Anabolic Pathways and Synthesis - Biochemistry

In order to linearize using a linkage, there needs to be an unbound carbon on the 1 position. However, sucrose is a linkage and doesn't have a carbon available to linearize in the 1 position. It isn't a reducing sugar and therefore cannot be linearized. All of the other sugars have their anomeric carbon located at the 1 position and all of them are reducing sugars that can be linearized.

From the question stem, we're told that the enzyme phosphoglucomutase is responsible for interconverting two intermediate forms of glucose, both glucose-1-phosphate and glucose-6-phosphate. We're then asked to determine the most likely metabolic pathway that glucose-6-phosphate would be used for in a fasting individual.

First, it's important to remember that in an individual that is fasting, energy resources become more scarce. Therefore, the body tries to conserve as much energy as it can in this state. Furthermore, since the brain relies mostly on glucose for its metabolism, the body tries to keep a relatively stable level of glucose in the blood. As a result, many tissues in the body switch from using glucose to instead using other energy sources, such as fatty acids or ketone bodies. In order to help ensure that blood levels of glucose remain stable, the liver increases its rate of gluconeogenesis, which generates glucose from non-sugar substrates, such as pyruvic acid, certain amino acids, and glycerol. Therefore, we would expect glucose-6-phosphate to be funneled mostly into the gluconeogenesis pathway.

Even though glucose-6-phosphate can also be diverted to other pathways, such as glycolysis, glycerogenesis, or the pentose phosphate pathway, all of these pathways result in a net consumption of glucose. In a fasting state, this is the opposite of what we would want, since blood glucose levels need to be mostly stabilized in order to ensure that nervous tissue has an adequate supply.

From the question stem, we are told that glucose-6-phosphate dehydrogenase oxidized glucose into another compound, and also produces a molecule of NADPH in the process. In order to determine the way in which this enzyme is likely to be regulated, it's important to consider feedback mechanics.

Since this enzyme is producing NADPH when it is turned on, we would expect this product to negatively regulate the enzyme via feedback inhibition. Moreover, since we know that is a reactant, we can correctly assume that having a high concentration of this will likely drive the reaction forward by turning the enzyme on. Thus, would be expected to allosterically activate this enzyme. Furthermore, the question stem tells us nothing about the unphosphorylated forms of these cofactors, therefore we have no way of knowing how many NADH or affects this enzyme, if they do at all.

From the question stem, we are shown the reaction in which glucose-6-phosphate is transformed into ribulose-5-phosphate. We are then asked to determine which type of reaction is occurring in this process.

We can also notice from the reaction that is a reactant, and is a product. Therefore, the is being reduced to form . In order for this reduction reaction to happen, there needs to be a simultaneous oxidation reaction occurring, since the electrons need to come from somewhere. In this case, the electrons are coming from glucose-6-phosphate. Therefore, as is reduced to , glucose-6-phosphate is oxidized to ribulose-5-phosphate. Thus, this is an oxidation reaction.

Also, it's important to note that this is not a carboxylation reaction. In fact, it is actually a decarboxylation reaction, since one of the carbon atoms on glucose is converted into carbon dioxide.

Moreover, this is also not a phosphorylation reaction, as the reactant and products have an equal number of phosphate groups.

And lastly, this is not an isomerization reaction because glucose-6-phosphate and ribulose-5-phosphate have different molecular formulas, thus they cannot ever be structural isomers.

Glucose is converted into glycogen during the process called glycogenesis. Its structure consists of many linear alpha(14) glycosidic bonds, and also many branched alpha(16) glycosidic bonds. This heavily-branched structure means that there are many free ends, which are the substrates for glycogen phosphorylase. A debranching enzyme is needed to lyse the alpha(16) glycosidic bonds. Cellulose is a polymer glucose linked together via of beta(14) glycosidic bonds. Humans lack enzymes to catalyze the lysis of these bonds in cellulose.

Oxaloacetate is a metabolite of the citric acid cycle, which takes place in the mitochondrial matrix. Oxaloacetate cannot diffuse across the mitochondrial matrix, but malate can. So oxaloacetate is reduced to malate by malate dehydrogenase, and can now enter into the cytoplasm. Since malate dehydrogenase can catalyze the reverse reaction as well as the forward reaction, it can be used again to reform oxaloacetate. Once in the cytoplasm, oxaloacetate is converted into phosphoenolpyruvate (PEP) and continues gluconeogenesis.

For each molecule converted into carbohydrate, 3 ATPs and 2 NADPHs are consumed. The Calvin cycle is initiated when and ribulose 1,5 biphosphate combine. The enzyme ribulose biphospate carboxylase catalyzes the reaction in the stroma of chloroplasts, and is considered one of the most the world's most abundant proteins. Glyceraldehyde 3-phosphate, which is a by-product of the cycle, goes on to be a building block of sugar, fatty acid, and amino acid synthesis.

The major distinction between NADH and NADPH is that NADH is generally used in catabolic reactions meant to produce ATP. NADPH, on the other hand, is used primarily in anabolic reactions meant to build macromolecules from their smaller parts.

UDP-glucose is the activated form of glucose that works to build chains of glycogen. The other listed molecules do not serve this function.

Aspartate can form arginosuccinate, which can then release a fumarate molecule. The fumarate can enter into the Krebs cycle and eventually the pathway can lead to gluconeogenesis. The arginine from the arginosuccinate can continue through the urea cycle.

Gluconeogenesis is the production of glucose from other sources than carbohydrates, such as from pyruvate, amino acids, lactate and glycerol. Phosphoenolpyruvate carboxykinase converts oxaloacetate to phosphoenolpyruvate and carbon dioxide. It also produces GDP from GTP. It is regulated by hormones, such as glucagon and cortisol.

The pentose phosphate pathway produces NADPH and five-carbon sugars. The net reaction is:

The pathway is also important in purine precursor synthesis.

Ingestion of high amounts of ethanol leads to increased NADPH. High levels of NADPH inhibit gluconeogenesis followed by low glucose levels in the absence of dietary intake. In acute ingestion of alcohol, hypoglycemia (low levels of glucose in the blood) can follow due to inhibition of gluconeogenesis.

The monomers from which DNA is polymerized are deoxyribonucleoside triphosphates (dNTPs). When DNA is in its polymerized form, the monomers are deoxyribonucleoside monophospates (dNMPs). This means that each nucleotide that is layed down by DNA polymerase must first have two of its phosphates hydrolyzed (beta and gamma). It is this hydrolysis that drives the nonspontaneous reaction of DNA polymerization.

Prokaryotic DNA replication occurs in the cytoplasm, since these cells lack nuclei. Prokaryotic genomes are comprised of a single circular chromosome, with one origin of replication. Translation is the process of protein synthesis, which occurs on ribosomes free in the cytosol (or on ribosomes embedded in the rough endoplasmic reticulum in eukaryotes).

The only true statement is that prokaryotic DNA replication is faster than eukaryotic DNA replication.

Primase is actually an RNA polymerase, not a DNA polymerase. Primase creates an RNA primer which is used to replicate short-stranded DNA. Primers serve as the starting point for DNA synthesis, so RNA polymerases wouldn’t require them. There are a number of means by which DNA is repaired including direct repair, excision repair, and homologous recombination. DNA repair, however, does not involve the use of RNA polymerases. All rRNA (except 5S rRNA) is synthesized by RNA polymerase I (or to be specific, the polymerase creates a pre-RNA which matures into rRNA), while the precursors of mRNA are synthesized by RNA polymerase II. Primers are short, complementary RNA sequences that serve as the starting point for DNA synthesis; the DNA polymerase begins replication at the primer’s 3’ end, and uses the opposite strand as a template. Without the primer, the DNA polymerase would not have an existing strand of nucleotides onto which it could attach new nucleotides.

To answer this question, it's important to understand that DNA replicates in a semi-conservative fashion. This means that the two complementary strands of DNA split apart, and a new complementary strand is added to each of the parent strands. Thus, each daughter DNA molecule will be composed of one parent strand, and one newly synthesized strand. Since we know that adenine base pairs with thymine, and guanine base pairs with cytosine, the ratio of is expected to remain the same, provided no mutations occur.

DNA replication is the process of producing a duplicate copy of a DNA strand. DNA double helix is first unwound by breaking the hydrogen bonds between nitrogenous bases, giving two parent strands. Next, these unwound DNA strands are utilized as a template strand (parent strand) to create a daughter strand that is identical to the parent strand. After completion of the replication, the parent strand and daughter strand hybridize (hydrogen bonds re-form between bases) and form a double helix. Note that the original parent strands never re-hybridize.

Epigenetic changes refer to alterations in DNA molecules or histones. These alterations can enhance or suppress transcription of DNA to RNA. DNA replication is unaffected by epigenetic changes.

As mentioned, each parent strand produces an identical, daughter strand that ultimately re-hybridizes with the parent strand (forms double helix structure); therefore, each parent strand only produces one daughter strand.

Unwinding of the double helix involves breaking the hydrogen bonds between nitrogenous bases from adjacent DNA molecules. Recall that hydrogen bonds occur between a hydrogen atom and either a nitrogen, oxygen, or fluorine atom. The nitrogenous bases found in DNA and RNA molecules do not contain any fluorine atoms; therefore, fluorine (although it is involved in hydrogen bonds in other molecules) is not involved in hydrogen bonding between nitrogenous bases.