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THE BIOCHEMICAL ROLE OF VITAMINS AND THE METABOLIC PATHWAYS THEY PARTICIPATE IN

THE BIOCHEMICAL ROLE OF VITAMINS AND THE METABOLIC PATHWAYS THEY PARTICIPATE IN

INTRODUCTION
Vitamins are a diverse group of organic molecules required in very small quantitiesin the diet for health, growth, and survival (Latin vita, life). The absence of a vitamin from the diet or an inadequate intake results in characteristic deficiency signs and, ultimately, death. The amount of each vitamin required in the diet is small (in the microgram or milligram range), compared with essential amino acid requirements (in the gram range). The vitamins are often divided into two classes, water-soluble vitamins and fat-soluble vitamins. This classification has little relationship to their function but is related to the absorption and transport of fat-soluble vitamins with lipids.

Most vitamins are used for the synthesis of coenzymes, complex organic molecules that assist enzymes in catalyzing biochemical reactions, and the deficiency symptoms reflect an inability of cells to carry out certain reactions. However, some vitamins also act as hormones. Although the RDA or AI for each vitamin varies with age and sex, the difference is usually not very large once adolescence is reached.

For example, the RDA for riboflavin is 0.9 mg/day for males between 9 and 13 years of age, 1.3 mg/day for males 19 to 30 years of age, still 1.3 mg/day for males older than 70 years, and 1.1 mg/day for females aged 19 to 30 years. The largest requirements occur during lactation (1.6 mg/day).

Vitamins, by definition, cannot be synthesized in the body, or are synthesized from a very specific dietary precursor in insufficient amounts. For example, we can synthesize the vitamin niacin from the essential amino acid tryptophan, but not in sufficient quantities to meet our needs. Niacin is therefore still classified as a vitamin.

Excessive intake of many vitamins, both fat-soluble and water-soluble, may cause deleterious effects. For example, high doses of vitamin A, a fat-soluble vitamin, can cause desquamation of the skin and birth defects. High doses of vitamin C cause diarrhoea and gastrointestinal disturbances.


TYPES OF VITAMINS

Vitamins are of two types;
1. WATER SOLUBLE VITAMINS
2. FAT SOLUBLE VITAMINS
 
1.WATER SOLUBLE VITAMINS:
· Non- B- complex
- Ascorbic Acid

· B-complex

 Energy Releasing
-  Thiamine (Vitamin B1)
-  Ribroflavin( Vitamin B2)
-  Niacin(Vitamin B3)
-  Biotin
-  Panthotenic acid

Hematopoeitic
-  Follic acid
-  Vitamin B12
 
 Others
-  Pyridoxine( Vitamin B6)
-  Pyridoxal
-  Pyridoxamine


FOLLIC ACID
Plays a key role  in one-carbon metabolism, is essential for biosynthesis of several compounds. Follic acid deficiency is probably the most common deficiency especially among pregnant women and alchoholics.

VITAMIN B12 (Cobalamin)
Required in humans for two essential enzymatic reactions, synthesis of methionine and the isomerisation of methylmalonyl CoA, that is produced during the degradation of some amino acids and fatty acids with odd numbers of carbon atoms. When the vitamin is deficient, abnormal fatty acids accumulate and become incorporated into cell membranes. This may account for some neurologic manifestation of  Vitamin B12 deficiency.

VITAMIN C (Ascorbic acid)
The active form of Vitamin C is ascorbate acid. The main function of ascorbate is the reducing agent in several different reactions. It has a well- documented role as a co enyme in the hydroxylation reaction. For example, hydroxylation of prolyl and lysyl-residues of collagen. Vitamin C is therefore required for the maintenance of normal connective tissue as well as for wound healing. Vitamin C also facilitates the absorption of dietary IRON from the intestine.

VITAMIN B6
Is a collective term for pyridoxine, pyridoxamine and pyridoxal, all derivatives of pyridine. They differ only in the nature of the functional group attached to the ring. Pyridoxine occurs primarily in plant whereas Pyridoxal and Pyridoxamine are found in food obtained from animals. All three compounds can serve as precursors for the biologically active enzyme PYRIDOXAL PHOSPHATE. Pyridoxal phosphate functions as coenzyme for a large number of enzymes, particularly those that catalyze reactions involving amino acids.

VITAMIN B1 (Thiamine)
Thiamine pyrophosphate is the biologically active form of the vitamin, formed by the transfer of pyrophosphate group from ATP to thiamine. Thiamine pyrophosphate serves as a coenzyme in the formation or degradation of alpha ketols by transketolase and in the oxidative decarboxylation of alpha keto- acids.

NIACIN
Niacin or nicotinic acid is a substituted pyridine derivative. The biologically active co enzyme forms are Nicotinamide adenine dinucleotide (NAD+) and its pyrophosphorylated derivative, nicotinamide adenine dinucleotide diphosphate (NADP+). Nicotinamide, a derivative of nicotinic acid that contains an amide group instead of a carboxyl group, also occurs in the diet. Nicotinamide is readily deaminated in the body and therefore, is nutritionally equivalent to nicotinic acid.NAD+ and NADP+ serve as coenzymes in oxidation-reduction reactions in which coenzyme undergoes reduction of the pyridine ring by accepting a hydride. The reduced forms of NAD+ and NADP+ are NADH and NADPH, respectively.

VITAMIN B2 (Riboflavin)
Riboflavin fulfills its role in metabolism as the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). FMN is formed by ATP-dependent phosphorylation of riboflavin, whereas FAD is synthesized by further reaction of FMN with ATP in which its AMP moiety is transferred to the FMN. The main dietary sources of riboflavin are milk and dairy products. In addition, because of its intense yellow color, riboflavin is widely used as a food additive.

Flavin coenzymes are electron carriers in Oxidoreduction Reactions. These include the mitochondrial respiratory chain, key enzymes in fatty acid and amino acid oxidation, and the citric acid cycle.

BIOTIN
Biotin is a coenzyme of carboxylase enzymes. Biotin functions to transfer carbon dioxide in a small number of carboxylation reactions. A holocarboxylase synthetase acts on a lysine residue of the apoenzymes of acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, or methylcrotonyl-CoA carboxylase to react with free biotin to form the biocytin residue of the holoenzyme. Biotin also has a role in regulation of the cell cycle, acting to biotinylate key nuclear proteins.

PANTOTHENIC ACID
As part of CoA and ACP, pantothenic acid acts as a carrier of acyl radicals. Pantothenic acid has a central role in acyl group metabolism when acting as the pantetheine functional moiety of coenzyme A or acyl carrier protein (ACP). The pantetheine moiety is formed after combination of pantothenate with cysteine, which provides the SH prosthetic group of CoA and ACP. CoA takes part in reactions of the citric acid cycle, fatty acid synthesis and oxidation, acetylations, and cholesterol synthesis. ACP participates in fatty acid synthesis. The vitamin is widely distributed in all foodstuffs, and deficiency has not been unequivocally reported in human beings except in specific depletion studies.

FAT SOLUBLE VITAMINS:

-  Vitamin A (Retinol, Beta Carotene)
-  Vitamin D ( Cholecalciferol)
-  Vitamin K ( phylloquinone, menaquinones)
-  Vitamin E ( tocopherol)

VITAMIN A
Retinoids comprise retinol, retinaldehyde, and retinoic acid (preformed vitamin A, found only in foods of animal origin); carotenoids, found in plants, Vitamin a has a function in vision In the retina, retinaldehyde functions as the prosthetic group of the light-sensitive opsin proteins, forming rhodopsin (in rods) and iodopsin (in cones). Any one cone cell contains only one type of opsin and is sensitive to only one color. A most important function of vitamin A is in the control of cell differentiation and turnover.

VITAMIN D
Vitamin D is really a hormone and  not strictly a vitamin since it can be synthesized in the skin, and under most conditions that is its major source. Only when sunlight is inadequate is a dietary source required. The main function of vitamin D is in the regulation of calcium absorption and homeostasis; most of its actions are mediated by way.

VITAMIN K
Vitamin K is required for synthesis of blood- clotting proteins. Vitamin K was discovered as a result of investigations into the cause of a bleeding disorder hemorrhagic (sweet clover) disease—of cattle, and of chickens fed on a fat-free diet. The missing factor in the diet of the chickens was vitamin K, while the cattle feed contained dicumarol, an antagonist of the vitamin. Antagonists of vitamin K are used to reduce blood coagulation in patients at risk of thrombosis—the most widely used agent is warfarin.

Three compounds have the biologic activity of vitamin K, phylloquinone, the normal dietary source, found in green vegetables; menaquinones, synthesized by intestinal bacteria, with differing lengths of side-chain; menadione, menadiol, and menadiol diacetate, synthetic compounds that can be metabolized to phylloquinone.

Menaquinones are absorbed to some extent but it is not clear to what extent they are biologically active as it is possible to induce signs of vitamin K deficiency simply by feeding a phylloquinone deficient diet, without inhibiting intestinal bacterial action.

VITAMIN E
Vitamin E does not have a precisely defined metabolic function.  However, it does act as a lipid-soluble antioxidant in cell membranes, where many of its functions can be provided by synthetic antioxidants. VitaminE is the generic descriptor for two families of compounds, the tocopherols and the tocotrienols.

The main function of vitamin E is as a chain-breaking, free radical trapping antioxidant in cell membranes and plasma lipoproteins.


WATER SOLUBLE VITAMINS



 FAT SOLUBLE VITAMINS

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Guest-Emilia

Great Examination Experience For The USMLE Step 2 CS

WARNING:: LONG POST!

Hi guys, this page has been so invaluable in my prep and I wanted to say thank you to everyone who answered any of my questions and I want to give back to this community for those who are still on this journey.

Test Center: Chicago, I loved it, SPs were great, played their part well, they weren't always "nice", but I kept in mind that they have a part to play so I didn't take it personally.

I did not take any courses because
a)I could not afford it and
b) I don't think it's necessary. If you find good SPs that aren't afraid to dish out constructive criticism, you should be ok.

Study time: I studied for about a month, I was lucky to have dedicated SPs that I met with almost every day and who would give me honest feedback on where I can improve.

Study materials: FA! Obviously :D I also did cases from Amboss and some from the Kaplan book. Kaplan was very complex and on the real exam I did not find my patients as complicated as in the Kaplan book. I recommend you do a few cases from there to practice time management because their cases are long. I also had note cards with mnemonics that I jotted down from different sources, and I will upload my file with them. I also made note cards of different Chief Complaints with possible DDx, physical exams and workups. I typed the notes for EVERY case I practiced. My typing speed is normally around 65-70 wmp, let me tell you, on exam day I turned into my grandma with a keyboard. :D Try to find the correct ways to describe physical findings and believe me when I tell you, the SPs WILL have physical findings.

Now here are some of my personal pro tips for test day:

-I tried to be out of the room by 12 minutes because the keyboard at the test center was different from mine at home and I personally found it difficult to "compose" my PN and type it out in 10 minutes. My notes were VERY short and to the point and I always started from the DDx and Work up, then PE, and finally HPI. The Drs checking your PN are "intimately familiar with the case" (according to USMLE website) so I wanted to make sure they knew I ran out of time on HPI (which the SP documents what you've asked), rather than on DDx and look like I couldn't come up with differentials or workup or PE.

• My physicals exams were TO THE POINT, super focused. Took 2-3 minutes max. I ALWAYS started with the most relevant PE, for example, shortness of breath? I did full pulmo first and foremost, then whatever else I deemed necessary like CVS and maybe extremities. Only neuro and MMSE took longer.

• On the patient note I copy/pasted the vitals. Started typing from Ddx and workup, then PE, then HPI at the end.

• CLOSURE!!! I can't stress this enough. Skimp on the PE, but do NOT miss closure. As I said on another post, this is a real human with a real problem before you.You don't show empathy for repeatedly apologizing to the SP that they are ill, you show empathy by asking questions and responding in a way that makes them feel heard and validated. If they are afraid they have cancer for example, inquire why they think so? Tell them you can see they are worried about this, so it's the first thing you'll rule out. When mentioning different tests, ask if they know what that is or explain in layman's terms.

• I never used the word cancer even if it was my first diagnosis. I always said they could have (second non cancer ddx or third non-cancer ddx), but I also want to run let's say a CXR to rule out something potentially more serious. I did this with my actual patients in my practice, because I don't want my patients to spend the next few days scared shitless they might be dying :)

• Counseling: I know a lot of sources will tell you to counsel during closure, but I counseled the second they told me they smoke/drank/did drugs. I also asked screening tests here and there and counseled getting those. Some of my patients actually told me they want to quit and I said this: "That's great, I'm glad to see you want to take better care of yourself. How about today we figure out what's going on with your current complaint, and then we can meet up again in a week or two to come up with a plan together that will help you quit (smoking/drinking/whatever)?"

• Challenging questions: If you didn't get any on a case that is GOOD NEWS! It means you answered before they could even ask. I had a few cases like that. For the rest - keep it VAGUE! You haven't run any tests yet, you have no idea what they have/if they will get better/whatever (don't say it like that, I think FA has fantastic examples of how to answer challenging questions, review them.)

• Last but not least: STAY CALM! I'm a nervous test taker and I had a nervous breakdown the morning of the exam, but managed to transform the anxiety into excitement because I get to do the thing I love and it showed. I had A LOT OF FUN during this exam (people who were with me on test day said I was crazy for "enjoying" an exam haha), something I can't say about the other 2 steps. I enjoyed talking to the SPs and getting to know them. You will make mistakes! Don't let that derail you! I forgot to wash my hands for 1 encounter, and remembered halfway through PE. I said "Oops, I don't think I washed my hands, I'm very sorry, let me do that right now!" and my SP smiled and said "It's ok". I forgot to ask some questions and remembered after leaving the room. I just reminded myself that it's not fatal and to focus on the next case.

Ok that's about it. If anyone else has any questions, post them in the comments and I will do my best to answer them! :) I hope I was able to help some people that still have the test and I wish you all luck on this exam and on your journey to residency! :)