Metabolic Pathways and Thiamine Deficiency

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Questions and Answers

Which of the following is NOT a recognised syndrome associated with thiamine deficiency?

Wernicke-Korsakoff Syndrome

Beri-Beri

Parkinson's Disease (correct)

Lactic Acidosis

What are the main characteristics of 'dry' Beri-beri?

Edema and progressive paralysis

Tachycardia and convulsions

Muscle weakness and atrophy (correct)

Vomiting and extreme heart failure

Which of the following enzymes is NOT directly dependent on thiamine?

Lactate dehydrogenase (correct)

α-Ketoglutarate dehydrogenase

Pyruvate dehydrogenase

Transketolase

In individuals with alcohol dependence, how does alcohol consumption contribute to thiamine deficiency?

Alcohol displaces other nutrients and interferes with thiamine absorption. (C) Signup and view all the answers

Which area of the brain is most likely to be damaged due to thiamine deficiency?

Mammillary bodies (A) Signup and view all the answers

In beta-oxidation, where does the initial attack on a fatty acid occur?

At the beta carbon (A) Signup and view all the answers

How many carbon units are released per cycle of beta-oxidation?

One 2-carbon unit (C) Signup and view all the answers

What are the products of the complete beta oxidation of Palmitoyl CoA (16-carbons)?

8 Acetyl CoA, 7 FADH2, 7 NADH (B) Signup and view all the answers

What is the key connection between fatty acid oxidation and the citric acid cycle?

The production of Acetyl CoA (C) Signup and view all the answers

Approximately how many ATP molecules can be produced from one molecule of FADH2 during oxidative phosphorylation?

1.5 ATP (B) Signup and view all the answers

If there's an excess of Acetyl-CoA in the liver after a meal, what is the most likely metabolic fate?

Used for the synthesis of fatty acids (D) Signup and view all the answers

Which of the following B vitamins and their associated coenzymes are correctly paired?

B3, Niacin (NADH) (D) Signup and view all the answers

What is the primary function of Vitamin C (ascorbic acid) as it relates to collagen?

It is a cofactor needed for the synthesis of hydroxyproline. (B) Signup and view all the answers

Where does the energy conversion of NADH and FADH2 to ATP take place?

In the mitochondria (B) Signup and view all the answers

What type of fatty acid is characterized by the presence of double bonds that result in a 'kinked' structure?

Cis fatty acids (B) Signup and view all the answers

What is the typical carbon number composition of naturally occurring fatty acids?

An even number of carbons (B) Signup and view all the answers

Which statement best describes the effect of cis double bonds on the melting temperature of a fatty acid?

They lower the melting temperature, resulting in liquids at body temperature. (C) Signup and view all the answers

What is a significant source of trans fatty acids in the diet, other than minor natural occurrences?

Catalytic hydrogenation of vegetable oils (D) Signup and view all the answers

How does the consumption of trans fatty acids typically impact blood cholesterol levels?

It increases LDL cholesterol and decreases HDL cholesterol. (D) Signup and view all the answers

During fatty acid metabolism, what molecule is produced from the breakdown of fatty acids, consisting of two carbons?

Acetyl-CoA (C) Signup and view all the answers

Study Notes

Metabolic Pathways for Major Nutrient Conversion to Usable Energy

Major nutrients are carbohydrates, fats, and proteins
These nutrients are broken down into usable energy through metabolic pathways

Inadequate Diet: Thiamine Deficiency

Thiamine deficiency is more common in famine areas or alcohol-dependent populations in developed nations
Several syndromes are recognized:
- Wernicke-Korsakoff Syndrome: Symptoms include confusion, abnormal eye movements, and ataxia (lack of muscle coordination)
- Beri-Beri: Symptoms include cardiac failure and peripheral neuropathy (damage to peripheral nerves)
Symptoms of severe B1 deficiencies include vomiting, tachycardia (fast heart rate), heart failure, convulsions, muscle weakness and atrophy (dry beriberi), and edema (wet beriberi) as well as progressive paralysis

Thiamine-Dependent Enzymes

These enzymes are vital for energy generation
Key enzymes include pyruvate dehydrogenase, α-ketoglutarate dehydrogenase (part of the citric acid cycle), and transketolase (involved in ribose synthesis from glucose)

How Serious is Thiamine Deficiency?

Thiamine deficiency mainly affects individuals with malnutrition or alcohol dependence
In those with alcohol dependence, alcohol displaces other nutrients, interfering with thiamine absorption and accelerating thiamine turnover
This can result in brain damage (mammillary bodies and hypothalamus)
Subclinical cases of Wernicke's encephalopathy are often missed, delaying treatment

Many Water-Soluble Vitamins as Coenzymes

Several vitamins act as coenzymes
- B1 (thiamine): Thiamine pyrophosphate (TPP)
- B2 (riboflavin): Flavin nucleotides (FADH2)
- B3 (niacin): Nicotinamide nucleotides (NADH)
- B5 (pantothenic acid): Coenzyme A
- Biotin: Coenzyme for pyruvate carboxylase, and acetyl CoA carboxylase
- B6 (pyridoxine): Transamination reactions
- C (ascorbic acid): Synthesis of hydroxyproline in collagen

Major Nutrient Metabolism: Overview

Fats are broken down into fatty acids and glycerol which enter the metabolic pathway
Polysaccharides (complex carbohydrates) are broken down into glucose and other sugars entering the metabolic pathway
Proteins are broken down into amino acids, which also enter the metabolic pathway at various stages
All pathways ultimately connect to the citric acid cycle
Oxidative phosphorylation then generates ATP

Naturally Occurring Fatty Acids

Fatty acids are typically composed of an even number of carbon atoms
They can be saturated (no double bonds) or unsaturated (with one or more double bonds). Unsaturated fats can be either mono-unsaturated (MUFA) or poly-unsaturated (PUFA).
Unsaturated double bonds can be cis or trans

Comparing Cis and Trans Fatty Acids

The majority of fatty acids in nature are in the cis configuration
Cis double bonds create kinks in the fatty acid chain which lower the melting point, making them liquid at body temperature
Trans fatty acids have a straight structure which results in higher melting points

Trans Fatty Acids

Small amounts of trans fatty acids occur naturally in animal fats
However, large amounts of trans fats are created by the partial hydrogenation of vegetable oils
Trans fats raise LDL (low-density lipoprotein) cholesterol levels and lower HDL (high-density lipoprotein) cholesterol levels

Fatty Acid Metabolism

Triacylglycerols are broken into glycerol and three fatty acid chains
Glycerol enters glycolysis as a substrate for the formation of glucose
Fatty acids are oxidized and two carbon molecules break off as acetyl-CoA

Fatty Acid Oxidation

Fatty acid energy is extracted primarily by β-oxidation
Each cycle in β-oxidation produces one acetyl-CoA, FADH2, and NADH from a two-carbon unit in the fatty acid chain. Four steps per cycle are required
The Overall reaction for complete β-oxidation of palmitate releases 8 Acetyl CoA, 7 FADH2, and 7 NADH

Fatty Acid Oxidation and the Citric Acid Cycle

Fatty acid oxidation and the citric acid cycle are linked by the generation of acetyl CoA
The citric acid cycle generates NADH and FADH2
Both pathways generate NADH and FADH2 from acetyl-CoA which can generate 2.5 ATP for each NADH and 1.5 ATP for each FADH2

Metabolic Fates of Excessive Acetyl CoA

In a normal condition, excessive acetyl CoA produced after a meal is utilized to form fat and transported to adipose tissue via lipoproteins
In pathological conditions (e.g., diabetes mellitus or fasting), excess acetyl CoA is converted into ketone bodies used by other organs as energy sources

Amino Acid Metabolism

Proteins are digested into amino acids, which are absorbed into blood and extracellular fluid
Surplus proteins can serve as metabolic fuel sources, as do carbohydrates and fats
Excess nitrogen is removed, producing carbon skeletons, and ammonia. The carbon skeletons then enter the metabolic pathways.

Protein Breakdown and Catabolism of Amino Acids

Amino acids undergo transamination to yield keto acids
Keto acids then enter either glycolysis or the Krebs cycle and, via intermediates, are oxidized to generate ATP
Essential amino acids are used to build proteins in the body

Overview: Degradation of Amino Acid Carbon Skeletons

Amino acids are categorized as glucogenic or ketogenic based on their metabolic fate, determining their contribution to glucose or ketone production
Amino acid metabolism is part of the central pathway of energy metabolism

Amino Acid Metabolism: Overview

Dietary proteins are broken down into amino acids
The nitrogen atom of amino acids is removed by deamination to generate ammonia as a byproduct
The carbon skeletons of amino acids can be used to produce metabolic intermediates that feed into the citric acid cycle, or glucose
The nitrogen group is processed further and excreted as urea

Summary

ATP is used as a readily available energy source
ATP generation involves the metabolism of dietary carbohydrates and fats
Glucose, other sugars, fatty acids, and amino acids undergo specific catabolic pathways
These pathways generate intermediates such as acetyl CoA which enter the citric acid cycle and result in ATP production
Many important vitamins act as coenzymes for these biochemical reactions

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Description

Explore the metabolic pathways involved in converting major nutrients into usable energy, focusing on carbohydrates, fats, and proteins. Additionally, delve into the significance of thiamine, its deficiency, syndromes associated with it, and the important thiamine-dependent enzymes crucial for energy generation.