binding proteins when in the cytosol
• Regulation of trafficking occurs in the cytosol and between subcellular compartments

Fatty Acid Breakdown

• Short-chain fatty acids contain 2-4 carbons broken down in the mitochondrion with diffusion transport
• Medium-chain contain 4-12 carbons broken down in the mitochondrion with diffusion transport
• Long-chain contain 12-20 carbons broken down in the mitochondrion via the carnitine cycle
• Very long-chain contain >20 carbons are broken down in the peroxisome with unknown transport

Adipose Tissue

• Adipocyte fat content is altered by energy input (diet) and energy expenditure (exercise)
• It's an active endocrine organ, produces adipokines which have the following hormones:
  • Leptin relies on secretion linked to tissue mass and adipocyte size
  • Adiponectin enhances insulin sensitivity; its absence causes insulin resistance, and physical training boosts its production and receptor upregulation in muscle tissue
  • Resistin
  • Endothelial growth factor
  • Proinflammatory cytokines secreted in obesity enable monocyte recruitment with monocyte attractant protein-1 (MCP-1).

Adipose Tissue Storage

• Subcutaneous fat beneath the skin is a benign energy store
• Visceral fat within the abdominal cavity is an active endocrine organ
• Ectopic fat (i.e., cardiac fat pad, liver, myocytes) isn't well understood
• During weight loss, visceral and ectopic fat are used and depleted

Fatty Acid Oxidation Connection to TCA Cycle

• Lipid metabolism is entirely oxidative
  • Beta-oxidation occurs in the peroxisome and mitochondrion
  • Acetyl-CoA and reduced FADH2 and NADH are end products
• In muscles, Acetyl-CoA is metabolized through the TCA cycle and oxidative phosphorylation to yield ATP
• In the liver, Acetyl-CoA is converted into ketone bodies via ketogenesis

Carnitine Shuttle

• Some materials are too large to enter the mitochondrion, so shuttles are needed
• CPT-I in the outer membrane moves fatty acid to carnitine
• The inner membrane acyl-carnitine transporter (translocase) moves acyl-carnitine into the mitochondrion
• CPTII then regenerates acyl-CoA and releases carnitine

Carnitine Palmitoyl Transferase I (CPTI) in Fatty Acid Oxidation

• CPTI connects with ATP consumption and malonyl-CoA
• Carbohydrate-rich meals inhibit oxidation of fatty acids and increase malonyl-CoA
  • The fed state drives storage rather than use
• When ATP is depleted, AMP Activated Protein Kinase (AMPK) will be acivated
  • AMPK inhibits ACC2 and activates malonyl-CoA decarboxylase
  • It reduces malonyl-CoA and activates CPTI

β-Oxidation of Fatty Acids

• Oxidation happens in a cycle of reactions
• Oxidation to a ketone happens with the β-carbon (C-3)
• DEHYDROGENASE is involved
• Cleavage between α–carbon and β–carbon occurs, via a thiolase, which preserves the high-energy thioester bond
• One mole of Acetyl-CoA, FADH2, and NADH are made during each cycle
  • The fatty acyl-CoA at the end has 2 fewer carbons
  • The cycle is repeated

Peroxisomal Catabolism of Fatty Acids

• Essential for very long-chain fatty acids
• Releases medium-chain fatty acids for oxidation in the mitochondrion
• Can oxidize long-chain and branched-chain fatty acids
• Peroxisomes have a carnitine shuttle and are similar in pathway to β–oxidation with oxidase versus dehydrogenase
• FADH2 is produced
• Energetically less favorable versus β–oxidation from the mitochondrion

Unsaturated Fatty Acid Oxidation

• Yields less FADH2 because they're partially oxidized
• Shifts the position and changes the geometry of the double bonds require extra isomerases and oxidoreductases
• Mitochondrial metabolism of 2-trans-5-cis-octadienoylCoA happens via:
  • pathway A, which is isomerase-dependent
  • pathway B, which is reductase-dependent
  • pathway C, which is dependent on enoyl-CoA isomerase and dienoyl-CoA isomerase, but not on 2,4-dienoyl-CoA reductase

Odd Chain Fatty Acid Oxidation

• With odd carbon numbers, fatty acids go from carboxyl to propionyl-CoA
• Propionyl-CoA is converted to succinyl-CoA by a multistep process requiring both biotin and cobalamin
• Succinyl-CoA enters the TCA cycle immediately

Lipid Metabolism Hormonal Regulation

• Triglycerides in adipose tissue, which are converted into free fatty acids
• Free fatty acids are converted into Acyl-CoA in the liver mithochondrion which eventually turn into ketone bodies

Metabolites in Urine

• Urine is the primary route for water-soluble waste disposal
• Urine usually has high levels of urea, inorganic salts, creatinine, ammonia, organic acids, water-soluble toxins/products, and hemoglobin breakdown
• Medium-chain dicarboxylic fatty acids can show in urine when there is a lipid catabolism disorder
• Active fat metabolism and gluconeogenesis are indicated by ketone bodies (Ketonuria)
• Occurs typically with a high-fat, low-carb diet.
• Higher levels in urine indicate higher levels in plasma which may lead to metabolic acidosis

Ketone Bodies

• Acetoacetate, Acetone, and β-hydroxybutyrate are referred to as ketone bodies
• Created in the liver and transported to other tissues and processed into CoA derivatives
• Plasma concentration rises during fasting and especially starvation

Ketone Bodies and Fasting/Starvation (in Liver)

• Fat-derived ATP and NADH are high
  • Results in inhibition of isocitrate dehydrogenase and shifting oxaloacetate-malate equilibrium to malate
  • Malate then leaves the mitochondrion for gluconeogenesis
  • The TCA cycle is inhibited by low levels of oxaloacetate
  • Resulting from CoA depletion, inhibition of β–oxidation combined with excess acetyl–CoA needs CoA regeneration

Ketogenesis

• Ketogenesis is a pathway to regenerate CoA from excess acetyl-CoA.
• HMG-CoA is an intermediate
• Only the liver has HMG-CoA Synthase

Ketogenesis Regulation

• Ketogenesis is regulated by the ratio of glucagon to insulin
  • There is a switch between their utilization and storage
• Insulin inhibits ketogenesis, but stimulates acetyl-CoA carboxylase
• There is high insulin production, so glucose progresses to ATP and glycogen storage in hepatocytes
  • Fatty acids are converted to TAG and stored
  • Production of new fatty acids takes place for membranes and other needs during high insulin levels
• During decreased insulin rates:
  • Glycogen reserves are tapped as fuel
  • Release of fatty acids increases lipase activity to release stored TAGs
  • There is increased production of fatty acids for the main fuel source ketones

Diabetes Mellitus Types

• Type 1: B-cell destruction that is ~90% autoimmune mediated, making up ~10% of diabetes cases
• Type 2: Insulin resistance, progressing to insulin deficiency making up ~90% of diabeties cases
• Other: MODY encompasses to 1-5% of cases
• Gestational: Pregnancy related alteration in insulin production. As high as 12.8% in high-risk populations

Type 1 versus Type 2 Diabetes Quick Guide

• Type 1: 5-10% of cases featuring autoimmune destruction of β-cells
  • Has a genetic predisposition with more than 50 loci contributing to risk, HLA Class II equaling 40-50%, and Chromosome 11 VNTR polymorphisms equaling 10%
  • Environmental factors are prenatal and postnatal triggers, enteroviruses, and dietary factors in infants
• Type 2: 90-95% of cases resulting from the inability of endogenous insulin to overcome factors that contribute to hyperglycemia
  • Genetic predisposition with over 500 variants at risk that have been identified, estimating a 69% heritability
  • TCF7L2
  • Environmental factor considerations are stress, obesity, and lack of physical activity

Type 1 versus Type 2 Diabetes Quick Guide Comparison

• Type 1 characteristics include:
  • Usually occurring during childhood or puberty with rapid symptom development
  • Characterized with frequent undernourishment
  • β-cell destruction that causes loss of insulin production and low to absent plasma insulin
  • common ketosis and acute ketoacidosis is present
  • Unresponsiveness to oral hypoglycemic drugs
  • Requires insulin treatment
• Type 2 characteristics include:
  • Frequently occurs after age 35 with gradual symptom development expected
  • Obesity is present in 90% of cases
  • Presents with insulin resistance and high levels of plasma insulin
  • Can be treated by: Diet, exercise, oral hypoglycemic drugs, potentially insulin, and REDUCTION OF RISK FACTORS

Insulin Resistance and Type 2 Diabetes Mellitus

• Insulin resistance causes type 2 diabetes
• Type 2 diabetes can precede DM2 by 10-15 years
• Skeletal muscle is affected, taking more insulin to encourage glucose uptake after a high-calorie meal, causing a cycle of calories, hyperinsulinemia, and insulin resistance

Case Study

• 12 year old male came to the emergency room after two days of excessive urination and hunger. The patient also had pain and was nauseous
• Physical exam showed lethargy
• The patient had above normal glucose and a low pH
• The patient's condition was caused by undiagnosed/unmanaged diabetes mellitus type 1 with diabetic ketoacidosis

Diabetic Ketoacidosis

• This life threatening condition shows a deficit in insulin
• It is considered a compilation of diabetes with slow developing symptoms
• However, vomiting can speed onset
• Symptoms are dry mouth, frequent urination, high ketone count, constant tiredness, nausea, abdominal pain, difficulty breathing and confusion
• High fat use results in increase ketone levels
• Causes are missed doses of insulin in patients with DM1 and DM2