lipid biochem overview

Question 1

· Identify the structures of fatty acids, phospholipids, cholesterol and cholesterol esters.

Answer 1

fatty acids: most polar, carbon chain with carboxyl group at one end. Phospholipids: intermediate polarity, fatty acid connected to phosphate group. Cholesterol: intermediate polarity, steroid structure. Cholesterol ester: most non-polar, steroid structure with COOR

Question 2

· Describe the chemical properties of each of these lipids

Answer 2

Cholesterol esters are so non polar it exists away from water in a droplet within cells or in center of circulating lipoproteins in plasma. Phospholipids and free cholesterol tend to be located at the interface of lipid and aqueous in membranes or at the surface of lipoproteins. Free fatty acids move in aqueous environments complexed to protein.

Question 3

· Describe the sources of each lipid class and the general functions of each.

Answer 3

fatty acids: product of pancreatic digestion and bile salt emulsification of dietary lipids. Phospholipids: come from dietary lipids unchanged. Cholesterol: comes from breakdown of cholesterol ester by cholesteryl esterase and bile salts. Cholesterol ester: comes from dietary lipids unchanged

Question 4

describe de novo lipogenesis

Answer 4

excess glucose in liver or adipose > glycolysis >Acetyl CoA (mitochondria) > citrate > Acetyl CoA (cytoplasm) > malonyl CoA (acetyl CoA carboxylase,rate limiting) > units of Acetyl CoA put together to form fatty acid chain (fatty acid synthase)

Question 5

Malonyl CoA as regulator

Answer 5

Inhibits oxidation of fatty acids (the reverse of de novo lipogenesis)

Question 6

energy source for fatty acid synthase

Answer 6

NADPH

Question 7

fates of fatty acids

Answer 7

Fatty acids can be packaged with glycerol into triglyceride which circulates in the blood as VLDL triglyceride. This triglyceride can be taken up by adipose tissue through the action of lipoprotein lipase where it can be stored to be used when the body is in negative energy balance.

Question 8

describe beta oxidation

Answer 8

low insulin, high counter-regulatory hormones > triglyceride in adipose tissue broken down (hormone sensitive lipase) > fatty acids/glycerol taken up by liver > glycerol used as substrate for gluconeogenesis and fatty acid is converted to acyl-carnitine then transported into mitochondria by carnitine palmitoyl transferase 1 (CPT1- rate limiting) > Acetyl CoA from fatty acyl CoA enters TCA cycle

Question 9

When does beta oxidation occur

Answer 9

When the body is in negative energy balance such as occurs during short-term fasting or during exercise. Fat is used as alternate fuel for muscle and liver to preserve glucose for the brain, and provides ATP to fuel gluconeogenesis

Question 10

describe ketogenesis

Answer 10

fatty acids or ethanol is converted to ketones. Rate limiting step in ketogenesis is the synthesis of hydroxyl methylglutaryl CoA by HMG CoA synthase

Question 11

When does ketogenesis occur

Answer 11

If insulin is very low or absent and counter-regulatory hormones quite high such as occurs during long-term fasting, keto diets or diabetic/alcholic ketoacidosis. Ketones serve as alternate fuel for brain and other tissues in states of prolonged glucose insufficiency.

Question 12

3 conditions of ketogenesis

Answer 12

1. very low insulin levels, 2. high counter-regulatory hormones, 3. An abundant source of substrate: fatty acids in the case of fasting and diabetic ketoacidosis, and ethanol in the case of alcoholic ketoacidosis.

Question 13

ketones are only formed when…

Answer 13

acetyl CoA produced by fatty acid metabolism (or ethanol metabolism) exceeds the capacity of the TCA cycle to metabolize it (ATP/ADP is high)

Question 14

describe cholesterol synthesis

Answer 14

Cholesterol is synthesized from acetyl CoA through the formation of hydroxymethyl glutaryl CoA. The rate limiting step in this pathway is HMG-CoA reductase. This reaction occurs in the cytosol and uses NADPH for energy.

Question 15

List 4 classes of specialized lipids

Answer 15

phospholipids, sphingolipids, glycospingolipids, arachadonic acid derivatives (leukotrienes or prostaglandins/thromboxanes)

Question 16

phospholipid structure

Answer 16

glycerol backbone and a PO4 group attached to one of the positions of the glycerol backbone

Question 17

sphingolipid structure

Answer 17

ceramide backbone- contains nitrogen

Question 18

glycosphingolipid structure

Answer 18

ceramide backbone with sugar residues attached to head group

Question 19

function of arachidonic acid derivatives

Answer 19

regulatory molecules in inflammation- COX-1 and 2 are critical enzymes in their synthesis

Question 20

Lipoprotein pathways

Answer 20

1. The dietary fat pathway or Chylomicron pathway in which triglyceride rich particles deliver dietary fat to skeletal muscle and adipose tissue. 2. The VLDL pathway by which triglyceride derived from the liver is delivered to skeletal muscle and adipose tissue and 3. the HDL pathway that largely functions as a reservoir and transport system for a variety of lipids including cholesterol from the periphery to the liver.