Thiamine is an important vitamin required for the conversion of pyruvate to acetyl-CoA. It is an important cofactor for the pyruvate dehydrogenase, an enzyme important for the conversion of pyruvate to acetyl-CoA. Pyruvate and NADH from glycolysis will continue to be produced; however, they cannot go any further without thiamine. This means that cells can’t undergo Krebs cycle and oxidative phosphorylation to produce ATP.

The buildup of pyruvate and NADH will cause the pyruvate molecules to undergo fermentation and produce lactic acid. It also will oxidize NADH, the product of which is essential for several cellular processes and needs to be regenerated.

The question states that the cell undergoes aerobic respiration. This means that the products from anaerobic respiration (glycolysis) will go through Krebs cycle and electron transport chain (aerobic respiration) to generate ATP. Lactate dehydrogenase is an enzyme important for converting the pyruvate molecules (from glycolysis) to lactate and oxidizing NADH. This reaction occurs in anaerobic fermentation when there is tissue hypoxia (decrease in oxygen).

If this inhibitor was placed in a cell that is deprived of oxygen, then there would be a buildup of pyruvate and NADH; however, since the inhibitor is added to cells undergoing aerobic respiration there will be no buildup. The pyruvate and NADH will undergo aerobic respiration and generate ATP. Note that red blood cells (RBCs) are unique in that they only use anaerobic respiration for ATP; therefore, adding lactate dehydrogenase inhibitor to RBCs will lead to a buildup of pyruvate and NADH.

Glycolysis has three irreversible enzymatic steps that help the substrate intermediates proceed in one direction through the glycolytic pathway: the enzymes are hexokinase, phosphofructokinase 1, and pyruvate kinase. Of these three enzymes, the most important enzyme that controls the rate of glycolysis is phosphofructokinase 1, or PFK-1. Pyruvate carboxylase is not used in glycolysis, but in gluconeogenesis.

The Krebs cycle is responsible for creating three molecules of NADH, and one molecule of both ATP and NADH. Acetyl CoA is the 2-carbon molecule that enters the Krebs cycle following pyruvate decarboxylation.

Each of the enzymes is involved in the citric acid cycle except for phosphoglycerate kinase, which is an enzyme that is utilized in glycolysis and gluconeogenesis, and catalyzes the reversible transfer or a phosphate group from 1,3-bisphosphoglycerate to ADP, producing and/or consuming 3-phosphoglycerate and ATP.

Gluconeogenesis is the process in which glucose can be generated from carbon structures that are not the canonical carbohydrate inputs. This is required for many organisms to maintain appropriate blood glucose levels, and for vertebrates gluconeogenesis primarily occurs in the liver, although it has been found to occur in the kidneys as well. 

The end product of glycolysis is two molecules of pyruvate, however the molecule used to starts the Krebs cycle is acetyl-CoA. Before going into the Krebs cycle then these 2 pyruvate molecules need to be converted into 2 acetyl-CoA molecules via pyruvate dehydrogenase complex, which occurs in the mitochondria and releases carbon dioxide.

During glycolysis, 2 molecules of NADH are produced per glucose. During the Krebs cycle, 3 molecules of NADH are produced per acetyl-CoA. With 2 molecules of glucose, glycolysis can run twice and produce 4 molecules of acetyl-CoA. Since 2 pyruvates are produced from glucose during glycolysis, a total of 4 are made from our 2 glucose molecules. These 4 pyruvates are then converted to 4 acetyl-CoA molecules. Each of these acteyl-CoA molecules runs through the Krebs cycle yielding a total 12 molecules of NADH. 4 from glycolysis, 12 from the TCA cycle so 16 molecules of NADH total.

Each turn of Krebs cycle produces 3 NADH and 1 ATP. Recall that each turn of Krebs cycle requires a molecule of acetyl-CoA (two-carbon molecule). Acetyl-CoA comes from pyruvate, which in turn comes from glucose. During glycolysis, a molecule of glucose (six-carbon molecule) is converted into two pyruvate molecules; therefore, a molecule of glucose will eventually lead to two acetyl-CoA molecules. This means that there are two turns of Krebs cycle for every glucose molecule and, therefore, for a given molecule of glucose Krebs cycle produces 6 NADH and 2 ATP.

Glycolysis produces a net of 2 ATP and 2 NADH; therefore, Krebs cycle produces three times the NADH and the same amount of ATP as glycolysis.

The question states that the enzyme inhibitor slows down the Krebs cycle. This suggests that the inhibitor blocks the rate-determining step of the cycle. Recall that the rate-determining step of Krebs cycle is the isocitrate dehydrogenase step. This converts the six-carbon isocitrate to five-carbon alpha-ketoglutarate. Blocking this step leads to the build up of six-carbon isocitrate and decrease in all of the downstream molecules (including alpha-ketoglutarate).