Flashcards in Biochemistry- TCA cycle Deck (29)
Why does NAD+ which was reduced in glycolysis to NADH need to be recycled?
There are limited amounts of NAD+ in the cell. We cannot synthesise it ourselves- it derived from a vitamin niacin.
Therefore NADH must be re-oxidised to allow glycolysis to continue.
Describe ways in which NAD+ is recycled (i.e. NADH is re-oxidised) in the absence of oxygen, and name the organisms in which these pathways occur.
Alcoholic fermentation: yeast and some other microorganisms can form ethanol from pyruvate in the absence of oxygen by the action of alcohol dehydrogenase. This is a reductive enzyme so requires the input of electrons. These are donated by NADH, which is regenerated to NAD+. Humans cannot form ethanol from pyruvate.
Lactic acid fermentation:
some microorganisms and humans can form lactate from pyruvate. The reaction is catalysed by lactate dehydrogenase which is a reductive enzyme and requires an input of electrons, which are donated by NADH. This then allows glycolysis to continue to occur when there is a lack of oxygen (e.g. in a hardworking muscle cell). In a human, this will not continue for long as there will be a build up of lactic acid which is harmful to the cell and H+ will inhibit glycolytic enzymes.
What are the three names for the citric acid cycle?
Citric acid cycle, TCA cycle, Krebs cycle.
How many compartments do mitochondria have?
4- the outer membrane, the inner membrane, the intermembrane space and the matrix.
Where in the mitochondria does the vast majority of energy metabolism occur?
What are the invaginations of the inner mitochondrial membrane called?
Where does the Krebs cycle occur?
In the matrix of the mitochondria
What is the substrate for the krebs cycle?
What happens to pyruvate after it has been formed in glycolysis?
It was formed by glycolysis in the cytoplasm of the cell. It moves into the matrix of the mitochondria via a specific transporter.
There the pyruvate dehydrogenase complex catalyses the oxidative decarboxylation of pyruvate to acetyl coA. One CO2 molecule is released and the remaining 2 carbon atoms end up as the acetyl group in acetyl coA.
This is an oxidation reaction so NADH + H+ is formed.
Where does the pyruvate dehydrogenase complex reside?
In the matrix of the mitochondria.
How many enzymes make up the pyruvate dehydrogenase complex?
How many enzymes are involved in the control of the pyruvate dehydrogenase complex?
2: a kinase and a phosphatase in a single polypeptide.
How many coenzymes are involved in the action of the pyruvate dehydrogenase complex?
What are they?
They are thiamine, lipoic acid, coenzyme A, FAD and NAD+.
Is the reaction catalysed by the pyruvate dehydrogenase complex reversible?
No, it is irreversible. Acetyl coA cannot be converted back to pyruvate.
What is the major determinant of glucose oxidation in well oxygenated tissues in vivo?
The activity of PDC
How many reactions make up the TCA cycle?
Describe the TCA cycle.
A two carbon unit (from acetyl coA) condenses with a four carbon unit (oxaloacetate) to produce citrate (a 6 carbon unit). Citrate is oxidised and decarboxylated (which released a CO2 and produces an NADH + H+ from NAD+) 2 times to form a 4 carbon unit. Thus 2 CO2s have been released, and 2NADH + 2H+ have been produced.
A GTP ( which can be converted to ATP) is produced from GDP, by substrate level phosphorylation. There are 2 more oxidation reactions. In one of these oxidation reactions, FAD is reduced to FADH2, and in the other, NAD+ is reduced to NADH + H+.
Oxaloacetate is then regenerated and the cycle can start again.
In total, 3NADH + 3H+, 1 FADH2, one GTP and 2 CO2 are produced in one round of the TCA cycle.
Which two molecules in the TCA cycle have 3 carboxyl groups? (I.e. are tricarboxylic)
Citrate and isocitrate
What are the two ways of phosphorylating nucleotides?
Substrate level phosphorylation and oxidative phosphorylation.
Where are the enzymes of the TCA cycle located?
What is the one exception, and what else makes this enzyme different?
They are all located in the matrix of the mitochondria, except for Succinate dehydrogenase, which is integrated in the inner mitochondrial membrane. This enzyme is also different because it is the one enzyme in the TCA cycle which uses FAD as a cofactor instead of NAD+.
How do carbohydrates enter the TCA cycle?
They are converted to pyruvate, then Acetyl coA, and enter the TCA cycle in this form.
How do lipids enter the TCA cycle?
Lipids are converted to fatty acids by beta oxidation. These are converted to acetyl coA, which enters the TCA cycle.
How do proteins enter the TCA cycle?
Proteins are converted to amino acids. The amino acids are either converted to pyruvate, or to acetyl coA, or are converted to intermediates in the TCA cycle.
How many substrate level phosphorylation reactions occur in the TCA cycle?
One, in the conversion of GDP to GTP.
Which molecules inhibit the TCA cycle?
Remember- the TCA cycle has two functions: to provide energy, and to provide precursor molecules for biosynthetic reactions.
High ATP, NADPH and acetyl coA inhibits the TCA cycle, as these all mean that there is a large supply of energy.
High acetyl coA and high succinyl coA also inhibit the TCA cycle as these mean that there is a good supply of precursor molecules for biosynthetic reactions.
Which molecules activate some steps in the TCA cycle?
High ADP and high NAD+ as these signify a lack of energy.
What is generated in each turn of the TCA cycle?
3 x NADH + H+
1 x FADH2
1 x GTP
2 x CO2
How can GTP be converted to ADP?
GTP + ADP --> GDP + ATP